Figure from Caulier, S., Gillis, A., Colau, G., Licciardi, F., Liepin, M., Desoignies, N., Modrie, P., Legreve, A., Mahillon, J., and Bragard, C. (2018). Versatile antagonistic activities of soil-borne Bacillus spp. against Phytophthora infestans and other potato pathogens. Frontiers in Microbiology.

Phytophthora are ubiquitous and present in most soils. These fungi reproduce asexually and grow in the soil, feeding off of decaying organic matter. The growth and reproduction of Phytophthora capsici is favored by free water and high relative humidity. The species seems unable to survive in a plant host when the relative humidity is below 47%. However, fluctuating saturation levels favor the spread of Phytophthora compared to constant saturation. When conditions are harsh or their resources are limited, they reproduce sexually, allowing them to quickly adapt to their changing environment. They spread their spores, which can lay dormant for long periods of time in soil, giving them the means to continually plague fields. Sporangia, the spore-releasing structures of Phytophthora, are only formed when moisture is abundant, and the spores can only be released when there is free water or humidity is approaching 100%.

Although pathogenic fungi such as Phytophthora are conventionally managed with fungicides, frequent fungicide applications give rise to resistant strains that reproduce as clones and dominate on a local or regional scale. Fungicides also kill the beneficial fungi that compete with Phytophthora. Similarly, water supplies contaminated with Phytophthora may be successfully treated with 2 mg/L chlorine, but this can negatively affect indigenous microbes that are beneficial to plant health. Saline soils and irrigation water have been shown to predispose many crops to infection by pathogenic fungi as well. Managing salt levels is necessary when preventing Phytophthora from taking hold in a field.

Supporting microbial diversity can be the best way to prevent crop infections by Phytophthora. Treatments that increase carbon in the soil, such as straw mulching, create conditions that favor the natural antagonists and predators of Phytophthora. Mulches rich in cellulose support microbes that release cellulase enzymes. These enzymes degrade the mulch, but also suppress Phytophthora spp. by breaking down their cellulose-based cell walls.

Many different types of bacteria and fungi can suppress plant diseases, but some of them also boost plant growth. Bacillus, for example, are known to be among the plant growth-promoting bacteria (PGPR). Additionally, arbuscular mycorrhizal fungi (AMF) are known to prevent pathogens from entering the plant by fortifying the plant’s structures.

Figure from Li, H., Cai, X., Gong, J., Xu, T., Ding, G, and Li, J. (2019). Long-term organic farming manipulated rhizospheric microbiome and Bacillus antagonism against pepper blight (Phytophthora capsici). Frontiers in Microbiology.

In a greenhouse experiment, Phytophthora infections were 34.2% less prevalent in peppers grown organically, compared to those grown conventionally. Bacillus antagonists were isolated from the soil and 18 isolates used to inoculate the soil of an integrated farming system. The Phytophthora prevalence in this inoculated soil was only 38.9%, compared to 97.2% in the non-inoculated integrated farming system. In another study, tomato plants inoculated with three bacterial isolates showed a lower pathogen load in the roots, as well as enhanced growth.