RESUMO
In growth chamber experiments using field soil naturally infested with Phytophthora sojae, addition of KCl at 150 to 400 µg of K per gram of dry soil increased the incidence of Phytophthora root and stem rot on susceptible soybean (Glycine max) compared with a distilled water control. Other potassium salts at 150 to 400 µg of K, phosphorus salts at 20 to 317 µg of P, and sulfate salts of K, Ca, Mg, Na, and Al at 491 µg of SO4 per gram of soil did not affect disease incidence. Application of chloride salts of K, Na, Mg, Ca, NH4, Al, Fe, and Sr at 250 µg of Cl per gram of soil all increased disease incidence compared with the distilled water control. Addition of other ammonium salts also increased disease incidence. Disease incidence increased when KCl at 300 µg of K per gram soil was applied to naturally infested Blount silt loam but not to naturally infested Hoytville silty clay. The effects of chloride salts on plant growth and on natural inoculum of P. sojae were investigated. At 450 µg Cl/g of soil, addition of NaCl increased plant fresh weight but had no effect on germination, emergence, transpiration, height, and dry weight of plants grown in pasteurized, pathogen-free soil. When soybean plants were grown without a deplasmolytic shock and without promoting zoospore release by saturating the soil, the addition of chloride salts at 304 µg of Cl per gram of soil increased disease. In a leaf-disc-baiting bioassay, leaf infection by P. sojae zoospores decreased when 0.01 or 0.02 M KCl was added to soil extracts from flooded soil but was unaffected by KCl at 150 to 600 µg of K per gram of soil applied to the soil 5 days before baiting. Energy-dispersive X-ray analysis of taproots of 5-day-old soybean plants indicated a reduction in the calcium content of the outer cell layers with KCl at 400 µg of K per gram of soil but not with K2SO4. Disease increases with addition of KCl appear to be due to the presence of chloride and may be associated with changes in the micropartitioning of root calcium.
RESUMO
Cotton and snap bean were selected for a multi-year, multi-state regional (south-eastern USA) research project to evaluate the efficacy of both commercial and experimental bacterial and fungal biological control agents for the management of damping-off diseases. The goal for this portion of the project was to determine the viability and stability of biological agents after application to seed. The biological seed treatments used included: (1) Bacillaceae bacteria, (2) non-Bacillaceae bacteria, (3) the fungus Trichoderma and (4) the fungus Beauveria bassiana. Seed assays were conducted to evaluate the following application factors: short-term (< or = 3 months) stability after seed treatment; quality (i.e. isolate purity); compatibility with chemical pesticides and other biocontrol agents; application uniformity between years and plant species. For the bacterial treatments, the Bacillaceae genera (Bacillus and Paenibacillus) maintained the greatest population of bacteria per seed, the best viability over time and the best application uniformity across years and seed type. The non-Bacillaceae genera Burkholderia and Pseudomonas had the least viability and uniformity. Although Beauveria bassiana was only evaluated one year, the seed fungal populations were high and uniform. The seed fungal populations and uniformity for the Trichoderma isolates were more variable, except for the commercial product T-22. However, this product was contaminated with a Streptomyces isolate in both the years that it was evaluated. The study demonstrated that Bacillaceae can be mixed with Trichoderma isolates or with numerous pesticides to provide an integrated pest control/growth enhancement package.
