Biological Control of Plant Diseases
Biological control of plant diseases can be broadly defined as the use of one organism to influence the activities of a plant pathogen. Biocontrol organisms can be fungi, bacteria, or nematodes. Most are natural inhabitants of the soil and the environment and are not pathogenic to birds, mammals (including humans), and fish. They are not genetically modified and generally have short re-entry and days to harvest intervals. Biocontrol organisms work by competing with the pathogen for space and nutrients, by parasitism or predation, by inducing the plant’s natural defense system, and/or by the production of antimicrobial substances (antibiotics like streptomycin). Often several mechanisms function together to make an organism effective. These products are living organisms or dried spore preparations and must be handled differently than conventional fungicides. They are sensitive to temperature extremes and must be applied immediately after mixing with water. They may also require special attention to pH, exposure to chlorine or UV light, and their shelf life may be limited.
There is a lot on interest in these products and few replicated University field research trials on which to base recommendations. Product efficacy claims are often based upon company sponsored research that occurred in greenhouses or other controlled environments. A brief summary of field trials found in recent literature follows:
Actinovate (Streptomyces lydicus) reduced root and seed rot severity in peas and resulted in significantly higher final emergence and significantly lower final disease in spinach challenged by Pythium and Fusarium (soil-borne fungi). No effect on Phytophthora fruit rots of pepper and pumpkin was obtained as well as no effect on Powdery Mildew on pumpkin. Compete Plus (Six species of Bacillus, Streptomyces griseoviridis, Trichoderma harzianum plus organic nutrients) significantly reduced incidence of potato tubers with both Black Scurf (Rhizoctonia solani) and common scab (Streptomyces scabies). BioYield (plant growth promoting rhizobacteria) significantly reduced incidence of root rot (Pythium, Rhizoctonia) and wirestem (Rhizoctonia) on broccoli, resulted in significantly less post emergence disease on spinach (soil-borne fungi), but had no effect on tomato foliar diseases such as Septoria, Alternaria (Early Blight), and Sclerotinia (White Mold). Compost Tea significantly reduced potato tubers with both Black Scurf and Common Scab and reduced scab severity, while resulting in significantly lower final biomass and final emergence when applied to spinach to combat soil-borne diseases .Bi-nucleate Rhizoctonia are effective against diseases caused by Rhizoctonia and significantly reduced Black Scurf and stem canker on potatoes as well as root rot and wirestem incidence on broccoli. Contans (Coniothyrium minitans) is applied to the soil before cropping and can significantly reduce lettuce drop caused by Sclerotinia species. Kodiak (Bacillus subtilis) is a seed or soil treatment that significantly reduced Black Scurf and stem canker on potato, resulted in significantly less post emergence disease and significantly higher biomass in spinach challenged by soil-borne diseases, and significantly reduced seed and root rot in peas. Treatment with Kodiak had no effect on Rhizoctonia on bean, Phytophthora on pumpkin, or Common Scab on potato. Muscador (Muscador albus) is a novel biocontrol organism that acts as a biofumigant by producing gaseous compounds. It has shown good efficacy against storage insect pests of apples and potatoes. Application of Muscador to radish resulted in significantly less root and hypocotyl rot and less Phytophthora fruit rot on pepper. Combining Muscador treatment with a resistant pepper cultivar significantly reduced Phytophthora disease severity. No effect of Muscador was noted with Clubroot (Plasmodiophora brassicae) on radish and Phytophthora on winter squash. Plant Shield (Trichoderma harzianum) had no effect on Rhizoctonia on bean or potato, Common Scab on potato, Botrytis on tomato, or Early Blight on tomato, although a trend toward reduced defoliation was noted. Serenade (Bacillus subtilis) failed to control the tomato diseases anthracnose, bacterial canker, and bacterial spot; had no effect on Sclerotinia sclerotiorum (White Mold) on lettuce or beans; and did not reduce Powdery Mildew on pumpkin or winter squash. Serenade did significantly increase yield and lower the incidence of root rot caused by Rhizoctonia on both beans and radish. SoilGard (Trichoderma virens) significantly reduced Black Scurf incidence and Common Scab severity on potato, resulted in significantly less post emergence disease on spinach from soil-borne disease, while having no effect on spinach and Pythium damping-off.
If one word could be used to describe research trials with these materials, that word would be inconsistent. The environment and application techniques have large impacts on their efficacy. Biocontrol organisms are only effective as preventive control and proper timing of application is critical. An unfavorable environment for their establishment or an environment too favorable for the pathogen can result in control failure. These organisms perform best at low pathogen populations; once disease is established, they will have little positive effect. Establishment of biocontrol organisms on foliar surfaces is difficult; most positive research results come against soil borne problems such as root, fruit, crown and seed rots. Combining these products with a naturally resistant or tolerant cultivars is a promising avenue for their use. Biocontrol organisms can also be integrated with naturally suppressive composts, improved sanitation and other cultural controls, and with conventional fungicides to reduce disease control chemical applications.
- M. B. Dicklow, UMass Extension
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