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Article

Impact of Nitrogen on Downy Mildew Infection and Its Effects on Growth and Physiological Traits in Early Growth Stages of Cucumber

by
Yafei Wang
1,*,
Qiang Shi
2,*,
Xiaoxue Du
3,
Tianhua Chen
4 and
Mohamed Farag Taha
1,5
1
School of Agricultural Engineering, Jiangsu University, Zhenjiang 212013, China
2
School of Science and Technology, Shanghai Open University, Shanghai 200433, China
3
School of Optoelectronic Engineering, Changzhou Institute of Technology, Changzhou 213032, China
4
College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, China
5
Department of Soil and Water Sciences, Faculty of Environmental Agricultural Sciences, Arish University, Arish 45516, Egypt
*
Authors to whom correspondence should be addressed.
Horticulturae 2025, 11(10), 1182; https://doi.org/10.3390/horticulturae11101182
Submission received: 12 August 2025 / Revised: 16 September 2025 / Accepted: 30 September 2025 / Published: 2 October 2025
(This article belongs to the Section Developmental Physiology, Biochemistry, and Molecular Biology)
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Versions Notes

Abstract

Nitrogen is a critical nutrient that influences plant growth and resistance to pathogens; however, its impact on disease dynamics, particularly downy mildew infection, and the associated physiological responses in cucumber during early growth stages remains poorly understood. To evaluate the combined effects of downy mildew (caused by Pseudoperonospora cubensis) infection and nitrogen application on cucumber growth and physiological traits during the seedling and vine development stages, two downy mildew treatments— infected (B0) and non-infected(B1)—and three nitrogen levels—T1 (N-50%), T2 (N-100%), and T3 (N-150%)—were applied. Significant differences were observed between all treatments (p < 0.05). Among them, the B1T3 treatment had the most pronounced stimulatory effect, particularly on growth parameters (such as plant height, stem diameter, and leaf area). Without any disease infection (B1), the B1T2 treatment showed an increasing trend in photosynthetic rate and a more notable rise in stomatal conductance. In contrast, with downy mildew infection (B0), photosynthetic rates declined under B0T1 and B0T2. Moreover, with downy mildew infection (B0), the intracellular CO2 concentration, stomatal conductance, and transpiration rate of cucumber leaves decreased in the B0T1, B0T2, and B0T3 treatments. Plant height, stem diameter, and leaf area responded variably to nitrogen levels and downy mildew infection. The total root length, root surface area, average root diameter, total root volume, and total root tips of cucumber plants were significantly different under different experimental conditions (p < 0.05). Consequently, this study provides a theoretical basis for stress-resistant cucumber cultivation in greenhouses and has practical implications for advancing the sustainable development of the greenhouse cucumber industry.

1. Introduction

China possesses an extensive greenhouse infrastructure, with a total area—excluding small- and medium-sized arched greenhouses—exceeding 2.1 million hectares, including 51,800 hectares of multi-span greenhouses, offering substantial space for cucumber cultivation [1,2,3]. As the world’s leading cucumber producer, China has cultivated cucumbers on over 1.2 million hectares. In 2020, cucumber production reached 73.36 million tons, accounting for 79.8% of the global output [4,5]. Cucumber (Cucumis sativus L.) is a perennial herbaceous plant of the Cucurbitaceae family belonging to the Cucumis genus. According to its uses, it can be classified into fresh consumption types (such as common cucumbers and fruit cucumbers) and processing types (such as pickled cucumbers); based on its maturity, there are early-maturing, mid-maturing, and late-maturing varieties. Cucumber is a diploid plant with a total of 14 chromosomes. Cucumber is a key component of China’s “vegetable basket” project and a vital source of income for farmers [6,7]. During growth, cucumbers are frequently exposed to abiotic stresses such as salinity, drought, high and low temperatures, and UVB radiation, as well as biotic stresses like bacterial and fungal infections [8,9]. Additionally, greenhouse conditions—characterized by high temperature and humidity—promote the occurrence, spread, and prevalence of airborne fungal pathogens responsible for downy mildew and powdery mildew [10,11]. Thus, understanding cucumber growth and physiological responses under these stresses is crucial for implementing timely and effective interventions to maintain yield and quality in greenhouse cultivation, while also supporting the sustainable development of the industry [12,13].
Downy mildew, caused by Pseudoperonospora cubensis, is a major biotic stress threatening greenhouse cucumber production [14,15]. The disease primarily affects leaves, with symptoms first appearing during the seedling stage as chlorotic spots on cotyledons, which develop into irregular yellow lesions [16,17]. Under moist conditions, a grayish-black mold forms on the underside of the cotyledons. As the disease progresses, cotyledons rapidly yellow and desiccate. Prior to flowering, pale-green water-soaked lesions appear on true leaves. These lesions expand within the confines of leaf veins, becoming polygonal and turning from yellow–green to light brown [18,19]. During fruiting, these lesions coalesce into large necrotic areas, leading to leaf curling and wilting from the margins. Under humid conditions, grayish-black fungal growth appears on the undersides of infected leaves. In severe cases, complete defoliation occurs. The high-humidity and low-light conditions typical of greenhouses facilitate rapid outbreaks and dissemination of downy mildew, resulting in substantial yield losses for growers [20,21].
Nitrogen is a key macronutrient essential for plant growth [22,23]. Proper nitrogen application supplies adequate nutrients for cucumbers, promotes stem and leaf development, enhances leaf thickness and dark green coloration, and improves photosynthetic efficiency, thereby supporting overall growth and fruit development [24]. An optimal nitrogen level fosters vigorous seedlings and strengthens stress resistance [25]. Sufficient nitrogen during growth ensures nutrient availability for fruit formation, increases fruit set and growth rates, improves fruit plumpness, and ultimately enhances yield [26,27]. Additionally, appropriate nitrogen application elevates levels of soluble proteins, free amino acids, and other nutrients in the fruit, thereby improving cucumber taste and nutritional value [28]. However, excessive nitrogen fertilization adversely affects cucumber growth. It promotes excessive vegetative growth, characterized by enlarged leaf area, thin and pale leaves, elongated stems, reduced stress tolerance, and increased susceptibility to frost and disease [29]. Overuse also results in abnormally tall and weak plants with impaired fruit set [30,31], delayed fruit development, and a higher incidence of deformed or bitter fruits. Furthermore, excess nitrogen application thins cell walls and softens tissues, thereby diminishing resistance to diseases, pests, and lodging. This increases vulnerability to biotic stress and environmental stressors. Prolonged over-application also raises soil salt concentration, hindering water and nutrient uptake [32,33]. Consequently, cucumber growth declines, and soil degradation issues such as compaction and acidification arise, compromising long-term soil sustainability.
Many factors can affect the occurrence and severity of downy mildew in cucumber seedlings, for example, weather conditions, such as humidity, temperature, and rainfall, and crop management. One of the most important factors is the amount of nutrients supplied to the plant. It is well known that there is a relationship between nutrients and the incidence of plant diseases. For example, when cucumbers are infected with downy mildew, reducing the irrigation volume of the nutrient solution can increase the root parameters of the cucumber plants [14], and increasing the application amount of potassium fertilizer can promote the photosynthesis of the cucumber plants and the growth of their roots [34]. Therefore, proper nutrient management may be useful for developing control strategies in sustainable agriculture [35]. Wang et al. [14] conducted an experiment to observe the responses of cucumber plants to drought stress and cucumber downy mildew infection. They set up two levels of downy mildew infection with disease infection and without disease infection, and three fertilizer requirement levels (full fertilization (T1), moderate nutrient solution deficiency (T2), and severe nutrient solution deficiency (T3)) in a greenhouse. The research results showed that the ratio of roots to stems of cucumbers exhibited different responses under different nutrient solution irrigation treatments and downy mildew infection conditions. Under the disease infection and severe nutrient solution deficiency treatment, the ratio of roots to stems of cucumbers was the highest. According to Mandal et al. [36], higher levels of nitrogen fertilization increased the incidence of downy mildew on isopagus (Plantago ovate L.), and the same result was found for apple scab [37] and powdery mildew in tomato [38]. In contrast, Sivaprakasam [39] reported that downy mildew on pearl millet (Pennisetum glaucum L.) was not significantly affected by nitrogen supplementation. From the above, we conclude that there are complex relationships between the type and timing of fertilizer application and various cucumber diseases.
Currently, limited research exists on the combined effects of nitrogen stress and downy mildew infection on photosynthesis and growth in greenhouse cucumbers. Therefore, this study aimed to explore the combined effect of nitrogen fertilization and downy mildew infection on cucumbers, as well as their effects on plant physiology and growth. This study employed two levels of downy mildew infection—infected (B0) and non-infected (B1)—and three nitrogen treatments: T1 (N-50%), T2 (N-100%), and T3 (N-150%). A total of six experimental treatments were set up, namely, B0T1, B0T2, B0T3, B1T1, B1T2, and B1T3. The effects of these treatments on photosynthetic, growth, and root parameters of cucumber leaves were examined. Among them, in the case of fungal blight infection (B0), three nitrogen levels were set, namely, B0T1, B0T2, and B0T3, which were the experimental treatments. The effects of single nitrogen fertilizer stress on the growth and physiological parameters of cucumbers were evaluated. In order to provide an important reference for formulating scientific and reasonable pest and disease control strategies and fertilization management measures, thereby reducing the adverse effects on both the photosynthesis and growth parameters of greenhouse cucumber plants, improving the yield and quality of cucumbers has important practical significance.

2. Materials and Methods

2.1. Study site and Plant Material

Cucumber plant samples were cultivated in a Venlo-type greenhouse at Jiangsu University (119.515293° E, 32.19940° N) using the Jinyou No. 1 variety (developed by the Tianjin Academy of Agricultural Sciences, Tianjin, China). The greenhouse is oriented east–west. The roof height is 4.73 m, the shoulder height is 4.0 m, each span is 3.2 m, and the length of the greenhouse is 45 m. The greenhouse has a thermal insulation curtain. It is heated by pipelines and cooled by shading nets, wet curtain fans, and other equipment. The environmental control is regulated by a computer automatic control system. The Jinyou No. 1 variety is a type of cucumber with wide adaptability, rapid growth, abundant fruiting nodes, and high yield. It is widely cultivated in the local area. The experiments were conducted from 25 July to 18 August 2024. During this period, the average greenhouse temperature was 25.8 °C (16.83~36.47 °C), with the relative humidity averaging at 85.6% RH (ranging from 23.7% to 95.6%). As shown in Figure 1, two downy mildew treatments—infected (B0) and non-infected (B1)—were combined with three nitrogen levels—T1 (N-50%), T2 (N-100%), and T3 (N-150%)—resulting in six treatment groups: B0T1, B0T2, B0T3, B1T1, B1T2, and B1T3. Each treatment comprised six cucumber plants. There were 36 cucumber plants in total across six experimental treatments. Among them, in the case of fungal blight infection (B0), three nitrogen levels were set, namely, B0T1, B0T2, and B0T3, which were the experimental treatments. The effects of single nitrogen fertilizer stress on the growth and physiological parameters of cucumbers were evaluated.
To prevent the spread of downy mildew, cucumber plants without downy mildew infection (B1) and three nitrogen levels—T1 (N-50%), T2 (N-100%), and T3 (N-150%)— were planted in three separate cultivation troughs on the west side of the greenhouse, with two rows in each trough. Similarly, cucumber plants with downy mildew infection (B0) and three nitrogen levels—T1 (N-50%), T2 (N-100%), and T3 (N-150%)—were planted in three separate cultivation troughs on the east side of the greenhouse, also with two rows per trough and the same spacing. Each trough was spaced 60 cm apart, with a plant spacing of 50 cm and a row spacing of 32 cm. A glass wall separated the east and west sides of the greenhouse. All seedlings were grown in perlite pots. On 29 July 2024, 10 L flowerpots were filled with perlite, which was rinsed with tap water to remove dust that could affect cucumber seedling growth. The perlite used had a particle density of 2.2 g/cm3, a bulk density of 80 kg/m3, a total porosity of 60.3%, and was primarily composed of SiO2. On 30 July 2024, cucumber seedlings at the “two leaves and one heart” stage were removed from plugs and transplanted into the pots.

2.2. Downy Mildew and Nitrogen Treatments

2.2.1. Downy Mildew Inoculation

In order to obtain a suspension of cucumber downy mildew spores, fresh diseased leaves from pre-inoculated and infected cucumber plants were selected to ensure that they were free from bacterial pollution and had typical symptoms. Then, a single “fully developed” cucumber downy mildew lesion (with a diameter of 3–5 mm and a yellow–brown polygonal shape and a white dense mold layer on the lower side of the leaf) and “far away from other lesions” (with a distance of ≥2 cm to avoid mixing with spores of other lesions) were accurately cut with scissors sterilized by high-pressure steam. The cut lesion was placed in a sterile centrifuge tube containing 10 mL of sterile water and gently shaken for 5 min (100 rpm) to make spores fall off, and then a sterile pipette was used to collect the supernatant (discarding the remaining diseased leaf tissue). This led to the initial preparation of the spore suspension. Then, counting was performed with a blood cell counter, wherein 100 μL of the supernatant was added to the counting room, it was observed under a 10× microscope, the complete spores in the four corners and five squares in the center were counted, it was diluted with sterile water according to the statistical results, and, finally, the spore suspension of pseudoperonospora cubensis was adjusted to a concentration of 1 × 106 spores/mL (repeating counting twice after adjustment to ensure that the concentration error was less than 5%) [14]. From 18:00 to 19:00 on the 8th day after the cucumber plants were transplanted, the bottom of the fresh cucumber leaves was sprayed with a spray bottle. Cucumber plants without any disease infection were sprayed with sterile water [40]. The cucumber disease classification followed the GB/T179800.26-2000 standard (in Table 1) [41].

2.2.2. Nitrogen Application

Since perlite was used as the cucumber substrate, 400 mL of a nutrient solution was added to each flowerpot daily between 8:00 and 9:00 a.m. to ensure consistent absorption. The Kawasaki nutrient solution formula was modified according to the experimental objectives. In the N-50% treatment, the amounts of Ca(NO3)2·4H2O, KNO3, and NH4H2PO4 were 413 g, 303 g, and 57 g, respectively. In the N-100% and N-150% treatments, the corresponding dosages were 826 g, 606 g, and 114 g, respectively. Additionally, in the N-50% treatment, potassium chloride and CaCl2 were applied at 223 g and 194 g, respectively. In the N-150% treatment, 552 g of NaNO3 was added. The complete nutrient solution composition is shown in Table 2.

2.3. Experimental Data Acquisition

The photosynthetic parameters (photosynthetic rate, intercellular CO2 concentration, stomatal conductance, and transpiration rate) of cucumber leaves were measured using a portable photosynthesis meter (LI-6400, LI-COR Inc., Lincoln, NE, USA) at three time points: before treatment (day 8), when the disease reached level 3 (15th day), and when it reached level 7 (20th day). All measurements were taken after a 20 min acclimation period at a 1000 μmol/m2·s photosynthetic photon flux density (PPFD), a 400 μmol/mol CO2 concentration, and a 500 μmol/s flow. The red and blue LED lights served as the illumination source. All leaves were selected from uniform positions across all plants. Plant height, leaf area, and stem diameter were recorded every two days before and during the experiment. A standard ruler was used to measure plant height, and calipers were used for the stem diameter. The leaf area of a single leaf was measured by the leaf-drawing method. The leaf was placed on a copy sheet of paper, the copy sheet was cut into the shape of a leaf, and weighed, and then the single leaf area was calculated using the area of the copy sheet, the weight of the leaf, and the weight of the leaf-shaped paper [34]. After the experiment, root parameters—including total root length, surface area, average diameter, total volume, and total root tips—were assessed. The perlite was washed off with tap water, and the roots were washed according to a previous method [34]. The cleaned roots were scanned using a Perfection V-700 root scanner produced by EPSON of Japan (SeikoEpson, Nagano, Japan), and the root traits were analyzed using the WinRHIZO software v7.6.8.

2.4. Statistical Analyses

The data were analyzed using SPSS 29.0.2.0. One-way analysis of variance (ANOVA) was performed to assess differences between groups, and the least-significant difference (LSD) test was used to determine significance at p < 0.05. Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at p < 0.05. The test results are considered significant when p < 0.05 and are marked with an asterisk (*). When p < 0.01, the significance is even more pronounced and is marked with double asterisks (**). The figures were created using Origin 2018.

3. Results

3.1. Photosynthetic Parameters

The results show significant differences between most treatments (p < 0.05). As shown in Figure 2, cucumber leaves under the B1T2 treatment exhibited an upward trend in photosynthetic rate and stomatal conductance, with a more pronounced increase in stomatal conductance. In all three measurements, the photosynthetic and transpiration rates of B1T3-treated plants initially increased and then declined. In contrast, the photosynthetic rate in the B0T1, B0T2, and B0T3 treatments showed a consistent decreasing trend across the three measurements. Similarly, the intercellular CO2 concentration, stomatal conductance, and transpiration rate decreased over time in the B0T1, B0T2, B0T3, and B1T1 treatments. No significant difference (p > 0.05) was observed in intercellular CO2 concentration in B1T2-treated cucumber leaves. The application of nitrogen fertilizer and the occurrence of powdery mildew had an interactive effect on the photosynthetic parameters of cucumber plant leaves (p < 0.01).
It can be seen from Figure 2a that on the 15th day after the transplantation of the cucumber plants, the leaf photosynthetic rates of plants treated with B0T1, B0T2, and B1T1 decreased by 26.42%, 13.56%, and 9.14% compared with plants in the experimental treatment with B1T2, respectively. On the 15th day after the transplantation of the cucumber plants, the leaf photosynthetic rates of plants treated with B0T3 and B1T3 increased by 0.44% and 7.51% compared with plants in the experimental treatment with B1T2, respectively. On the 20th day after transplantation, the photosynthetic rates of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 42.41%, 38.57%, 26.2%, 27.53%, and 7.61% compared with plants in the experimental treatment with B1T2, respectively. On the 15th and 20th days after transplantation, there were significant differences in the photosynthetic rates of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1, B1T2, and B1T3. It can be seen from Figure 2b that on the 15th day after the transplantation of the cucumber plants, the intercellular CO2 concentrations in the leaves of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 20.66%, 6.37%, 15.47%, 12.99%, and 11.18% compared with plants in the experimental treatment with B1T2, respectively. On the 20th day after the transplantation of the cucumber plants, the intercellular CO2 concentrations in the leaves of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 44.75%, 27.79%, 35.21%, 25.09%, and 19.92% compared with plants in the experimental treatment with B1T2, respectively. On the 15th and 20th days after transplantation, there were significant differences in the intercellular CO2 concentrations of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1, and B1T3. It can be seen from Figure 2c that on the 15th day after the transplantation of the cucumber plants, the stomatal conductance of the leaves of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 44.14%, 24.76%, 32.52%, 27.09%, and 17.92% compared with plants in the experimental treatment with B1T2, respectively. On the 20th day after the transplantation of the cucumber plants, the stomatal conductance of the leaves of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 51.97%, 39.88%, 55.01%, 45.52%, and 34.74% compared with plants in the experimental treatment with B1T2, respectively. On the 15th and 20th days after transplantation, there was a significant difference in the stomatal conductance of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1, and B1T3. It can be seen from Figure 2d that on the 15th day after the transplantation of the cucumber plants, the leaf transpiration rates of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 30.93%, 28.74%, 19.03%, 17.04%, and 4.11% compared with plants in the experimental treatment with B1T2, respectively. On the 20th day after the transplantation of cucumber plants, the leaf transpiration rates of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 43.71%, 36.62%, 32.36%, 15.69%, and 19.37% compared with plants in the experimental treatment with B1T2, respectively. On the 15th and 20th days after transplantation, there were significant differences in the leaf transpiration rates of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1, and B1T3.

3.2. Cucumber Plants’ Growth Parameters

The effects of downy mildew infection and varying nitrogen application treatments on cucumber plant height, stem diameter, and leaf area are shown in Figure 3. These parameters responded differently to nitrogen levels and downy mildew infection. It can be seen from Figure 3a that on the 20th day after the transplantation of the cucumber plants, the plant heights of cucumbers treated with B0T1, B0T2, B0T3, and B1T1 decreased by 24.64%, 19.24%, 13.46%, and 16.39% compared with those treated with B1T2, respectively. The plant height of cucumbers treated with B1T3 increased by 8.2% compared with those treated with B1T2. The growth rate of cucumber plants treated with B1T3 was significantly higher than that of other treatments from the fourth measurement, while the growth rate of cucumber plants treated with B0T3 was always lower than that of plants in the other treatments. Starting from the second measurement, the plant height of the cucumber plants treated with B1T1 was consistently lower than that of the plants treated with B0T3. It can be seen from Figure 3b that on the 20th day after transplantation, the stem diameters of cucumbers treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 13.51%, 9.15%, 17.75%, 6.81%, and 14.84% compared with those of plants treated with B1T2, respectively. From the second measurement, the stem diameter of cucumber plants treated with B1T2 was always larger than that of plants in the other treatments. The stem diameter of cucumber plants treated with B0T3 was always smaller than that of plants in the other treatments. It can be seen from Figure 3c that on the 20th day after transplantation, the leaf areas of cucumbers treated with B0T1, B0T2, and B1T1 decreased by 26.43%, 10.05%, and 15.24% compared with those of plants treated with B1T2, respectively. The leaf areas of cucumbers treated with B0T3 and B1T3 decreased by 7.56% and 19.04% compared with those of plants treated with B1T2, respectively. From the second measurement, the leaf area of cucumber plants treated with B1T3 was larger than that of plants in the other treatments. The leaf area of cucumber plants treated with B0T1 was always smaller than that of plants treated with the other treatments. The leaf area sizes of cucumber plants treated with B0T2 and B1T1 were similar but lower than those of plants treated with B0T3 and B1T3.

3.3. Parameters of Root Systems

The effects of downy mildew infection and varying nitrogen treatments on cucumber root parameters are shown in Figure 4. It can be seen from Figure 4a that on the 20th day after transplantation, the total root lengths of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 29.76%, 17.01%, 37.75%, 3.59%, and 20.76% compared with those of plants treated with B1T2, respectively. It can be seen from Figure 4b that on the 20th day after transplantation, the root surface areas of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 26.26%, 12.39%, 37.17%, 5.18%, and 11.38% compared with those of plants treated with B1T2, respectively. It can be seen from Figure 4c that on the 20th day after transplantation, the average root diameters of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 36.21%, 20.03%, 29.71%, 13.52%, and 8.84% compared with those of plants treated with B1T2, respectively. It can be seen from Figure 4d that on the 20th day after transplantation, the total root volumes of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 43.18%, 12.13%, 18.54%, 11.29%, and 4.8% compared with those of plants treated with B1T2, respectively. It can be seen from Figure 4e that on the 20th day after transplantation, the total root tips of cucumber plants treated with B0T1, B0T2, B0T3, B1T1, and B1T3 decreased by 35.84%, 16.38%, 46.67%, 6.05%, and 28.39% compared with those of plants treated with B1T2, respectively.

4. Discussions

4.1. Effects of Downy Mildew Stress and Nitrogen Stress on Gas Exchange Parameters of Cucumber Plants

Nitrogen fertilizer is a vital nutrient element in the growth of cucumbers. When cucumber plants suffer from nitrogen fertilizer stress, it significantly impacts their photosynthetic rate, stomatal conductance, intercellular CO2 concentration, and transpiration rate [42]. Under low nitrogen stress, the photosynthetic rate of cucumber plants was reduced, resulting in the inhibition of cucumber plant growth. As shown in Figure 2a, on the 15th and 20th days after cucumber plant transplantation, the photosynthetic rates of the leaves of the plants treated with B1T1 were 9.14% and 27.53% lower than those of the plants treated with B1T2, respectively. As shown in Figure 3a, on the 20th day after the transplantation of cucumber plants, the plant height of cucumbers treated with B1T1 decreased by 16.39% compared with those treated with B1T2. When nitrogen application is insufficient, chlorophyll synthesis is limited, the leaf color becomes lighter, and the photosynthetic pigment content decreases. This directly weakens the plant’s ability to capture light energy and leads to a decline in the photoreaction rate [43,44].
Stomata serve as channels for gas exchange between plants and the external environment. As shown in Figure 2c, on the 15th and 20th days after transplantation, the stomatal conductance of the cucumber leaves in treatment B1T1 was 27.09% and 45.52% lower than that of the leaves in treatment B1T2, respectively. Under low-nitrogen conditions, plants close some stomata to reduce water loss and maintain internal physiological balance, thereby reducing stomatal conductance [45]. However, as the pressure continued and the stomatal conductance kept decreasing, the CO2 entering the leaves was restricted, and the intercellular CO2 concentration dropped again. The transpiration rate also decreased as the stomatal conductance decreased. As shown in Figure 2, on the 15th and 20th days after transplantation, the transpiration rates of the cucumber leaves in treatment B1T1 and treatment B1T2 were, respectively, 17.04% and 15.69% lower than that of plants in treatment B1T2. This is because transpiration mainly occurs through stomata [46]. At the same time, nitrogen deficiency affects the activity of photosynthesis-related enzymes and protein synthesis, slowing carbon assimilation and reducing the efficiency of CO2 fixation. In addition, under low-nitrogen conditions, plants grow slowly, leaf development is poor, and both stomatal density and stomatal conductance may decrease. This limits CO2 entry and water loss, thereby affecting the gas exchange process [47,48].
Downy mildew disease can affect the normal opening and closing of the stomata in cucumber plants [40]. After infection by downy mildew, the stomatal conductance decreases, restricting the entry of CO2 into the leaves and affecting photosynthesis. As shown in Figure 2c, on the 15th and 20th days after transplantation, the stomatal conductance of cucumber leaves in treatment B0T2 was 24.76% and 39.88% lower than that of plants in treatment B1T2, respectively. As shown in Figure 2a, on the 15th and 20th days after cucumber plant transplantation, the photosynthetic rates of the plants treated with B0T2 were 13.56% and 38.57% lower than those of the plants treated with B1T2, respectively. In the early stages of infection, the intercellular CO2 concentration may increase, but as disease severity increases, stomatal conductance continues to decline, reducing the CO2 supply and causing the intercellular CO2 concentration to decrease again [49,50]. In diseased plants, physiological functions are impaired by infection, and low-nitrogen conditions further hinder growth and metabolism, significantly reducing gas exchange parameters in the leaves. Plants expend too much energy on vegetative growth, while synthesis of disease resistance compounds declines, further weakening disease resistance and increasing disease severity [51,52].

4.2. Effects of Downy Mildew Stress and Nitrogen Stress on Cucumber Plants’ Growth Parameters

When nitrogen fertilizer is excessive, the height of cucumber plants increases significantly. The height of the plants treated with excessive nitrogen fertilizer was 8.2% higher than that of the normal nitrogen-fertilized cucumber plants (Figure 3a). Conversely, under nitrogen deficiency, plant height increases more slowly, and plants remain short. The cucumber plants treated with this fertilizer reduced by 16.39% compared with those treated with normal nitrogen fertilizer (Figure 3a). Downy mildew infection impairs leaf photosynthesis, reducing the production of photosynthetic products that supply energy and metabolites for growth, thereby inhibiting plant height [22,42]. The more severe the infection, the more pronounced the suppression of plant height. In severely infected plants, growth may cease altogether, and plant height is significantly reduced compared with healthy plants, with a reduction rate of up to 19.05% [14], which is similar to the 19.24% result of this study. When excessive nitrogen fertilization coincided with downy mildew, the plant height of cucumber plants decreased by 13.46% (Figure 3a). This is because excessive vegetative growth weakens plant vigor, reduces their resistance, and increases their susceptibility to rapid disease progression [25,27]. When nitrogen deficiency and downy mildew occurred together, the plant height of cucumber plants decreased by 24.64% (Figure 3a). This is because the plants were subjected to both nutritional limitations and disease stress, which further slowed down their growth in height, resulting in smaller and less-developed plants [29,50].
Excessive nitrogen promotes the thickening of the cucumber stems [31]. On the contrary, nitrogen deficiency restricts stem growth, causing the stems to become weak and unable to support the aboveground biomass of the plant. The root–shoot ratio of cucumbers also decreased by 4.47% [14]. Due to the impact of downy mildew on the overall plant growth, the development of the stems is also impaired [44]. In infected plants, the growth rate of stem thickness slows down [14]. Ultimately, it reduced the thickness of cucumber stems by 9.1% (Figure 3b). This is because when nitrogen deficiency and downy mildew occur simultaneously, the stems lack nutrients and are further damaged by the disease [44]. When nitrogen fertilizer is adequately supplied, it provides sufficient nutrition for cucumber plant growth. The leaves grow vigorously, and the leaf area increases by 19.04%. When the nitrogen supply is insufficient, the leaf area decreases by 15.24% (Figure 3c). The results are inconsistent with previous studies. This might be due to the differences in fertilizer application [14,53,54,55]. After infection with downy mildew, lesions spread more rapidly, senescence and leaf drop intensified, and the reduction in the leaf area was more pronounced.

4.3. Effects of Downy Mildew Stress and Nitrogen Stress on Cucumber Plants’ Root System Parameters

Nitrogen significantly affects cucumber root growth (Figure 4). Under deficiency, root development slows [56]. As shown in Figure 4a, after the cucumber plants were infected with downy mildew, when nitrogen was deficient, the total root length of the cucumber plants decreased by 15.38%. When nitrogen deficiency and downy mildew occur simultaneously, root growth is further suppressed, and the total root length is significantly shorter than in healthy plants [57]. Furthermore, if nitrogen fertilizer is insufficient, root surface area increases primarily through slender root elongation and increased branching, which expand soil contact and enhance nutrient absorption. In contrast, excessive nitrogen application leads to a significant decrease in the root surface area. As shown in Figure 4b, after cucumber plants were infected with downy mildew, the root surface area of the plants in the nitrogen-deficient condition decreased by 28.29%. This is because of the rapid growth of roots and the proliferation of lateral roots and root hairs [58]. Following downy mildew infection, cucumber root growth is suppressed, with reduced development of lateral roots and root hairs, resulting in a marked decline in root surface area. The disease impairs plant physiology, diminishing the root system’s ability to absorb water and nutrients. This reduction in surface area further worsens the condition, increasing the risk of water and nutrient deficiencies [59].
Under the combined influence of nitrogen fertilizer stress and downy mildew, the average root diameter usually becomes thinner. When nitrogen is insufficient, the root system becomes weaker and thinner [60,61]. When the nitrogen supply is adequate, the overall volume of the root system of cucumber plants gradually increases (Figure 4d), as sufficient nitrogen fertilizer can support the cell division and growth of the plant’s root system [40]. Under insufficient nitrogen fertilization, cucumber plants may increase the number of root tips to create additional absorption sites and improve nitrogen uptake [62]. When nitrogen fertilizer stress and downy mildew occur concurrently, the number of root tips decreases significantly. As shown in Figure 4e, compared with plants in the B1T1 treatment, the total number of root tips of the cucumber plants in the B0T1 treatment decreased by 31.7%. Compared with the plants in the B1T3 treatment, the total number of root tips of the cucumber plants in the B0T3 treatment decreased by 25.53%. When nitrogen is insufficient, root tip growth is restricted. Downy mildew further inhibits root tip formation and growth, and may also cause root tip cell death, resulting in a marked reduction in root tip number. This severely impairs the absorption and sensing functions of the root system and reduces the plant’s adaptability to the environment [63].

5. Conclusions

Understanding the effects of different stresses on the physiological growth of cucumber plants is crucial for implementing effective measures to enhance cucumber production. To examine the impact of downy mildew (caused by Pseudoperonospora cubensis) infection and varying nitrogen treatments on cucumber growth and physiology, two levels of downy mildew infection—infected (B0) and non-infected (B1)—and three nitrogen application levels—T1 (N-50%), T2 (N-100%), and T3 (N-150%)—were applied in a greenhouse setting.
The plant height, stem diameter, and leaf area responded differently to the various nitrogen treatments and downy mildew infection levels. Root traits, including total length, surface area, average diameter, total volume, and number of root tips, also showed significant variations under different experimental conditions (p < 0.05). Among all treatments, B0T3 had the most pronounced effect on the physiological growth of cucumber plants.
This study focused only on analyzing how nitrogen stress and downy mildew infection affected the physiological growth parameters of cucumber plants before flowering, but did not study their effects on cucumber fruit and yield. Future research should explore the effects of other stresses, such as diseases and the failure to apply nitrogen fertilizer, on hormone metabolism and fruit quality to provide a more comprehensive basis for optimizing cucumber cultivation.

Author Contributions

Conceptualization, Y.W. and Q.S.; methodology, Y.W., Q.S., M.F.T. and T.C.; software, Y.W., Q.S. and X.D.; validation, Y.W., Q.S., X.D. and T.C.; formal analysis, Y.W.; investigation, Y.W. and Q.S.; resources, Y.W. and Q.S.; data curation, Y.W., Q.S., X.D., M.F.T. and T.C.; writing—original draft preparation, Y.W., Q.S., X.D. and T.C.; writing—review and editing, Y.W., Q.S., X.D., M.F.T. and T.C.; visualization, Y.W., Q.S., X.D., M.F.T. and T.C.; supervision, Y.W.; project administration, Y.W. and Q.S.; funding acquisition, Y.W. and Q.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by the National Natural Science Foundation of China (grant nos. 32501779 and 32201686). Natural Science Foundation of Jiangsu Province for Youth(Grant No. BK20250866). The Shanghai Oriental Talent Program Youth Project (QNJY2024095).

Data Availability Statement

The data are contained within this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Horticulturae 11 01182 g001
Figure 1. Flowchart of proposed methodology.
Figure 1. Flowchart of proposed methodology.
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Figure 2. The effects of downy mildew infection and different nitrogen application treatments on leaf photosynthetic parameters of cucumber plants. Note: Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at (p < 0.05). The test results are considered significant when p < 0.05 and are marked with an asterisk (*). When p < 0.01, the significance is even more pronounced and is marked with double asterisks (**). B0: this indicates that the cucumber plants were infected with downy mildew; B1: indicates that the cucumber plants were not infected with downy mildew; T1: indicates that the nitrogen fertilizer application rate was 50% of the normal level; T2: indicates that the application amount of nitrogen fertilizer was normal; and T3: indicates that the nitrogen fertilizer application rate was 150% of the normal level. (a) Photosynthetic rate; (b) intercellular CO2 concentration; (c) stomatal conductance; and (d) transpiration rate.
Figure 2. The effects of downy mildew infection and different nitrogen application treatments on leaf photosynthetic parameters of cucumber plants. Note: Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at (p < 0.05). The test results are considered significant when p < 0.05 and are marked with an asterisk (*). When p < 0.01, the significance is even more pronounced and is marked with double asterisks (**). B0: this indicates that the cucumber plants were infected with downy mildew; B1: indicates that the cucumber plants were not infected with downy mildew; T1: indicates that the nitrogen fertilizer application rate was 50% of the normal level; T2: indicates that the application amount of nitrogen fertilizer was normal; and T3: indicates that the nitrogen fertilizer application rate was 150% of the normal level. (a) Photosynthetic rate; (b) intercellular CO2 concentration; (c) stomatal conductance; and (d) transpiration rate.
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Figure 3. The effects of downy mildew infection and different nitrogen application treatments on cucumber plants’ growth parameters. Note: Error bars indicate standard deviations; B0: indicates that the cucumber plants were infected with downy mildew; B1: indicates that the cucumber plants were not infected with downy mildew; T1: indicates that the nitrogen fertilizer application rate was 50% of the normal level; T2: indicates that the application amount of nitrogen fertilizer was normal; and T3: indicates that the nitrogen fertilizer application rate was 150% of the normal level. (a) Plant height; (b) stem diameter; and (c) leaf area.
Figure 3. The effects of downy mildew infection and different nitrogen application treatments on cucumber plants’ growth parameters. Note: Error bars indicate standard deviations; B0: indicates that the cucumber plants were infected with downy mildew; B1: indicates that the cucumber plants were not infected with downy mildew; T1: indicates that the nitrogen fertilizer application rate was 50% of the normal level; T2: indicates that the application amount of nitrogen fertilizer was normal; and T3: indicates that the nitrogen fertilizer application rate was 150% of the normal level. (a) Plant height; (b) stem diameter; and (c) leaf area.
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Figure 4. The effects of downy mildew infection and different nitrogen application treatments on root system parameters of cucumber plants. Note: Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at (p < 0.05). The test results are considered significant when p < 0.05 and are marked with an asterisk (*). When p < 0.01, the significance is even more pronounced and is marked with double asterisks (**). B0: indicates that the cucumber plants were infected with downy mildew; B1: indicates that the cucumber plants were not infected with downy mildew; T1: indicates that the nitrogen fertilizer application rate was 50% of the normal level; T2: indicates that the application amount of nitrogen fertilizer was normal; and T3: indicates that the nitrogen fertilizer application rate was 150% of the normal level. (a) Total root length; (b) root surface area; (c) average root diameter; (d) total root volume; and (e) total root tips.
Figure 4. The effects of downy mildew infection and different nitrogen application treatments on root system parameters of cucumber plants. Note: Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at (p < 0.05). The test results are considered significant when p < 0.05 and are marked with an asterisk (*). When p < 0.01, the significance is even more pronounced and is marked with double asterisks (**). B0: indicates that the cucumber plants were infected with downy mildew; B1: indicates that the cucumber plants were not infected with downy mildew; T1: indicates that the nitrogen fertilizer application rate was 50% of the normal level; T2: indicates that the application amount of nitrogen fertilizer was normal; and T3: indicates that the nitrogen fertilizer application rate was 150% of the normal level. (a) Total root length; (b) root surface area; (c) average root diameter; (d) total root volume; and (e) total root tips.
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Table 1. Grading of cucumber downy mildew severity.
Table 1. Grading of cucumber downy mildew severity.
Disease LevelSymptoms Described
Level 0Asymptomatic
Level 1Diseased spot area occupies less than 5% of the leaf area
Level 3Diseased area accounts for 6% to 10% of the leaf area
Level 5Diseased area accounts for 11% to 25% of the leaf area
Level 7Diseased area accounts for 26% to 50% of the leaf area
Level 9Diseased spot area accounts for more than 50% of the leaf area
Table 2. Component of nutrient solution (unit/g).
Table 2. Component of nutrient solution (unit/g).
Nutrient Solution GroupChemical ReagentN-50%N-100%N-150%
ACa(NO3)2·4H2O413826826
KNO3303606606
BNH4H2PO457114144
MgSO4·7H2O492492492
NaNO300552
KCl22300
CaCl219400
CNaFe-EDTA777
MnSO41.71.71.7
ZnSO41.451.451.45
CuSO40.190.190.19
Na2MoO40.120.120.12
Na2B4O72.452.452.45
Note: Each standard liquor was prepared in a 10 L aqueous solution and used after diluting 100 times.
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MDPI and ACS Style

Wang, Y.; Shi, Q.; Du, X.; Chen, T.; Taha, M.F. Impact of Nitrogen on Downy Mildew Infection and Its Effects on Growth and Physiological Traits in Early Growth Stages of Cucumber. Horticulturae 2025, 11, 1182. https://doi.org/10.3390/horticulturae11101182

AMA Style

Wang Y, Shi Q, Du X, Chen T, Taha MF. Impact of Nitrogen on Downy Mildew Infection and Its Effects on Growth and Physiological Traits in Early Growth Stages of Cucumber. Horticulturae. 2025; 11(10):1182. https://doi.org/10.3390/horticulturae11101182

Chicago/Turabian Style

Wang, Yafei, Qiang Shi, Xiaoxue Du, Tianhua Chen, and Mohamed Farag Taha. 2025. "Impact of Nitrogen on Downy Mildew Infection and Its Effects on Growth and Physiological Traits in Early Growth Stages of Cucumber" Horticulturae 11, no. 10: 1182. https://doi.org/10.3390/horticulturae11101182

APA Style

Wang, Y., Shi, Q., Du, X., Chen, T., & Taha, M. F. (2025). Impact of Nitrogen on Downy Mildew Infection and Its Effects on Growth and Physiological Traits in Early Growth Stages of Cucumber. Horticulturae, 11(10), 1182. https://doi.org/10.3390/horticulturae11101182

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