Downy mildew is a widespread and economically impactful plant disease that affects various crops, including cucumbers, grapes, cantaloupes, and other cucurbits, across different agricultural regions globally. This disease is caused by obligate fungal-like pathogens, such as Plasmopara viticola(Heger et al., 2022); (Clippinger et al., 2024) and Pseudoperonospora cubensis (Wallace et al., 2020); (Sun et al., 2022), which predominantly infect green plant tissues, especially the leaves. Typical symptoms include irregular spots on the leaves, which can range from pale green to yellow or brown in color (Unknown Author, 2019) (Purayannur et al., 2021). Favorable conditions, including high humidity and moderate temperatures, facilitate rapid disease spread through wind-borne spores or rain splashes, often resulting in large-scale outbreaks.

In severe cases, downy mildew can lead to defoliation, stunted growth, poor fruit quality, and even total crop failure, making it a critical challenge for growers (Purayannur et al., 2021); (E et al., 2022). As global temperatures and weather patterns shift, the range and frequency of downy mildew outbreaks are projected to increase, emphasizing the need for integrated disease management strategies that combine resistant crop varieties, cultural practices, and sustainable control measures (Singh et al., 2023).

Globally, downy mildew significantly impacts food security and agricultural output, particularly in areas where susceptible crops are grown extensively. Plasmopara viticola for example, continues to challenge grape production in Europe, North America, and Australia, leading to reduced harvests and increased dependency on fungicides (Koledenkova et al., 2022). Similarly, P. cubensis is a key limitation for cucurbit farming in Asia, the Americas, and parts of Africa, where recurrent outbreaks cause severe losses to commercial yields (Salcedo et al., 2020).

Downy mildew, a destructive disease affecting the foliage of many crops, has traditionally been managed using several strategies. These include crop rotation, which interrupts the pathogen’s lifecycle by alternating host availability, and the deployment of resistant plant varieties that utilize genetic traits to fend off infections (Tör et al., 2023) (Clippinger et al., 2024). Efficient irrigation practices, such as drip systems and proper drainage, reduce moisture levels conducive to the disease’s growth (Tör et al., 2023); (Clippinger et al., 2024). Chemical fungicides remain a common solution, offering rapid disease suppression during critical periods (Unknown Author, 2024); (Clippinger et al., 2024). However, their frequent use has raised concerns over environmental pollution, public health, and the evolution of fungicide-resistant pathogens (Ons et al., 2020); (Islam et al., 2024).

Current research underscores the value of integrated disease management (IDM) approaches that combine various practices to achieve effective and sustainable disease control. Studies by (Corkley et al., 2022) and (Wang et al., 2022) demonstrated the advantages of integrating resistant crop varieties with targeted fungicide applications to manage disease outbreaks. This dual approach, leveraging genetic resistance to reduce pathogen pressure and applying fungicides strategically to mitigate severe infections, minimizes reliance on chemicals while sustaining crop yields. Such strategies align with contemporary goals of reducing the environmental impact of agriculture.

In growing concerns about fungicide overuse, biological control options are becoming increasingly popular within IDM frameworks (Ons et al., 2020). Compost teas, liquid extracts produced through compost fermentation, represent one promising strategy (Coker & Ozores-Hampton, 2021). These solutions are rich in beneficial microorganisms and bioactive compounds that combat pathogens and enhance plant immunity (Ramírez-Gottfried et al., 2023). They offer a natural and sustainable alternative or complement to synthetic chemicals, integrating seamlessly into environmentally conscious agricultural practices.

Recent research also highlights the role of specific biocontrol agents, such as L. antibioticus, P. corrugata, and Trichoderma spp. in managing zucchini diseases such as downy mildew (Unknown Author, 2023). L. antibioticus produces antibiotics that suppress Pseudoperonospora cubensis (Unknown Author, 2023), the pathogen responsible for downy mildew, while P. corrugata inhibits spore germination and disease spread through antagonistic interactions (Ramona et al., 2020). Meanwhile, Trichoderma sp. not only protect plants by colonizing root systems but also enhance systemic resistance, making plants more resilient to infections (Chakraborty et al., 2020).

Combining traditional practices with these innovative biological tools provides a balanced way forward (Clippinger et al., 2024). This integrated approach reduces harmful inputs while addressing key challenges in plant health management, promoting sustainable and resilient agricultural systems.

The effectiveness of compost teas in managing plant diseases arises primarily from the diversity of their microbial communities, which actively suppress pathogens while supporting plant health (Barghouth et al., 2023). Beneficial microbes, such as Bacillus and Pseudomonas produce antimicrobial compounds that inhibit pathogen growth, while fungi like Trichoderma outcompete pathogens for space and nutrients and form protective root barriers (Martin CCG et al., 2020). Some microorganisms in compost teas also trigger systemic resistance, equipping plants with enhanced defense mechanisms against diverse pathogens (M et al., 2024).

(Sarmah et al., 2020) emphasized that enriching compost teas with targeted strains, including Trichoderma spp. and Bacillus spp., boosts their efficacy against a broader spectrum of diseases. This approach addresses common challenges like variability in compost tea effectiveness, which often results from differences in compost quality and brewing conditions. Introducing well-characterized microbial strains ensures consistency and reliability, enhancing the utility of compost teas in agriculture.

Beyond their role in disease management, compost teas promote healthier plants by improving soil quality and nutrient availability (Corato U, 2020);(Ramírez-Gottfried et al., 2023). Their microorganisms facilitate the release of essential nutrients like nitrogen and phosphorus, boosting plant growth and vitality (Singh et al., 2022). By combining disease suppression with nutrient enhancement, compost teas serve as a multifunctional tool that aligns with the goals of sustainable agriculture. Leveraging these biological solutions offers a scalable and eco-friendly way to address modern agricultural challenges.

Based on the above rationale, this research focused on assessing the effectiveness of bacterial suspensions of Lysobacter antibioticus Bali G and Pseudomonas corrugata SAJ 6 in TSB, as well as a fungal spore suspension of Td₂₂ in saline, for controlling downy mildew in zucchini plants. The study aimed to offer alternatives to conventional “compost teas” by using selective microbial antagonists as active agents for disease management in important crops, including, zucchini, grapes, and poppies.

The efficacy of the selected antagonistic fungus and bacteria in preventing zucchini leaves from downy mildew infection, as evaluated by the percentage of infected leaves and the disease severity index is presented in Figure 1. The use of antagonists, excluding treatment A₂B₁ (plants treated with L. antibioticus and the pathogen), significantly lowered disease incidence compared to the untreated-pathogen control (A0B₁) two weeks post-infection (P < 0.05) Figure 1.

The most effective disease suppression was achieved with the fungal antagonist Trichoderma sp. Td₂₂ (A₃B₁), providing an 83% reduction in disease (calculated from disease incidence) compared to the control group at two-weeks post-inoculation. However, this effect declined and became statistically insignificant after five weeks. A non-significant synergistic effect (P > 0.05) was noted when L. antibioticus and P. corrugata were applied in combination (treatment A₄B₁), resulting in 57% disease protection (calculated from disease incidence; Figure 1 A) relative to the untreated-pathogen control. This was higher (higher protection) than when each bacterial antagonist was applied individually (A₁B₁ or A₂B₁; Figure 1). When the Trichoderma sp. Td₂₂ was combined with the bacterial antagonists, its effectiveness diminished, possibly due to reduced levels of the primary biocidal compounds on leaf surfaces (Poromarto et al., 2021). Additionally, the lack of synergy between the fungal and bacterial antagonists could be from antagonistic interactions, as dual-culture tests showed inhibition zones produced by both bacteria against Trichoderma sp. Td₂₂ Figure 2. In contrast to our findings, (Poveda & Eugui, 2022) suggested synergic effect when they were applied in combination in a sustainable agriculture system.

An interesting phenomenon was observed in this glasshouse trial where some plants in the control group (no antagonist or pathogen applied or A0B0) showed disease symptom Figure 1. This could be due to cross contamination from the infected leaves nearby (plants in pots treated with pathogen). Water splash during irrigation or the blowing wind could be the main cause of this cross contamination.

The protection shown by the antagonists against downy mildew was no longer significant (P > 0.05) after five weeks. At that time, 60% of the leaves in the untreated control group were infected. The survival rate of the applied antagonists on the leaf surfaces was observed to be low. For instance, Trichodermasp. Td₂₂ was recovered from only two out of 20 leaf samples taken from treatment A₅B₁ (plants treated with L. antibioticus, Trichoderma sp. Td₂₂, and the pathogen) five weeks post-application, and it was undetectable in samples from other treatments. Likewise, the bacterial antagonists applied to the leaves were not found (exist) from any treatments after the five-week exposure.

Applying fungal and bacterial antagonists, either individually or in combination, showed potential for managing foliar diseases, such as downy mildew in zucchini plants. Significant reductions in disease incidence (P < 0.05) were observed two weeks after the pathogen was introduced Figure 1. These findings indicate the possibility of modifying compost to develop specialized “compost teas” or cultivating specific biocontrol agents to manage foliar pathogens.

The application of Trichoderma sp. Td₂₂ yielded the most successful results in our study, particularly when the strain was cultivated in a mixture of wood fiber waste (WFW) compost and millet seed (80 : 20 w/w), as detailed in our previous research (Ramona et al., 2022). The biocontrol agent was stored for approximately 10 months at around 20 °C before being applied in the current study. In efforts to further improve the effectiveness of both fungal and bacterial antagonists, recent research by (Brost, 2020) suggested that the addition of chelating agents and detergents may enhance the activity of these biocontrol agents. By incorporating such additives, it may be possible to increase the stability, viability, and overall efficacy of microbial antagonists under various application conditions.

Previous studies have pointed out the compatibility challenges between bacterial and fungal antagonists, which can significantly influence the success of biocontrol strategies. (El-Sharkawy et al., 2021) for example, reported that the combination of T. harzianum with P. fluorescensreduced the efficacy of the fungus in controlling Aphanomyces euteiches, a root rot pathogen in peas. This finding highlights the importance of understanding microbial interactions, as incompatibility can undermine the individual effectiveness of biocontrol agents. On the other hand, research by (Amirthalingam et al., 2020) and (Ntakirutimana et al., 2024) showed that using mixed cultures of antagonistic microorganisms often improved disease control. These contrasting outcomes emphasize the need for comprehensive evaluations of compatibility when developing microbial formulations for disease management.

The broad-spectrum potential of Trichodermastrains, such as Td₂₂, is particularly significant in this regard. Its ability to combat various fungal pathogens has been consistently demonstrated in several studies, establishing it as a promising biocontrol agent (Ali et al., 2021); (Kumar et al., 2023). In our current study, Trichoderma sp. Td₂₂ achieved an 83% reduction in zucchini downy mildew during a two-week glasshouse trial, offering a level of protection comparable to chemical fungicides. Such results validate its potential as a sustainable alternative to synthetic treatments. However, achieving similar effectiveness in field conditions remains challenging, as environmental factors often necessitate frequent reapplication to maintain the agent’s activity and persistence.Reapplication intervals for biocontrol agents align closely with those recommended for chemical fungicides. (Jones et al., 2021) for example, suggested reapplying fungicides every 10 - 14 days to sustain protection against downy mildew. This similarity underscores the practical challenges of deploying biological control agents while highlighting their potential integration into established disease management practices. Optimizing formulations, improving the stability of biocontrol products, and addressing compatibility issues are essential for advancing the reliability and scalability of biological control solutions.Mildew symptoms observed in the nil-pathogen control group (A0B0) after five weeks Figure 1 were likely caused by natural infection from spores originating in a nearby vineyard with known disease presence or accidental transfer through human activity. Despite this unintended exposure, the infection levels remained minimal compared to those in inoculated plants, ensuring that the overall conclusions of the study were unaffected. This minimal infection demonstrates the robustness of the experimental setup in isolating key variables under investigation.

The limited persistence of biocontrol agents on leaf surfaces was expected due to several environmental challenges. Factors such as low moisture levels, ultraviolet (UV) radiation from sunlight, and the washing effect of overhead irrigation contributed to the reduced survival of these agents (Devi, 2024). These environmental conditions are well-known to limit the effectiveness of biocontrol organisms in outdoor applications. A thorough review by (Fedele et al., 2020) discussed the impacts of environmental stressors, including humidity, temperature variations, and irrigation practices, on the survival and performance of biocontrol agents in field environments. These findings emphasize the need for improved application techniques and protective formulations to enhance the stability and efficacy of biocontrol agents under field conditions.

The biological control agents evaluated in this study, with the exception of L. antibioticus Bali G, demonstrated significant effectiveness (P < 0.05) in protecting zucchini leaves from downy mildew during the first two weeks of the glasshouse trial. These findings highlighted their potential for managing foliar diseases. It is necessary to maintain protection beyond 14 days by reapplying the biocontrol agents.

The fungal antagonist Trichoderma sp. Td₂₂ was found to be incompatible with both L. antibioticus Bali G and P. corrugata SAJ6. In contrast, the combination of L. antibioticus Bali G and P. corrugata SAJ6 appeared to be compatible but did not significantly enhanced disease control when compared to P. corrugata SAJ6 alone. The survival of all three antagonists on zucchini leaf surfaces was notably very low.