VIEW ARTICLE

Ecology and Epidemiology

Effects of Flood Duration on the Development of Phytophthora Root and Crown Rots of Apple. G. T. Browne, Plant pathologist, Agricultural Research Service, U. S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616; S. M. Mircetich, Plant pathologists, Agricultural Research Service, U. S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 78:846-851. Accepted for publication 25 January 1988. 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, 1988. DOI: 10.1094/Phyto-78-846.

Delicious apple seedlings were grown for 10–12 wk in artificially infested or noninfested U.C. mix soil on tension plates. A –20 mbar soil water matric potential ( ψ ) was maintained constantly or interrupted once every 2 wk by 4-, 12-, 24-, or 48-hr flood intervals ( ψ _m = 0). At constant ψ _m = –20 mbar, Phytophthora cactorum caused relatively mild (mean 18% root rot) and P. cambivora caused relatively severe (mean 60% root rot) disease, but P. cryptogea caused no disease. As the duration of flood intervals lengthened from 4 to 48 hr, severity of disease caused by P. cryptogea increased progressively from mild (mean 1% root rot) to severe (mean 52% root rot), but severity of disease caused by P. cactorum and P. cambivora was moderate (mean 13–46% root rot) and severe (mean 80–89% root rot), respectively, and was not highly correlated with flood duration. When apple leaf disks colonized by P. cactorum, P. cambivora, or P. cryptogea were held for 2 days at ψ _m = –20 mbar, subsequent flooding was required for the production and release of significant numbers of zoospores. When host tissue pieces were used as bait for zoospores in the experiments on effects of flood duration on disease severity, the mean incidences of bait infection by P. cactorum (70, 15, and 13%) and P. cambivora (20, 7, and 8%) decreased over the intervals of baiting that were, respectively, 0–4 hr, 20–24 hr, and 44–48 hr after flooding onset, whereas the mean incidences of infection by P. cryptogea over the same baiting intervals increased (36, 44, and 62%, respectively).