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Article

Effects of Downy Mildew Infection and Potassium on Growth and Physiological Traits of Greenhouse Cucumber

by
Qiang Shi
1,
Lu You
1,
Yafei Wang
2,*,
Xiaoxue Du
3 and
Tianhua Chen
4
1
School of Science and Technology, Shanghai Open University, Shanghai 200433, China
2
School of Agricultural Engineering, Jiangsu University, Zhenjiang 212013, 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
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(5), 1017; https://doi.org/10.3390/agronomy15051017
Submission received: 31 March 2025 / Revised: 20 April 2025 / Accepted: 23 April 2025 / Published: 24 April 2025
(This article belongs to the Section Pest and Disease Management)
Browse Figures
Versions Notes

Abstract

:
Both abiotic and biological stresses reduce the quality and quantity of cultivated plants. In order to observe the response of cucumber plants to potassium stress and cucumber downy mildew infestation, two different levels of downy mildew infestation, B0 (disease infestation) and B1 (disease-free infestation), and three fertilizer requirement levels of potassium fertilization, T1 (K-50%), T2 (K-100%) and T3 (K-150%), were applied in the greenhouse. Results show that the photosynthetic rate and stomatal conductance of cucumber plants leaves treated with B1T2 had an increasing trend, and the increase in stomatal conductance was more significant. The intercellular CO2 concentration of cucumber leaves treated with B1T2 showed no significant difference. The plant height of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 32.21%, 16.93%, 21.59%, 53.54% and 6.31%, respectively, compared with that treated with B1T2. The leaf area of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 27.85%, 15.97%, 21.47%, 7.29% and 18.5%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B0T1, B0T2, B0T3 and B1T1 decreased by 13.43%, 8.72%, 6.01% and 7.06%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B1T3 increased by 6.83% compared with that treated with B1T2. The root total length, root surface area, root average diameter, root total volume and total root tips of cucumber plants were significantly different under different experimental conditions (p < 0.05). This study provides a theoretical basis for cucumber stress resistance cultivation in greenhouses and has important practical significance to ensure the sustainable development of the greenhouse cucumber industry.

1. Introduction

In recent years, China’s greenhouse area has shown a steady growth trend. Up to now, China’s greenhouse area has exceeded 4 million hectares [1,2]. Greenhouse cucumber cultivation is rapidly emerging throughout the country and has gradually become an important production method to ensure the annual supply of vegetables [3,4]. China is a major producer of cucumbers. Its cultivated area accounts for about 60% of the cultivated area of cucumber in the world, and its scale and output rank first in the world [5,6]. The growth environment of cucumber is complicated and easy to be subjected to various external environmental stresses [7,8]. Generally, the stress faced by cucumber can be divided into two categories: abiotic stress and biological stress. Abiotic stress includes nutrient deficiency, salt stress, drought, flood, heavy metal pollution and heat stress [9,10,11]. Under the condition of nutrient deficiency, the growth rate of cucumber plants is significantly slowed down, and the yield can be reduced by 20–30% [12,13]. However, in salt-stressed environments, the germination rate will decrease by 5–15%, which seriously affects the early growth [14,15]. Biological stress includes the invasion of viruses, fungi, bacteria, nematodes and insects. Both abiotic and biological stresses can lead to loss of cucumber yield [16,17]. With the intensification in the global warming trend, climate anomalies are becoming more frequent, and multiple stress combinations are increasing continuously and synchronously, which has aroused widespread concern [18,19]. Some studies have pointed out that under the combined stress of drought and high temperature, the photosynthetic efficiency of cucumber is greatly reduced, and the yield loss is increased by 15–20% compared with that under a single stress [20,21]. Therefore, an in-depth study of the response and adaptation mechanism of cucumber plants to different stress combinations is of pivotal significance for improving the production efficiency of cucumber and ensuring the stable development of the industry [22,23].
However, behind the booming development of the greenhouse cucumber industry, there are also many challenges. Among them, the frequent occurrence of pests and diseases and the imbalance of soil nutrients seriously restrict the sustainable development of greenhouse cucumbers. Among many pests and diseases, downy mildew has become one of the most serious diseases threatening greenhouse cucumber production [24]. Cucumber downy mildew is caused by the infection of Pseudoperonospora cubensis, a subphylum fungus, which is the most common and seriously harmful fungal disease in cucumber production [25]. The disease has the characteristics of fast spread, rapid onset and wide prevalence, etc. Under appropriate environmental conditions, a large area of cucumber plants can be infected within a short period of time, bringing huge losses to cucumber production [26]. When cucumber plants are infected by downy mildew, the leaves are the most important parts damaged. In the early stage of the disease, small spots like water stains appear on the surface of the leaves. With the development of the disease, the spots gradually expand and form polygonal spots restricted by the leaf veins. Under wet conditions, the abaxial leaf surface will develop a gray–black mold layer, which is the sporangium of downy mildew. When the disease is severe, multiple spots are connected to each other, causing the leaves to become yellow, curl, and eventually lose the ability of photosynthesis and dry out and die [27]. The effect of downy mildew on the photosynthesis of cucumber plants was significant. The chlorophyll content of infected cucumber leaves decreased significantly. Chlorophyll is the key pigment for photosynthesis in plants, and the decrease in chlorophyll content directly affects the absorption and conversion efficiency of light energy in leaves [28]. At the same time, downy mildew will also destroy the chloroplast structure of the leaves, damage the thylakoid membrane of the chloroplast, and affect the electron transfer and photophosphorylation process of photosynthesis, thus inhibiting both the light and dark reactions of photosynthesis [29]. As a result, the plant’s ability to absorb carbon dioxide is weakened, and the synthesis and accumulation of carbohydrates are reduced, which seriously affects the growth and development of cucumber plants [30,31].
Potassium, as one of the essential elements for plant growth, plays a crucial role in the growth and development of cucumber plants. Potassium not only participates in a variety of physiological and biochemical reactions in plants, such as enzyme activation, photosynthesis, respiration, carbohydrate metabolism, protein synthesis, etc., but also plays an important role in maintaining the osmotic pressure of plant cells, regulating stomatal opening and closing, and enhancing plant stress resistance [32,33,34]. In the cultivation of cucumber in greenhouses, due to the influence of long-term continuous cropping, unreasonable fertilization and other factors, providing the content of potassium in soil is often difficult to meet the growth demand of cucumber plants, resulting in the occurrence of potassium stress [35]. When cucumber plants are under potassium stress, their photosynthesis is first affected [36,37]. Potassium stress can reduce the content of chlorophyll in cucumber leaves and affect the synthesis and stability of chlorophyll. At the same time, potassium stress can also affect the activity of photosynthetic-related enzymes, such as carboxylase, phosphoenolpyruvate carboxylase, etc.; these enzymes play a key role in the carbon dioxide fixation and assimilation process of photosynthesis. The decrease in enzyme activity leads to the obstruction of the dark reaction of photosynthesis, which in turn affects the synthesis and accumulation of photosynthetic products [38]. In addition, potassium stress affects the stomatal conductance of leaves, closing stomata, limiting the entry of carbon dioxide, and further reducing the efficiency of photosynthesis. Potassium stress can inhibit the growth and development of cucumber plants [39]. The plants showed poor root growth, reduced number and length of roots, and decreased absorption capacity, which affected the absorption of water and nutrients by the plants. The aboveground part showed a weak stem and vine, small leaves, yellowing, slow growth, a short plant and shortened internode. At the same time, potassium stress can also affect flower bud differentiation and the flowering and fruit of cucumber, resulting in poor flower bud differentiation, a reduced number of female flowers, reduced fruit setting rate, delayed fruit development, small and malformed fruit, and substantial decline in yield and quality [40].
In the actual greenhouse cucumber production, cucumber plants often face the double pressure of downy mildew infection and potassium stress. However, studies on the effects of downy mildew and potassium stress on the photosynthesis and growth of cucumber plants are relatively few, and no systematic understanding has been formed. Therefore, to study the effects of downy mildew and potassium stress on the photosynthesis and growth of cucumber plants in a greenhouse not only helps to reveal the adaptation mechanism of cucumber plants under complex stress conditions, but also provides a theoretical basis for the stress-resistant cultivation of cucumber in a greenhouse, and provides an important reference for the formulation of scientific and reasonable pest control strategies and fertilization management measures, which will improve the yield and quality of cucumber in greenhouses. It is of great practical significance to ensure the sustainable development of the greenhouse cucumber industry.

2. Materials and Methods

2.1. Study Site and Treatment Details

In order to study the effects of downy mildew and potassium on the growth physiological traits of cucumber plants in a greenhouse, cucumber plant samples were cultivated in a Venlo-type greenhouse at Jiangsu University. The cucumber variety tested was “Jinyou No. 1” (developed by Tianjin Academy of Agricultural Sciences). The experiment was conducted from 25 July 2024 to 18 August 2024. The average temperature of the greenhouse during the experiment was 25.8 °C (16.83~36.47 °C). The relative humidity of the greenhouse was 85.6% RH (23.7~95.6% RH). On 15 July 2024, nutrient soil was placed in the pot, watered and cucumber seeds were placed in the pot with nutrient soil for cultivation. On 29 July 2024, perlite was poured into the flowerpot with a volume of 10 L, and the dust in the perlite was rinsed with tap water. On 30 July 2024, the “two-leaf one heart” cucumber seedlings with good growth and similar state were removed from the pot and transplanted into the pot. From 30 July 2024 to 5 August 2024, from 7:00 am to 8:00 am, the standard nutrient solution was poured on cucumber seedlings every day, and the nutrient solution formula and watering amount was carried out according to the literature [16].

2.2. Treatments and Experiment Design

In order to study the effects of downy mildew (It is caused by the infection of Pseudoperonospora cubensis) and potassium on the photosynthesis, plant height, stem diameter, leaf area and root parameters of cucumber in a greenhouse, as shown in Figure 1, the experimental design consisted of 6 treatments, each of which was repeated 6 times. B0 indicates that cucumber plants are infested with downy mildew. B1 indicates that cucumber plants are free of downy mildew. The application rates of potassium fertilizer were T1 (K-50%), T2 (K-100%) and T3 (K-150%). Therefore, the six experimental treatments were B0T1 (cucumber plants with downy mildew infestation and 50% of the normal application of potassium fertilizer), B0T2 (cucumber plants with downy mildew infestation and the normal application of potassium fertilizer) and B0T3 (cucumber plants with downy mildew infestation and 150% of the normal application of potassium fertilizer), and B1T1 (cucumber plants without frost mildew infection; the potassium fertilizer application rate was 50% of normal fertilizer), B1T2 (cucumber plants did not have downy mildew infection; the potassium fertilizer application rate was normal) and B1T3 (cucumber plants did not have downy mildew infection; the potassium fertilizer application rate was 150% of the normal fertilizer rate). In order to avoid the spread of downy mildew between cucumber plants, the infected cucumber plants were cultivated in the western greenhouse, and the non-infected cucumber plants were cultivated in the eastern greenhouse. The ridge spacing of cucumber plants was 32 cm and the plant spacing was 50 cm. In addition, 400 mL of nutrient solution was poured on the experimental cucumber plant samples from 8:00 am to 9:00 am every day during the experimental treatment. The formula of the nutrient solution is shown in Table 1.

2.3. Extraction and Inoculation of Pathogenic Fungi

In order to obtain the pathogen of cucumber downy mildew, fresh diseased leaves were collected from cucumber plants with natural disease before the experiment, and a single cucumber downy mildew spot with sufficient disease was cut with scissors. After dipping in sterile water, the spot was facing downward, and then gently pasted on the front of the cucumber leaf cultured in advance. Since cucumber downy mildew spores and cucumber powdery mildew spores cannot be cultured in vitro, in order to preserve the samples of cucumber downy mildew pathogens, cucumber downy mildew was regularly transferred from old infected cucumber plants to newly cultivated cucumber plants without disease, so as to achieve the purpose of expanding culture. The experimental treatment began on 6 August 2024, the eighth day after the cucumber was transplanted. Plant leaves cultivated with cucumber downy mildew were put into sterile water to prepare cucumber downy mildew spore suspension. The suspensions of cucumber downy mildew pathogen spores prepared in advance were sprayed onto cucumber leaves with disposable sterile syringe. The inoculation time of pathogen spores was from 6:00 pm to 7:00 pm. Cucumber plants that have not been treated for disease are sprayed with sterile water. The severity of cucumber diseases in this study was graded according to GB/T17980.26-2000 [41] (In Table 2).

2.4. Experimental Data Acquisition

During the experiment, before the experimental treatment (when the disease degree of cucumber was grade 0, that is, on the 8th day of the experiment), the gas exchange parameters of cucumber leaves were measured by portable photosynthesizers (LI-6400, LI-COR Inc., Lincoln, NE, USA) when the disease degree was grade 3 (on the 15th day of the experiment) and grade 7 (on the 20th day of the experiment). The plant height, leaf area and stem diameter of cucumber were measured every 2 days before and during the experiment. The measuring method of cucumber plant height is a straight ruler. The diameter of cucumber stem is measured by vernier caliper. The measurement method of cucumber plant leaf area was the method in Reference [16]. After the experiment, in order to measure the root system parameters of cucumber plants, tap water was used to clean the perlite on the root surface of the plants, and the roots of the plants were cleaned according to the method mentioned in Reference [31]. After the roots of the cucumber were cleaned, a Perfection V700 root scanner produced by EPSON of Japan (SeikoEpson, Nagano Prefecture, Japan) was used to scan the root parameters, and the root analysis software Win RHIZO was used to measure the root parameters after scanning.

2.5. Statistical Analyses

The data were analyzed using the statistical analysis software program SPSS_29.0.2.0. The statistical differences between groups were analyzed by using ANOVA. The least significant difference (LSD) test was used to determine a significance level of p < 0.05.

3. Results and Discussion

3.1. Leaf Gas Exchange Parameters

Leaf gas exchange parameters were measured on the 8th (Level 0), 15th (Level 3) and 20th (Level 7) days after seedlings were transplanted. The effects of potassium treatments and different stages after downy mildew infection on (8th, 15th and 20th days after transplanting) the gas exchange parameters of cucumber leaves are shown in Figure 2. As can be seen from Figure 2, the photosynthetic rate and stomatal conductance of cucumber plants leaves treated with B1T2 showed an increasing trend, and the increase in stomatal conductance was more significant. The intercellular CO2 concentration of cucumber leaves treated with B1T2 showed no significant difference. During the whole experiment period, the leaf photosynthetic rate, transpiration rate, intercellular CO2 concentration and stomatal conductance of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 were significantly decreased. There were no significant differences in the photosynthetic rate, transpiration rate, intercellular CO2 concentration and stomatal conductance of cucumber leaves measured on the eighth day after transplanting (before different potassium treatments and downy mildew infection).
As can be seen from Figure 2a, on the 15th day after the transplantation of cucumber plants, the leaf photosynthetic rate of plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 50%, 37%, 32.56%, 33.46% and 13.78%, respectively, compared with the experimental treatment with B1T2. On the 20th day after transplantation, the photosynthetic rate of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 67.89%, 50.94%, 42.19%, 47.8% and 30.47%, respectively, compared with the experimental treatment with B1T2. On the 15th and 20th days after transplantation, there was no significant difference in the photosynthetic rate of cucumber leaves treated by B0T2 and B1T1. As can be seen from Figure 2b, on the 15th day after the transplantation of cucumber plants, the intercellular CO2 concentration in the leaves of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 30.64%, 38.31%, 34.23%, 8.49% and 10.8%, respectively, compared with the experimental treatment with B1T2. On the 20th day after transplantation, the intercellular CO2 concentration of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 44.52%, 48.31%, 42.48%, 16.01% and 13.97%, respectively, compared with that treated with B1T2. On the 15th and 20th days after transplantation, there was no significant difference in the intercellular CO2 concentration of cucumber leaves treated by B0T2 and B0T3. As can be seen from Figure 2c, on the 15th day after the transplantation of cucumber plants, the stomatal conductance of the leaves of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 48.09%, 34.71%, 25.72%, 38.1% and 14.92%, respectively, compared with the experimental treatment with B1T2. On the 20th day after transplantation, the stomatal conductance of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 72.87%, 57.48%, 44.12%, 57.07% and 36.35%, respectively, compared with B1T2. There was no significant difference in stomatal conductance between B0T2 and B1T1 on the 15th and 20th days after transplantation. As can be seen from Figure 2d, on the 15th day after the transplantation of cucumber plants, the leaf transpiration rate of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 29.26%, 17.49%, 18.95%, 34.62% and 27.11%, respectively, compared with the experimental treatment with B1T2. On the 20th day after transplantation, the transpiration rate of cucumber leaves treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 45.67%, 39.2%, 36.01%, 47.56% and 44.73%, respectively, compared with B1T2. On the 15th and 20th days after transplantation, there was no significant difference in the leaf transpiration rate between B0T1 and B1T3.
Potassium can improve the efficiency of crop photosynthesis, promote the stomatal opening of leaves, regulate stomatal conductance and increase chlorophyll content, thus increasing the production and transport of nutrients in plants. A lack of potassium will reduce stomatal opening, resulting in the obstruction of water and gas entering and leaving the stomata, and the insufficient supply of carbon dioxide, thus affecting the photosynthesis of plant leaves [42,43]. When cucumber was infected with downy mildew, water spots appeared at the initial position of leaf veins, and then gradually expanded to form polygonal brown spots restricted by leaf veins. As the disease progresses, the plaque dries up. The destruction of the leaf structure reduces the photosynthetic area of the leaves and directly affects the process of photosynthesis. At the same time, the existence of disease spots will also lead to the impairment of stomatal functions on the leaf surface, affecting the absorption of carbon dioxide and the discharge of oxygen, and thus affecting the gas exchange process of photosynthesis [44,45]. Downy mildew also affects the content and function of photosynthetic pigments in cucumber leaves. On the one hand, the infection of bacteria may cause the synthesis of chlorophyll to be blocked and the chlorophyll content to decrease. On the other hand, leaf aging and lesions caused by diseases will also accelerate the decomposition of chlorophyll. The reduction in chlorophyll content will reduce the absorption and conversion ability of leaves to light energy, thus weakening the light reaction stage of photosynthesis, reducing the generation of ATP and NADPH, and affecting the fixation and reduction of carbon dioxide in the dark reaction [46]. When downy mildew infects cucumber plants, it interferes with physiological metabolism and inhibits the activity of photosynthetic enzymes. Carboxylase and other key enzymes involved in carbon dioxide fixation decreased, resulting in a decrease in the fixation efficiency of carbon dioxide, which in turn affected the dark reaction of photosynthesis and reduced the synthesis of photosynthetic products. After cucumber was infected with downy mildew, the vascular bundle tissue of the leaves may be affected by the pathogen, which affects the transport of photosynthetic products. The inability of photosynthates to be transported to fruits and other organs in a timely and effective manner will lead to the accumulation of photosynthates in leaves, and the feedback will inhibit the progress of photosynthesis [31,47].
The infection of downy mildew can destroy the leaf structure and physiological function of cucumber plants, and weaken the photosynthetic capacity and growth potential of cucumber plants, and potassium stress can further aggravate this adverse effect. On the one hand, potassium stress can reduce the stress resistance of plants, making plants more susceptible to downy mildew infection, and the disease may be more serious after onset. On the other hand, the occurrence of downy mildew will affect the absorption and utilization of potassium in plants, and further aggravate the degree of potassium stress. Under the combined stress of downy mildew and potassium, the chlorophyll content of cucumber plants decreased more obviously, photosynthesis was more inhibited and photosynthetic parameters such as net photosynthetic rate, stomatal conductivity and transpiration rate were significantly lower than those under single stress treatment.

3.2. Cucumber Plants Growth Parameters

The parameters of plant height, stem diameter and leaf area of cucumber were measured five times during the experiment. The tests were conducted on the 8th, 11th, 14th, 17th and 20th days after the transplantation of cucumber plants. The effects of potassium treatment and downy mildew infection on the plant height, stem diameter and leaf area of cucumber are shown in Figure 3. The plant height, stem diameter and leaf area had different responses to different potassium treatments and downy mildew infection. As can be seen from Figure 3, on the 20th day after the transplantation of cucumber plants, the plant height of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 32.21%, 16.93%, 21.59%, 53.54% and 6.31%, respectively, compared with that treated with B1T2. The growth rate of cucumber plants treated with B1T2 was significantly higher than that of other treatments from the fourth measurement, while the growth rate of cucumber plants treated with B0T1 was always lower than that of other treatments. On the 20th day after transplantation, the leaf area of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 27.85%, 15.97%, 21.47%, 7.29% and 18.5%, respectively, compared with that treated with B1T2. From the second measurement, the leaf area of cucumber plants treated with B1T2 was larger than that of other treatments. The leaf area of cucumber plants treated with B0T1 was always smaller than that treated with other treatments. The leaf area sizes of cucumber plants treated with B0T3, B1T3 and B0T2 were similar, but lower than that treated with B1T1. On the 20th day after transplantation, the stem diameter of cucumbers treated with B0T1, B0T2, B0T3 and B1T1 decreased by 13.43%, 8.72%, 6.01% and 7.06%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B1T3 increased by 6.83% compared with that treated with B1T2. From the second measurement, the stem diameter of cucumber plants treated with B1T3 was always larger than that of other treatments. The stem diameter of cucumber plants treated with B0T1 was always smaller than that of other treatments. The stem sizes of B0T3-, B1T1- and B0T2-treated cucumber plants were similar, but lower than that of B1T2-treated cucumber plants.
The cucumber plant growth rate decreased and plant height increased slowly when potassium fertilizer was insufficient. Because potassium is involved in the synthesis of plant auxin, a lack of potassium will reduce the auxin content and inhibit cell elongation and division, thus limiting the longitudinal growth of the plant [45,46]. When potassium is excessive, it may affect the absorption of other cations, which will indirectly affect plant growth and lead to an abnormal plant height. Potassium plays an important role in maintaining the turgor pressure of plant cells. A lack of potassium can cause dysplasia in the mechanical tissue of the cucumber stem, so that the thickness of the fiber secondary wall become thin, the stem becomes weak, the stem diameter growth is limited and the anti-lodging ability is reduced. The thickness of the fiber secondary wall can be increased by applying potassium fertilizer in a proper amount, which makes the stem more robust. However, excess potassium can affect the absorption and utilization of phosphorus, resulting in phosphorus deficiency, which may make the stem elongated. In the early stage of potassium deficiency, the tip or edge of cucumber old leaves will appear mottled or chlorosis areas. With the progression of potassium deficiency, the whole leaves will turn yellow, photosynthesis will decrease, the accumulation of assimilates will decrease and the expansion of the leaf area will be hindered. In addition, potassium participates in the osmotic regulation of plants, and potassium deficiency will destroy the osmotic balance of cells, affect the extension and expansion of leaf cells, and limit the increase in leaf area [47,48].
After cucumber was infected with downy mildew, the leaf function was damaged, photosynthesis was weakened, and the energy and material for plant growth were reduced, thus affecting the growth of plant height. When the disease was severe, the plant grew slowly and the plant height was significantly lower than that of healthy plants [26,29]. Because downy mildew mainly harms the leaves, resulting in a reduction in photosynthetic products produced by the leaves and a corresponding reduction in nutrients delivered to the stems, the growth and thickening of the stems are inhibited, the stem thickness may become thinner, and the strength and toughness of the stems are reduced [24,25]. Downy mildew forms polygonal spots on the leaves. With the development of the disease, multiple spots are connected into pieces, and the whole leaves turn yellow brown and dry and die. The normal growth and expansion of the leaves are hindered, and the new leaves grow slowly, the leaf area decreases and the old leaves fall off in advance, resulting in the overall leaf area of the plant being reduced [25,28].
Both potassium stress and downy mildew had adverse effects on the plant height, stem diameter and leaf area of cucumber, and the harm degree was more serious when they occurred simultaneously [49]. On the one hand, potassium stress leads to weak stems; on the other hand, downy mildew damages the photosynthesis of leaves and reduces nutrient delivery to stems. Together, the growth of stem size is more restricted, the stems are thinner and weaker, and the lodging resistance is worse. Potassium stress affects the normal growth and photosynthesis of the leaves, while downy mildew directly harms the leaves, causing leaf lesions, drying and falling off. The synergistic effect of the two can seriously hinder the growth and development of cucumber leaves, significantly reduce the leaf area, and then affect the overall photosynthesis and growth of the plant, resulting in a decrease in cucumber yield. In the process of cucumber planting, reasonable fertilization should be applied to ensure a sufficient supply of potassium, and pest control should be strengthened to reduce the harm of potassium stress and downy mildew on the growth and development of cucumber, and to improve the yield and quality of cucumber [50].

3.3. Parameters of Root Systems

The effects of potassium stress and downy mildew infection on the root parameters of cucumber are shown in Figure 4. The root total length, root surface area, root average diameter, root total volume and total root tips of cucumber plants were significantly different under different experimental conditions (p < 0.05). As can be seen from Figure 4, on the 20th day after transplantation, the root total length of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 44.73%, 19.94%, 24.43%, 24.1% and 15.18%, respectively, compared with that treated with B1T2. The root surface area of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 63.98%, 36.19%, 18.82%, 31.27% and 2.22%, respectively, compared with that treated with B1T2. The root average diameter of cucumber plants treated with B0T1, B0T2, B0T3 and B1T1 decreased by 61.67%, 50.22%, 23.04% and 30.03%, respectively, compared with that treated with B1T2. The root average diameter of cucumber plants treated with B1T3 was increased by 12.07% compared with that treated with B1T2. The root total volume of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 51.46%, 37.14%, 19.97%, 25.07% and 2.85%, respectively, compared with that treated with B1T2. The total root tips of cucumber plants treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 54.48%, 43.91%, 53.86%, 27.41% and 23.07%, respectively.
Under potassium deficiency, the root growth of cucumber was slow, the elongation of taproot was inhibited, the number of lateral roots and root hairs was reduced, and the absorption area and capacity of roots were decreased. This is because potassium is involved in a variety of physiological processes in plant cells, and potassium deficiency will destroy the normal metabolic and physiological functions of cells, affecting the division and elongation of root cells. In addition, the lack of potassium will reduce the vitality of the root system, and the ability of the root system to absorb water and nutrients is weakened, resulting in poor plant growth. When potassium is excessive, it may affect the absorption balance of other elements, such as the absorption of calcium, magnesium and other elements being inhibited, which indirectly affects the normal development of the root system and causes abnormal root morphology and physiological function [48,49]. After cucumber is infected with downy mildew, although the main damage site is the leaf, it also has an indirect effect on the root system. Downy mildew causes the leaf photosynthesis to weaken, the production of photosynthates to decrease, the delivery of nutrients to the root system is also reduced, and the energy and material supply required for root growth is insufficient, thus affecting the growth and development of the root system [28]. The growth of the root system was slow, the fresh weight and dry weight of roots decreased, the root activity decreased and the absorption function was affected. At the same time, due to the disease of the aboveground part, the plant growth is hindered and the hormone balance is changed, which will also inhibit the growth of the root system, and may lead to changes in the root morphology, such as the reduction in the number of lateral roots and the reduction in the distribution range of roots [50,51]. When potassium stress and downy mildew were present at the same time, the damage to cucumber roots was synergistic. On the one hand, potassium stress itself inhibited the growth and development of roots and reduced the resistance of roots to stress. Downy mildew, on the other hand, leads to impaired leaf function, further reducing the supply of photosynthetic products to the root system. The combined effect of the two factors severely hindered root growth, significantly reduced root parameters such as root length, root volume, root fresh weight and dry weight, significantly decreased root vitality, and severely damaged root absorption and transport functions, thus affecting the overall growth and development of plants [52].

4. Conclusions

In order to observe the response of cucumber plants to potassium stress and cucumber downy mildew infestation, two different levels of downy mildew infestation, B0 (disease infestation) and B1 (disease-free infestation), and three fertilizer requirement levels of potassium fertilization, T1 (K-50%), T2 (K-100%) and T3 (K-150%), were applied in the greenhouse. The photosynthetic rate and stomatal conductance of cucumber plants leaves treated with B1T2 showed an increasing trend, and the increase in stomatal conductance was more significant. The intercellular CO2 concentration of cucumber leaves treated with B1T2 showed no significant difference. The plant height of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 32.21%, 16.93%, 21.59%, 53.54% and 6.31%, respectively, compared with that treated with B1T2. The leaf area of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 27.85%, 15.97%, 21.47%, 7.29% and 18.5%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B0T1, B0T2, B0T3 and B1T1 decreased by 13.43%, 8.72%, 6.01% and 7.06%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B1T3 increased by 6.83% compared with that treated with B1T2. The root total length, root surface area, root average diameter, root total volume and total root tips of cucumber plants were significantly different under different experimental conditions (p < 0.05). The B0T1 test treatment (that is, cucumber plants with downy mildew infestation and 50% of the normal application of potassium fertilizer) had the greatest impact on the growth and physiological traits of greenhouse cucumbers.

Author Contributions

Conceptualization, Q.S. and Y.W.; methodology, Q.S., L.Y., Y.W. and T.C.; software, Q.S., L.Y., Y.W. and X.D.; validation, Q.S., L.Y., Y.W., X.D. and T.C.; formal analysis, Q.S.; investigation, Q.S., L.Y. and Y.W.; resources, Q.S. and Y.W.; data curation, Q.S., L.Y., Y.W., X.D. and T.C.; writing—original draft preparation, Q.S., L.Y., Y.W., X.D. and T.C.; writing—review and editing, Q.S., L.Y., Y.W., X.D. and T.C.; visualization, Q.S., L.Y., Y.W., X.D. and T.C.; supervision, Y.W.; project administration, Q.S. and Y.W.; funding acquisition, Q.S. and Y.W. 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 No. 32201686) and the Shanghai Oriental Talent Program Youth Project (QNJY2024095).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 15 01017 g001
Figure 1. Cucumber planting. B0T1: Cucumber plants with downy mildew infestation and 50% of normal application of potassium fertilizer; B0T2: Cucumber plants with downy mildew infestation and normal application of potassium fertilizer; B0T3: Cucumber plants with downy mildew infestation and 150% of normal application of potassium fertilizer; B1T1: Cucumber plants without frost mildew infection, potassium fertilizer application rate was 50% of normal fertilizer; B1T2: Cucumber plants did not have downy mildew infection, potassium fertilizer application rate was normal; B1T3: Cucumber plants did not have downy mildew infection, potassium fertilizer application rate was 150% of normal fertilizer rate.
Figure 1. Cucumber planting. B0T1: Cucumber plants with downy mildew infestation and 50% of normal application of potassium fertilizer; B0T2: Cucumber plants with downy mildew infestation and normal application of potassium fertilizer; B0T3: Cucumber plants with downy mildew infestation and 150% of normal application of potassium fertilizer; B1T1: Cucumber plants without frost mildew infection, potassium fertilizer application rate was 50% of normal fertilizer; B1T2: Cucumber plants did not have downy mildew infection, potassium fertilizer application rate was normal; B1T3: Cucumber plants did not have downy mildew infection, potassium fertilizer application rate was 150% of normal fertilizer rate.
Agronomy 15 01017 g001
Agronomy 15 01017 g002aAgronomy 15 01017 g002b
Figure 2. The effects of downy mildew infection and potassium 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). (a) Photosynthetic rate; (b) intercellular CO2 concentration; (c) stomatal conductance; (d) transpiration rate.
Figure 2. The effects of downy mildew infection and potassium 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). (a) Photosynthetic rate; (b) intercellular CO2 concentration; (c) stomatal conductance; (d) transpiration rate.
Agronomy 15 01017 g002aAgronomy 15 01017 g002b
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Figure 3. The effects of downy mildew infection and potassium on cucumber plants’ growth parameters. (a) Plant height; (b) leaf area; (c) stem diameter.
Figure 3. The effects of downy mildew infection and potassium on cucumber plants’ growth parameters. (a) Plant height; (b) leaf area; (c) stem diameter.
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Figure 4. Root system parameters of cucumber plant. Note: Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at (p < 0.05). (a) Total length; (b) surface area; (c) average diameter; (d) total volume; (e) total tips.
Figure 4. Root system parameters of cucumber plant. Note: Error bars indicate standard deviations, with different lowercase letters between treatments indicating significant differences at (p < 0.05). (a) Total length; (b) surface area; (c) average diameter; (d) total volume; (e) total tips.
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Table 1. Component of nutrient solution (Unit/g).
Table 1. Component of nutrient solution (Unit/g).
Nutrient Solution GroupChemical ReagentK-50%K-100%K-150%
ACa(NO3)2·4H2O826826826
KNO3303606606
BNH4H2PO4114114114
MgSO4·7H2O492492492
NaNO325500
KCl00223
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. Use after diluting 100 times.
Table 2. Grading of cucumber downy mildew severity.
Table 2. 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
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Shi, Q.; You, L.; Wang, Y.; Du, X.; Chen, T. Effects of Downy Mildew Infection and Potassium on Growth and Physiological Traits of Greenhouse Cucumber. Agronomy 2025, 15, 1017. https://doi.org/10.3390/agronomy15051017

AMA Style

Shi Q, You L, Wang Y, Du X, Chen T. Effects of Downy Mildew Infection and Potassium on Growth and Physiological Traits of Greenhouse Cucumber. Agronomy. 2025; 15(5):1017. https://doi.org/10.3390/agronomy15051017

Chicago/Turabian Style

Shi, Qiang, Lu You, Yafei Wang, Xiaoxue Du, and Tianhua Chen. 2025. "Effects of Downy Mildew Infection and Potassium on Growth and Physiological Traits of Greenhouse Cucumber" Agronomy 15, no. 5: 1017. https://doi.org/10.3390/agronomy15051017

APA Style

Shi, Q., You, L., Wang, Y., Du, X., & Chen, T. (2025). Effects of Downy Mildew Infection and Potassium on Growth and Physiological Traits of Greenhouse Cucumber. Agronomy, 15(5), 1017. https://doi.org/10.3390/agronomy15051017

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