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VIEW ARTICLE
Ecology and Epidemiology
Influence of Calcium Nitrate and Ammonium Sulfate on Phytophthora Root Rot of Persea indica. B. S. Lee, Graduate student, Department of Plant Pathology, University of California, Riverside 92521, Present address of senior author: Crop Protection Research Branch, MARDI, Malaysia; G. A. Zentmyer, professor, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 72:1558-1564. Accepted for publication 20 April 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1558.
In greenhouse experiments with soil naturally infested with Phytophthora cinnamomi, Ca(NO3)2 at 100, 200, and 300 μg nitrogen and (NH4)2SO4 at 300 μg nitrogen per gram of air-dried soil significantly reduced root rot of Persea indica, a close relative of avocado. Nitrapyrin at 1 μg/g of air-dried soil also resulted in higher NH4 levels and significantly reduced root rot and completely prevented stem canker development, indicating direct toxicity of this chemical to Phytophthora. Seedlings grown in Hoagland’s solution containing Ca++ at 160 μg/ml were more resistant to infection by P. cinnamomi than those grown in lower Ca++ levels. Reduction in inoculum density following application of Ca(NO3)2 and high levels of (NH4)2SO4 also contributed to the reduction of root rot. Sporangium production in mycelial mats incubated in extracts of these soils was also inhibited. Approximately 2.5 to 3 times more bacteria and actinomycetes were present in these soils compared to the nontreated soil or soil treated with a lower level of (NH4)2SO4. In separate experiments, addition of (NH4)2SO4 to nonsterile and sterile soil extracts suppressed sporangium production and release of zoospores. Addition of Ca(NO3)2 stimulated sporangium production, but release of zoospores was poor. In nonsterile soil extracts supplemented with Ca(NO3)2, large numbers of bacteria were attached to the walls of sporangia and hyphae, and these were associated with breakdown of sporangia and lysis of hyphae.
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