VIEW ARTICLE

Ecology and Epidemiology

Seasonal Variation in Susceptibility of Juglans hindsii and Paradox Rootstocks of English Walnut Trees to Phytophthora citricola. M. E. Matheron, Former graduate student, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616, Present address of senior author: Department of Plant Pathology, University of Arizona, Yuma Mesa Agricultural Center, 6425 West Eighth Street, Yuma 85364; S. M. Mircetich, professor, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 75:970-972. Accepted for publication 26 March 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/Phyto-75-970.

Juglans hindsii and Paradox (J. hindsii x J. regia) walnut seedlings were evaluated for seasonal variation in susceptibility to Phytophthora citricola. At monthly intervals, seedlings were removed from a lathhouse, placed in a growth chamber, and flooded for 48 hr in soil artificially infested with P. citricola. Disease severity was assessed 14 days after commencement of flooding under controlled conditions. The incidence and severity of crown rot in J. hindsii was highest in June; 60% of the trees were affected by crown rot and the stem girdling index was 1.9. Lowest disease incidence and severity occurred in January; 3% of the trees were affected by crown rot and the stem girdling index was 0.1. Similarly, disease incidence and severity in Paradox rootstocks were highest in June (50% of the trees were affected by crown rot and the stem girdling index was 1.0), while the lowest disease incidence and severity occurred in October-November and January-May, when no crown rot was observed. Direct inoculation of stems and excised stem tissue of J. hindsii with mycelium of P. citricola yielded seasonal susceptibility variation results similar to those obtained in infested soil. The suitability of each assay method for investigations of relative rootstock resistance and the implications of seasonal variation in susceptibility for disease control in California walnut orchards are discussed.