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Influence of Soil Water Matric Potential on the Development of Phytophthora Root and Crown Rots of Mahaleb Cherry. W. F. Wilcox, Former graduate research assistant, 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, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; S. M. Mircetich, research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 75:648-653. Accepted for publication 17 December 1984. 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-75-648.

Mahaleb cherry seedlings were grown for 3 mo in UC mix (UCM) artificially infested with Phytophthora cryptogea, P. cambivora, P. megasperma, or P. drechsleri and subjected to various levels of soil moisture. When the soil water matric potential (φm) was maintained at ?10 millibars (mb), ?25 mb, or ?25 mb with 4-hr flooding (φm = 0) interruptions once every 2 wk, P. cryptogea, P. megasperma, and P. drechsleri caused no crown rot, negligible root rot, and usually insignificant (P=0.05) shoot and root growth reductions. In contrast, P. cambivora caused a 20?60% incidence of crown rot, moderate root rot, and moderate reductions in shoot and root growth at these same φm values. When φm was normally maintained at ?25 mb but the soil was flooded (φm = 0) for a 48-hr period once every 2 wk, P. cryptogea and P. cambivora caused 80 and 60% incidences of crown rot, respectively, and all four Phytophthora spp. caused severe root rot and significant (P=0.05) growth reductions on Mahaleb seedlings. The same soil water regimes similarly influenced disease severity when seedlings were grown in a clay loam infested with P. cryptogea, while P. cambivora caused less root rot in clay loam than in the coarser UCM. When colonized leaf disks were buried for 3 days in UCM held at ?25 mb φm, P. cryptogea formed numerous sporangia, whereas P. cambivora, P. drechsleri, and P. megasperma formed few or no sporangia. One hour after the UCM was subsequently flooded, sporangia of P. cryptogea began releasing zoospores, and sporangium production by the other three Phytophthora spp. was initiated. These sporangia completed development and began discharging zoospores within 3?6 hr. Thus, soil water conditions that were optimum for disease development were also optimum for sporangium production and/or zoospore discharge.

Additional keywords: Prunus avium, Prunus mahaleb, soilborne diseases, sweet cherry, wet feet.