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Verticillium Wilt of Pistachio: The Influence of Potassium Nutrition on Susceptibility to Infection by Verticillium dahliae. L. J. Ashworth, Jr., Plant pathologist, Plant Pathology Department, University of California, Berkeley 94720; S. A. Gaona(2), and E. Surber(3). (2)(3)Blackwell Management Company, P. O. Box 2393, Bakersfield, CA 93303. Phytopathology 75:1091-1093. Accepted for publication 6 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1091.

The incidence of Verticillium wilt of pistachio, grown in soils with low inoculum densities of Verticillium dahliae (0.02- 0.2 microsclerotia per gram of soil) increased dramatically during 1981- 1983. Increased incidence of disease was associated with unthrifty trees having low levels of leaf K (0.5- 0.8%) during midsummer. During 1982, 39.6% of K-deficient trees surveyed were infected. Infection was rare (0.37%) among thrifty trees having 1% or more leaf K. Infection percentages were 18.8, 13.8, and 9.7 in 1983 for, respectively, 1.0, 1.5, and 3.0 kg of K per tree treatments. Tree survival, compared with the 1.0 kg of K per tree treatments, was improved 8% and 13% in, respectively, the 1.5 and 3.0 kg of K per tree treatments during 1983. Results were similar in 1984 except the 1.5 and 3.0 kg of K per tree treatments were equally effective. Annual increases of infection observed during 1981-1983 were reversed in 1984 following treatments of 1,620 ha with 1.5 kg of K per tree in 1983. No infection of thrifty trees was observed in 1984. Incidence of disease in 1984 was 35% less than that during 1983. Phosphorus deficiency, discovered in 1984, also may influence susceptibility to infection, however, P deficiency was corrected following the peak infection period. Reduced susceptibility to infection appears to be a klendusic response of actively growing trees in low inoculum density soils.

Additional keywords: phosphorus deficiency, potassium deficiency.