Efficacy of Biological Products in Managing Root Pathogens in Melons

Abstract

Biological control represents a sustainable alternative that can be used to reduce the impacts of soilborne diseases in melon cultivation, which are major constraints to productivity. This study evaluated the effectiveness of four biological products formulated with Bacillus and Trichoderma species in suppressing symptoms caused by root pathogens in melon crops, including Fusarium spp., Macrophomina phaseolina, Monosporascus cannonballus, and Rhizoctonia solani. Two greenhouse experiments were conducted to simulate successive crop cycles using two naturally infested soils (A and B). Bombardeiro/Lastro, Quality^®, TrichobiolMax, and TrichonemateMax were applied using two management strategies: (1) a tray application 8 days after sowing (DAS) + four pot applications at 7-day intervals, totaling five applications, and (2) a tray application 8 DAS + two pot applications at 14-day intervals, totaling three applications. The yellow melon cultivar ‘Goldex’ was used in the experiments. Forty-five days after transplanting, the treatments showed statistically significant differences compared to the positive control (naturally infested soil without products), both in disease incidence and severity and in plant growth parameters. In Soil A, three applications of Quality^® and TrichobiolMax resulted in 50% and 60% disease incidences, respectively. In Soil B, five applications of Lastro and TrichobiolMax led to 60% of plants showing disease symptoms. These products also reduced disease severity in both soils, and TrichonemateMax showed potential for nematode control. Additionally, these products resulted in a 21% reduction in the frequency of Fusarium spp. in Soil A. These findings are valuable for developing sustainable practices in melon cultivation, promoting more efficient and environmentally sound management of root diseases.

1. Introduction

Melon (Cucumis melo L.), a plant species belonging to the Cucurbitaceae family, is currently Brazil’s most exported fresh fruit, generating approximately USD 31,297,334.00 in revenue during the first two months of 2025 [1]. Production is concentrated in the Northeast region, mainly in the states of Rio Grande do Norte, Bahia, and Ceará, which together account for nearly 90% of the country’s total output [2]. Brazil ranks fifth among the world’s largest melon producers, with a cultivated area of 30,635 ha and an estimated production of 862,387 t [2,3].

Melon production in northeastern Brazil benefits from favorable edaphoclimatic conditions, such as high solar radiation, low rainfall, and low relative humidity [4]. These factors facilitate the adoption of advanced technologies, including hybrid cultivars, plastic mulch, seedling transplantation, drip irrigation, and high planting densities, allowing for crop intensification [5]. However, the expansion of cultivation, combined with intensive and continuous farming practices without adequate integrated management, has contributed to increased incidence and severity of root diseases, which pose a major threat to crop productivity [6].

Root diseases are particularly concerning for melon growers due to their high destructive potential and economic impact. Several soilborne pathogens limit production by simultaneously affecting the root system and stem base, leading to plant death either early in development (as in seedling damping-off) or near harvest [6,7]. Among the most important fungal pathogens are Stagonosporopsis cucurbitacearum (synonym: Didymella bryoniae), which causes gummy stem blight or stem canker; Fusarium spp., which are responsible for fusarium wilt; Macrophomina phaseolina, the causal agent of charcoal rot; Monosporascus cannonballus, which is associated with sudden vine collapse; and Rhizoctonia solani, which causes rhizoctoniosis and seedling damping-off [8].

Current strategies for managing these pathogens largely rely on synthetic fungicides. Although effective and easy to apply, excessive fungicide use can lead to pathogen resistance and residual contamination in food and the environment [9]. To mitigate these impacts, integrated disease management (IDM) is recommended. IDM combines multiple control measures, prioritizing biological, natural, and biotechnological approaches to reduce reliance on chemical inputs [10].

Among these methods, biological control is a sustainable strategy that employs beneficial microorganisms—biological control agents (BCAs)—to suppress phytopathogens and reduce disease incidence [11]. BCAs also promote plant growth and protect against fungal diseases and nematodes [12]. Key genera used in biocontrol include Bacillus and Pseudomonas (bacteria) and Trichoderma (fungi) [13]. Strains and isolates of Bacillus and Trichoderma species have already demonstrated efficacy in reducing the incidence and severity of root rot caused by M. phaseolina and R. solani in melon crops [14,15,16]. Trichoderma species have also shown the potential to suppress nematode populations in soil [17,18]. However, the colonization and activity of these microorganisms can be influenced by environmental factors such as the soil pH, temperature, humidity, organic matter content, and native microbiota [19].

Currently, more than 80 commercial biological products based on Bacillus and Trichoderma species are registered in Brazil for use in melon cultivation [20]. These products support the development of organic farming and are suitable for use in sensitive areas where synthetic pesticides are restricted, promoting environmentally responsible agriculture [21,22]. When incorporated into IDM programs, they can enhance plant disease control while reducing dependency on chemical inputs and minimizing environmental impacts [23]. Therefore, the performance of available biological products should be assessed during successive crop cycles using naturally infested soils in order to expand our understanding of their efficacy, application strategies, and effectiveness against key root diseases affecting melon crops in this region.

Thus, this study aimed to evaluate the effectiveness of four biological products based on Bacillus spp. strains and Trichoderma spp. isolates in suppressing symptoms caused by root diseases in melons. We hypothesized that these products, applied during successive crop cycles in naturally infested soils, would significantly reduce disease incidence and severity compared to untreated controls.

2. Materials and Methods

2.1. Experiment Overview

The experiments were conducted in a greenhouse located in the municipality of Mossoró, Rio Grande do Norte, Brazil, from July to December 2024, with an average temperature of 37.4 °C and a relative humidity of 39%.

The soils used in this study were collected from two commercial melon-producing areas on different farms and are referred to as Soil A and Soil B. The farms were located 4.9 km apart, and both had a documented history of root pathogen occurrence (4°54′26.40″ S, 37°24′5.46″ W; 4°54′11.9″ S, 37°21′58.8″ W, respectively). The two soils were classified as Dystrophic Red-Yellow Latosols with medium texture. The chemical characteristics of Soil A were as follows: pH (water) = 6.94, electrical conductivity (EC) = 0.07 dS.m⁻¹, P = 273.00 mg.dm⁻³, K⁺ = 59.40 mg.dm⁻³, Na⁺ = 16.80 mg.dm⁻³, Ca²⁺ = 2.60 cmol_c.dm⁻³, Mg²⁺ = 0.71 cmol_c.dm⁻³, sum of bases (SB) = 3.54 cmol_c.dm⁻³, cation exchange capacity (CEC) = 3.87 cmol_c.dm⁻³, and percentage of exchangeable sodium (PES) = 2%. Soil B showed the following characteristics: pH (water) = 6.64, EC = 0.29 dS.m⁻¹, P = 132.80 mg.dm⁻³, K⁺ = 42.60 mg.dm⁻³, Na⁺ = 97.00 mg.dm⁻³, Ca²⁺ = 1.76 cmol_c.dm⁻³, Mg²⁺ = 0.51 cmol_c.dm⁻³, SB = 2.80 cmol_c.dm⁻³, CEC = 3.30 cmol_c.dm⁻³, and PES = 13%.

2.2. Biological Products

Four biological products based on Bacillus spp. bacteria and Trichoderma spp. fungi that are available in the Brazilian market were used in this study (

Table 1. Biological products used in this study.

Product	Composition	Formulation	Dose	Company
Bombardeiro/Lastro	Bacillus subtilis, B. velezensis, B. pumilus	Concentrated suspension	1.0 L.ha−1	Total Biotecnologia Indústria e Comércio S.A., Curitiba, Brazil
Quality® WG	Trichoderma asperellum	Water-dispersible granules	150.0 g.ha−1	Lallemand Soluções Biológicas Ltd.a, Patos de Minas, Brazil
TrichobiolMax	Trichoderma asperellum	Concentrated suspension	1.5 L.ha−1	Biofungi Controle Biológico, Eunápolis, Brazil
TrichonemateMax	Trichoderma longibrachiatum	Concentrated suspension	1.5 L.ha−1	Biofungi Controle Biológico, Eunápolis, Brazil

). The doses were based on the manufacturers’ recommendations but were adjusted to reflect those commonly applied in commercial melon production areas in this region.

2.3. Methodology

The experiments were conducted using 10 L pots. Soil preparation included sieving to remove crop residues and break up large aggregates. The treatments involved applying the four biological products to both Soil A and Soil B using two management strategies: (1) application in seedling trays 8 days after sowing (DAS) + four pot applications at 7-day intervals, totaling five applications (5A), and (2) application in trays 8 DAS + two pot applications at 14-day intervals, totaling three applications (3A).

Each soil included both positive and negative controls. The positive control consisted of naturally infested soil without product application. The negative control used sterilized soil, which was autoclaved twice for 60 min at 1 atm, with a 24 h interval between the sterilizations.

Seeds of the ‘Goldex’ yellow melon cultivar (Semillas Fitó, Fortaleza, Brazil) were sown in trays with 128 cells, each measuring 5 × 5 cm (length × width), filled with a commercial substrate composed of pine bark, peat, charcoal, vermiculite, and nutrients (Basaplant Hortaliças—Base Soluções em Substratos Ltd.a, Artur Nogueira, Brazil). Then, 8 DAS, the products were applied directly in each tray cell using a 5 mL graduated syringe, targeting individual seedlings. Two days after application, the seedlings were transplanted into pots containing Soil A or Soil B.

In the pots, product application was carried out using a 50 mL graduated cylinder directed at the stem base and root zone. Applications occurred 7, 14, 21, and 28 days after transplanting (DAT) for the five-application strategy (5A) and 14 and 28 DAT for the three-application strategy (3A). Irrigation was performed daily with a manual watering can.

The experiment followed a completely randomized design, with 10 replicates per combination of soil type, biological product, and application method, which resulted in 200 pots, each containing one plant. A second experiment was carried out using the soils from the first experiment to simulate successive crop cycles typical of melon production in this region, where fields are usually cultivated continuously without fallow periods or crop rotation, reflecting real field scenarios.

Evaluations were performed 45 DAT in both experiments and included measurements of disease incidence and severity, fresh and dry biomass, and root and shoot length. The plants were carefully removed from the pots, and their roots were washed under running water. The root disease incidence was determined based on the number of symptomatic plants per treatment and expressed as a percentage (%). Disease severity was assessed using a diagrammatic scale adapted from Ambrósio et al. [24], where 0 = asymptomatic tissue; 1 = <3% infected tissue; 2 = 3–10% infected tissue; 3 = 11–25% infected tissue; 4 = 26–50% infected tissue; and 5 = >50% infected tissue (Figure 1).

Data normality was assessed using the Shapiro–Wilk test. Disease incidence and severity were analyzed using the non-parametric Kruskal–Wallis test followed by Dunn’s post hoc test at a 5% significance level (p < 0.05) with Assistat software version 7.7 [25]. Root length (RL) and shoot length (SL) were measured in centimeters (cm) using a measuring tape. Fresh and dry root weights (FRW and DRW) and fresh and dry shoot weights (FSW and DSW) were determined using a semi-analytical balance and expressed in grams (g). To determine dry weight, plant tissues were placed in paper bags and dried in a forced-air oven at 60 °C until reaching a constant weight. These data were subjected to an analysis of variance (ANOVA), and the means were compared by Tukey and Dunnett’s tests (p < 0.05).

To assess fungal frequency, seven fragments from the root and stem base tissues of each symptomatic plant were surface-sterilized in 2% sodium hypochlorite for 1 min, rinsed twice with distilled water, and plated in Petri dishes containing a Potato Dextrose Agar (PDA) medium supplemented with 500 mg.L⁻¹ streptomycin sulfate. At least three replicates were used for each treatment. The presence of nematodes in the soil was evaluated following the method described by Jenkins [26]. Briefly, 100 g of soil was mixed with water and agitated. Then, the suspension was passed sequentially through 20- and 400-mesh sieves to remove debris and retain nematode eggs and juveniles. The retained material was centrifuged and separated using a sucrose flotation technique. The extracted eggs were collected and counted under a microscope. Juvenile and adult nematodes were identified by morphological analysis using standard taxonomic keys.

3. Results

3.1. Incidence of Root Diseases

The treatments applied to Soils A and B resulted in statistically significant differences in root disease incidence (p < 0.05) (Figure 2). In Experiment 1, the lowest incidences were observed in Soil A with the application of Quality^® (50%) and TrichobiolMax (60%), both using the three-application (3A) management strategy. These results were statistically different from the other treatments but not significantly different from the control (Figure 2A). In Soil B, the lowest incidence (60%) was recorded following five applications (5A) of Lastro and TrichobiolMax (Figure 2C). In Experiment 2, however, no significant differences were observed among the treatments (p ≥ 0.05), and none of the products were effective in reducing disease incidence in either soil (Figure 2B,D).

3.2. Severity of Root Diseases

The severity of root diseases varied significantly among the treatments applied to Soils A and B (p < 0.05) (Figure 3). In Experiment 1, the lowest severities in Soil A were observed with the application of Quality^® (0.70) and TrichobiolMax (0.90), both using the three-application (3A) management strategy. These results were consistent with the reduced incidence observed and were statistically similar to the control. Additionally, Lastro, Quality^®, and TrichobiolMax applied using the five-application (5A) strategy also produced severity levels that were not significantly different from the control (Figure 3A). In Soil B, the lowest severity (0.60) was recorded with TrichobiolMax using the 5A strategy. This treatment, along with Lastro using 5A management and both Quality^® and TrichobiolMax using 3A management, showed performance that was statistically similar to the control (Figure 3C).

In Experiment 2, the severity in Soil A was higher than in Experiment 1. The lowest severity (2.44) in this soil was observed with TrichobiolMax using 3A management, followed by Lastro using both management strategies. All of these treatments showed results comparable to the control (Figure 3B). In Soil B, the lowest severity (2.30) was achieved with Quality^® using both the 3A and 5A management strategies (Figure 3D).

3.3. Effects of Biological Products on Root and Shoot Growth

The application of the biological products significantly affected the root and shoot growth of the melon plants cultivated in Soils A and B in both experiments (p < 0.05) (

Table 2. The effects of the biological products on the root length (RL), shoot length (SL), fresh root weight (FRW), fresh shoot weight (FSW), dry root weight (DRW), and dry shoot weight (DSW) of ‘Goldex’ yellow melon plants as a function of the biological product applied.

Soil A
Experiment 1
Product	RL (cm)	FRW (g)	DRW (g)	SL (cm)	FSW (g)	DSW (g)
Bombardeiro/Lastro	25.60 a	2.30 a	0.36 a	167.50 a	81.10 a	8.21 a
Quality®	22.55 a	2.17 ab	0.33 ab	172.55 a	80.15 a	8.20 a
TrichobiolMax	18.20 b	2.04 ab	0.33 ab	175.20 a	77.12 a	8.17 a
TrichonemateMax	21.80 ab	1.87 b	0.29 b	175.20 a	75.61 a	7.99 a
LSD	3.98	0.38	0.06	20.72	10.47	0.93
CV (%)	21.78	21.25	20.96	14.04	15.34	13.49
Experiment 2
Bombardeiro/Lastro	25.70 a	2.11 a	0.32 a	176.45 a	86.83 a	8.46 a
Quality®	25.80 a	1.71 b	0.30 a	175.15 a	78.58 ab	7.98 a
TrichobiolMax	26.80 a	1.71 b	0.30 a	180.85 a	80.94 ab	8.29 a
TrichonemateMax	25.15 a	1.51 b	0.27 a	179.65 a	75.83 b	7.86 a
LSD	4.47	0.39	0.08	23.29	9.71	0.89
CV (%)	20.73	26.55	31.43	15.48	14.47	13.20
Soil B
Experiment 1
Product	RL (cm)	FRW (g)	DRW (g)	SL (cm)	FSW (g)	DSW (g)
Bombardeiro/Lastro	27.00 ab	2.30 a	0.33 a	175.60 a	80.00 a	6.52 a
Quality®	32.80 a	1.99 a	0.22 b	177.50 a	71.69 a	5.40 b
TrichobiolMax	25.85 b	1.96 a	0.27 b	183.60 a	76.86 a	6.39 a
TrichonemateMax	27.15 ab	1.51 b	0.21 b	177.35 a	71.60 a	6.37 a
LSD	6.28	0.35	0.06	13.96	10.13	0.92
CV (%)	27.16	20.65	28.91	8.98	15.22	17.25
Experiment 2
Bombardeiro/Lastro	22.60 ab	1.61 a	0.29 a	176.45 a	77.90 a	7.18 a
Quality®	21.25 b	1.67 a	0.27 a	184.10 a	76.18 a	6.92 a
TrichobiolMax	21.10 b	1.42 a	0.26 a	181.30 a	73.57 a	6.91 a
TrichonemateMax	25.95 a	1.63 a	0.29 a	176.75 a	73.66 a	6.72 a
LSD	3.40	0.32	0.09	17.79	10.76	0.87
CV (%)	18.36	24.95	37.95	11.59	17.07	15.03

LSD = least significant difference value. CV (%) = coefficient of variation. Means of 10 replicates followed by the same lowercase letter do not differ statistically from each other according to Tukey’s test at 5% probability.

). In Experiment 1, Lastro and Quality^® resulted in the highest RLs in Soil A, with values of 25.60 and 22.55 cm, respectively. Lastro also provided the highest FRW (2.30 g) and DRW (0.36 g), significantly differing from the other treatments. In Experiment 2, Lastro again led to the highest values of FRW (2.11 g) and FSW (86.83 g). For the other variables evaluated, no significant differences were observed among the treatments.

In Soil B, in Experiment 1, Quality^® resulted in the highest RL (32.80 cm) but was also associated with the lowest DSW (5.40 g). Conversely, Lastro yielded the highest DRW (0.33 g). In Experiment 2, TrichonemateMax showed the highest RL (25.95 cm); however, no significant differences were observed among the treatments for the remaining variables.

The application of the biological products had no significant effect on the root and shoot growth of the melon plants cultivated in Soils A and B in either experiment (p < 0.05) (

Table 3. The effects of the biological products on the root length (RL), shoot length (SL), fresh root weight (FRW), fresh shoot weight (FSW), dry root weight (DRW), and dry shoot weight (DSW) of ‘Goldex’ yellow melon plants as a function of the management strategy used.

Soil A
Experiment 1
Application Strategy	RL (cm)	FRW (g)	DRW (g)	SL (cm)	FSW (g)	DSW (g)
Tray +7 + 7 + 7 + 7 (5A)	22.30 a	2.08 a	0.34 a	174.37 a	80.13 a	8.46 a
Tray +14 + 14 (3A)	21.77 a	2.11 a	0.32 a	170.85 a	76.86 a	7.84 b
LSD	2.13	0.21	0.03	11.11	5.61	0.50
CV (%)	21.78	21.25	20.96	14.04	15.34	13.49
Experiment 2
Tray +7 + 7 + 7 + 7 (5A)	26.40 a	1.68 a	0.29 a	176.17 a	80.78 a	8.10 a
Tray +14 + 14 (3A)	25.32 a	1.83 a	0.31 a	179.87 a	80.31 a	8.20 a
LSD	2.39	0.21	0.04	12.49	5.21	0.48
CV (%)	20.73	26.55	31.43	15.48	14.47	13.20
Soil B
Experiment 1
Application Strategy	RL (cm)	FRW (g)	DRW (g)	SL (cm)	FSW (g)	DSW (g)
Tray +7 + 7 + 7 + 7 (5A)	28.62 a	1.96 a	0.27 a	186.90 a	77.18 a	6.31 a
Tray +14 + 14 (3A)	27.77 a	1.92 a	0.25 a	170.12 b	72.89 a	6.03 a
LSD	3.37	0.19	0.03	7.48	5.43	0.49
CV (%)	27.16	20.65	28.91	8.98	15.22	17.25
Experiment 2
Tray +7 + 7 + 7 + 7 (5A)	21.30 b	1.49 b	0.25 b	173.25 b	72.82 a	6.73 a
Tray +14 + 14 (3A)	24.15 a	1.68 a	0.30 a	186.05 a	77.83 a	7.14 a
LSD	1.82	0.17	0.05	9.54	5.77	0.47
CV (%)	18.36	24.95	37.95	11.59	17.07	15.03

Five-application strategy = 5A. Three-application strategy = 3A. LSD = least significant difference value. CV (%) = coefficient of variation. Means of 10 replicates followed by the same lowercase letter do not differ statistically from each other according to Tukey’s test at 5% probability.

). In Soil A, a difference was observed in Experiment 1 only for the DSW, with a value of 8.46 g for the 5A strategy; however, no statistically significant differences were observed between the application management strategies in Experiment 2. In Soil B, the highest SL (186.90 cm) was recorded in Experiment 1 under 5A management. In Experiment 2, the highest values of RL (24.15 cm), FRW (1.68 g), DRW (0.30 g), and SL (186.05 cm) were observed for 3A management.

A comparative analysis of the treatments versus the positive and negative controls revealed significant differences in the root and shoot growth of the melon plants cultivated in both soils across the two experiments (p < 0.05) (

Table 4. The means of root length (RL), shoot length (SL), fresh root weight (FRW), fresh shoot weight (FSW), dry root weight (DRW), and dry shoot weight (DSW) of the ‘Goldex’ yellow melon plants as a function of the biological products and management strategies applied.

Soil A
Experiment 1
Treatment	RL (cm)	FRW (g)	DRW (g)	SL (cm)	FSW (g)	DSW (g)
Bombardeiro/Lastro—5 A	26.00 a	2.37 b	0.39 a	167.60 b	84.29 b	8.48 b
Bombardeiro/Lastro—3 A	25.20 a	2.23 b	0.33 a	167.40 b	77.91 b	7.97 b
Quality®—5 A	24.40 a	2.01 b	0.33 a	175.50 a	81.27 b	8.56 b
Quality®—3 A	20.70 a	2.33 b	0.34 a	169.60 b	79.02 b	7.85 b
TrichobiolMax—5 A	16.40 b	1.95 b	0.34 a	178.70 a	77.37 b	8.50 b
TrichobiolMax—3 A	20.00 a	2.13 b	0.32 a	171.70 b	76.86 b	7.83 b
TrichonemateMax—5 A	22.40 a	1.98 b	0.31 a	175.70 a	77.59 b	8.28 b
TrichonemateMax—3 A	21.20 a	1.76 b	0.27 b	174.70 b	73.63 b	7.70 b
Positive control	21.10 a	1.89 b	0.28 b	194.40 a	68.09 b	7.07 b
Control	22.80 a	3.18 a	0.37 a	205.50 a	127.74 a	10.65 a
LSD	5.88	0.57	0.08	30.67	15.50	1.37
CV (%)	21.78	21.25	20.96	14.04	15.34	13.49
Experiment 2
Bombardeiro/Lastro—5 A	25.60 a	2.09 a	0.35 a	179.80 a	87.36 a	8.63 a
Bombardeiro/Lastro—3 A	25.80 a	2.12 a	0.30 a	173.10 b	86.31 a	8.30 a
Quality®—5 A	25.50 a	1.66 a	0.29 a	173.10 b	76.37 b	7.66 a
Quality®—3 A	26.10 a	1.76 a	0.30 a	177.20 b	80.79 a	8.29 a
TrichobiolMax—5 A	29.40 a	1.46 b	0.24 b	174.40 b	80.33 a	8.08 a
TrichobiolMax—3 A	24.20 a	1.95 a	0.37 a	187.30 a	81.55 a	8.49 a
TrichonemateMax—5 A	25.10 a	1.51 b	0.28 a	177.40 b	79.07 b	8.02 a
TrichonemateMax—3 A	25.20 a	1.50 b	0.27 a	181.90 a	72.58 b	7.70 a
Positive control	27.10 a	1.37 b	0.25 b	177.60 b	72.32 b	7.33 b
Control	25.90 a	2.22 a	0.38 a	213.20 a	93.45 a	8.65 a
LSD	6.61	0.57	0.12	34.48	14.38	1.31
CV (%)	20.73	26.55	31.43	15.48	14.47	13.20
Soil B
Experiment 1
Treatment	RL (cm)	FRW (g)	DRW (g)	SL (cm)	FSW (g)	DSW (g)
Bombardeiro/Lastro—5 A	24.80 a	2.42 b	0.36 a	178.20 b	80.52 b	6.62 a
Bombardeiro/Lastro—3 A	29.20 a	2.19 b	0.31 a	173.00 b	79.48 b	6.41 b
Quality®—5 A	33.10 a	1.91 b	0.22 b	181.50 b	70.44 b	5.41 b
Quality®—3 A	32.50 a	2.06 b	0.21 b	173.50 b	72.94 b	5.40 b
TrichobiolMax—5 A	26.70 a	2.03 b	0.28 a	199.60 a	84.23 b	6.67 a
TrichobiolMax—3 A	25.00 a	1.89 b	0.26 a	167.60 b	69.50 b	6.11 b
TrichonemateMax—5 A	29.90 a	1.49 b	0.22 b	188.30 b	73.54 b	6.55 a
TrichonemateMax—3 A	24.40 a	1.52 b	0.21 b	166.40 b	69.66 b	6.20 b
Positive control	28.40 a	1.87 b	0.27 a	231.30 a	91.02 b	7.27 a
Control	25.10 a	2.99 a	0.35 a	216.60 a	111.61 a	7.86 a
LSD	9.30	0.52	0.09	20.66	14.99	1.36
CV (%)	27.16	20.65	28.91	8.98	15.22	17.25
Experiment 2
Bombardeiro/Lastro—5 A	23.10 b	1.61 a	0.28 a	169.30 b	73.66 b	7.00 b
Bombardeiro/Lastro—3 A	22.10 b	1.61 a	0.30 a	183.60 b	82.14 a	7.35 b
Quality®—5 A	20.40 b	1.52 a	0.26 b	178.10 b	72.95 b	6.82 b
Quality®—3 A	22.10 b	1.83 a	0.27 b	190.10 b	79.40 b	7.03 b
TrichobiolMax—5 A	18.40 b	1.18 b	0.23 b	170.50 b	70.98 b	6.54 b
TrichobiolMax—3 A	23.80 b	1.67 a	0.30 a	192.10 b	76.15 b	7.29 b
TrichonemateMax—5 A	23.30 b	1.64 a	0.23 b	175.10 b	73.68 b	6.56 b
TrichonemateMax—3 A	28.60 a	1.61 a	0.34 a	178.40 b	73.64 b	6.88 b
Positive control	20.30 b	1.17 b	0.20 b	193.50 b	62.07 b	5.32 b
Control	21.40 b	1.86 a	0.41 a	221.40 a	95.62 a	9.26 a
LSD	5.03	0.48	0.13	26.33	15.92	1.29
CV (%)	18.36	24.95	37.95	11.59	17.07	15.03

Five-application strategy = 5A. Three-application strategy = 3A. LSD = least significant difference value. CV (%) = coefficient of variation. Means of 10 replicates followed by the same lowercase letter do not differ statistically from each other according to Dunnett’s test at 5% probability.

). In Experiment 1, in Soil A, the treatments with Quality^® and TrichonemateMax—both applied using the 5A strategy—showed values for RL (24.40 and 22.40 cm), DRW (0.33 and 0.31 g), and SL (175.50 and 175.70 cm) that were statistically similar to the control and stood out for exhibiting the smallest reductions in these growth parameters. In Experiment 2, Lastro with five applications and TrichobiolMax with three applications provided FRW (2.09 and 1.95 g), SL (179.80 and 187.30 cm), and FSW (87.36 and 81.55 g) values that were statistically similar to the control.

In Experiment 1, in Soil B, Lastro with five applications showed DRW (0.36 g) and DSW (6.62 g) values similar to the control. Similar results were observed for TrichobiolMax, also applied five times, which also had DRW (0.28 g), SL (199.60 cm), and DSW (6.67 g) values comparable to the control. In Experiment 2, Lastro with three applications promoted DRW (0.30 g) and FSW (82.14 g) values statistically similar to the control, while TrichonemateMax (three applications) presented RL (28.60 cm) and DRW (0.34 g) values comparable to the control but differing from the positive control.

3.4. Fungal Pathogens and Nematode Egg Frequency

Soilborne fungal pathogens from four genera were isolated from the stem bases and roots of the melon plants across all treatments in both soils (Figure 4 and Figure 5). In Soil A, the Fusarium isolation rates ranged from 30 to 46%, the Macrophomina isolation rates ranged from 18 to 38%, and the Monosporascus isolation rates ranged from 2 to 7%. Rhizoctonia was only detected in treatments with TrichonemateMax, with a frequency of 2%. Trichoderma was only isolated from plants treated with Quality^® (14%) and TrichobiolMax (5%) (Figure 4A–E). In Soil B, Fusarium exhibited a markedly higher frequency than in Soil A, with isolation rates ranging from 64% to 86%. Macrophomina ranged from 2% to 12%, while Monosporascus and Rhizoctonia were each detected at frequencies between 2% and 4%. Trichoderma was isolated from plants treated with Lastro, Quality^®, and TrichonemateMax, each showing a frequency of 2% (Figure 5A–E). Additionally, saprophytic genera such as Aspergillus, Curvularia, Helminthosporium, Penicillium, and Rhizopus were also isolated and are grouped in the figures under the category ‘Others’.

At the beginning of the experiment, the nematode egg populations differed between the two soils. Soil A contained an average of 1185 eggs per 100 g of soil, corresponding to 89% of the sampled plants, whereas Soil B contained an average of 90 eggs per 100 g of soil, corresponding to 55% of the sampled plants. This population was lower in all treatments compared to the positive control in both soils. In Soil A, the treatments with Lastro showed the highest egg percentages (8%), followed by TrichobiolMax (3%) (Figure 4F). Moreover, although free-living nematodes were present in all treatments, no significant egg percentages were observed in the treatments with Quality^® and TrichonemateMax. In the same soil, phytopathogenic nematodes from the Meloidogyne, Mesocriconema, and Pratylenchus genera were also detected. In Soil B, Quality^® and TrichobiolMax showed the highest egg percentages (18%), while Lastro showed the lowest (9%). No significant egg percentages were observed in the treatments with TrichonemateMax (Figure 5F). Moreover, free-living nematodes were observed in all treatments of this soil, with no phytopathogenic individuals detected.

4. Discussion

Biological control of root diseases is a promising strategy, but its effectiveness depends on complex interactions among the pathogen, biological control agent, host, application method, and environment [27]. Products based on Bacillus spp. and Trichoderma spp. can effectively manage diseases and promote melon growth. Hashem et al. [14] demonstrated that Bacillus subtilis and Trichoderma viride can reduce the incidence and impacts of charcoal rot caused by M. phaseolina in melons. Additionally, Matloob and Al-Amri [15] observed that treatments with salicylic acid, Trichoderma harzianum, and B. subtilis reduced the incidence and severity of root rot caused by R. solani.

In field trials, Trichoderma-based products such as Quality^® and TrichonemateMax reduced disease incidence caused by root pathogens by 40% and 28%, respectively, and reduced disease severity by 1.03 and 0.95, respectively [28]. These trials were conducted in soils with characteristics similar to those used in the present study. They were obtained from the same production field, and the results were statistically comparable to those observed here, supporting the relevance of our findings. In in vitro assays, the same authors reported over 70% inhibition of mycelial growth of Fusarium solani, M. phaseolina, and M. cannonballus. Bakhshi et al. [16] also observed a reduction in the disease caused by M. phaseolina using strains of Bacillus amyloliquefaciens. Plants inoculated with the bacteria showed increases in fresh and dry root weights of 32.97% and 30.39% and increases in shoot weights of 36.27 and 34.44% compared to untreated plants.

In addition to pathogen control, B. subtilis strains have been shown to promote melon growth by producing phytohormones such as indole-3-acetic acid (IAA) and solubilizing phosphorus, resulting in more vigorous plants with enhanced nutritional contents [29]. IAA production and phosphorus solubilization are well-documented mechanisms by which Bacillus species enhance nutrient uptake and stimulate root development [30,31]. Similar growth-promoting effects of Bacillus spp. and Trichoderma spp. have also been reported in other crops, including soybeans, cowpeas, maize, and rice [32].

In this study, application of biological products reduced the incidence and severity of root diseases in ‘Goldex’ yellow melon plants across both evaluated soils. In Soil A, the lowest disease incidence was observed with three applications (3A) of Quality^® and TrichobiolMax, while in Soil B, five applications (5A) of Lastro and TrichobiolMax were most effective. These products also reduced disease severity in both soils. However, in the second (repeat) experiment, the treatment efficacy declined, suggesting that successive cultivation may have increased the inoculum pressure in both soils. The variability in effectiveness may be attributed to the specificity of the biological control agents toward the pathogen populations present in each soil or to environmental conditions that may have limited microbial activity. These findings highlight the importance of tailoring management strategies and application methods to optimize product performance under different soil and experimental conditions.

The biological treatments also slightly reduced the incidence of soilborne fungal pathogens compared to the positive control. In this untreated infested control, Fusarium was the most frequently isolated genus, followed by Macrophomina and Monosporascus. The high incidences of Fusarium spp. and Macrophomina spp. confirm their prevalence as soil pathogens in melon cultivation. They are already known to cause root diseases. This is likely due to their ability to form resistant structures that allow them to survive high temperatures and variable environmental conditions [28].

Despite the high incidence of phytopathogens, the presence of Trichoderma spp. in the treated plants suggests a potential long-term suppressive effect, contributing to stabilization of the local soil microbiota. Previous studies have already demonstrated that Trichoderma species may compete with pathogens such as F. pseudograminearum through multiple mechanisms, including the production of antifungal metabolites, activation of induced systemic resistance, mycoparasitism, and competition for space and nutrients [33,34].

Trichoderma koningii and T. polysporum isolates have previously been reported to be effective in controlling F. solani, F. oxysporum, and Sclerotium rolfsii in melons cultivated under similar environmental conditions. However, their efficacy can vary depending on the specific isolate and soil characteristics [35]. Factors such as the soil pH, temperature, humidity, organic matter content, and native microbiota composition can also influence the colonization and activity of these microorganisms [19].

In this study, the biological treatments influenced the fungal microbiota differently in the two soils. In Soil A, the products had a more pronounced effect in reducing Fusarium spp. than in Soil B. This difference can be attributed to variations in soil physicochemical properties, such as the organic matter content, pH, and nutrient availability, which were characterized after collection. These results highlight that interactions between biological control agents and native soil microbiota are strongly influenced by the chemical and biological properties of the soil.

Treatments with biological products showed varying efficacy in controlling nematode eggs, with reductions compared to the positive control. This confirms that, in the absence of biocontrol agents, nematodes proliferate freely in the soil. In treatments with Quality^® and TrichonemateMax, the complete absence of nematode eggs suggests a strong antagonistic effect, consistent with previous studies demonstrating that both Trichoderma and Bacillus species can suppress Meloidogyne spp. populations in vegetable crops [36,37].

Certain Trichoderma spp. isolates are known to parasitize nematode eggs and juveniles by producing chitinolytic enzymes that degrade egg cell walls and reduce hatching rates [38]. Trichoderma longibrachiatum, for instance, has shown strong inhibitory effects on the development and hatching of eggs and cysts of Heterodera avenae [17]. These findings support the potential for Trichoderma spp. to significantly reduce nematode populations through enzymatic degradation of eggs and juveniles [18]. The complete elimination of nematode eggs in both soils following treatment with TrichonemateMax suggests that this product may contain a particularly effective Trichoderma isolate, making it a promising alternative for managing nematodes in melon crops.

In summary, this study demonstrates the differential efficacy of four commercial biological control products during successive crop cycles in two naturally infested soils, providing practical insights for melon cultivation in northeastern Brazil. The findings highlight that product performance depends on soil characteristics, pathogen pressure, and the application strategy. Future studies should investigate the optimal application timing and frequency, as well as interactions with native soil microbiota, under field conditions to maximize the benefits of biological control in melon production.

5. Conclusions

Application of biological products based on Bacillus spp. and Trichoderma spp. demonstrated potential for reducing root disease symptoms in melon plants, particularly in Experiment 1. The products Lastro, Quality^®, and TrichobiolMax were the most consistent in reducing disease severity across both soils, while TrichonemateMax showed greater efficacy in suppressing nematodes. Detection of Trichoderma spp. in certain treatments, along with observed reductions in pathogens such as Fusarium, suggests effective colonization and potential antagonistic activity. These findings support the viability of biological control as a complementary strategy within integrated disease management, particularly when considering the prior presence of pathogens in the soil and the frequency of product application.