Effects of Flooding Duration on the Development of Phytophthora Root and Crown Rots of Cherry

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Abstract
Mahaleb [Prunus mahaleb] and Mazzard cherry [P. avium] seedlings were grown for 3 mo in UC mix artificially infested with Phytophthora cryptogea or P. megasperma and flooded for periods of 0, 8, 12, 24, or 48 hr at 2-wk intervals. For each host/pathogen combination, disease severity progressed from mild (2-7% of the root system rotted) in treatments that were not flooded to extreme (81-99% of the root system rotted) with 48-hr flooding periods. Similarly, P. cryptogea caused crown rot only after 48-hr flooding periods, whereas P. megasperma caused no crown rot. When seedlings were grown in uninfested UC mix, and a single 48-hr flooding period was partitioned to provide various flooding intervals before and after inoculation with zoospores of P. cryptogea, disease severity was proportional to the length of the postinoculation flooding interval. In contrast, preinoculation flooding periods (regardless of their length) had no apparent effect on disease development. Furthermore, a postinoculation treatment consisting of 6 hr of flooding followed by 42 hr at 0.5% soil O2 resulted in a disease rating comparable to that which occurred with 48 hr of continuous flooding. These data indicate that the severe Phytophthora root and crown rots that develop on cherry after prolonged flooding periods result in part from phenomena that occur in flooded soil after zoospore discharge. In addition, these data suggest that the susceptibility of Mahaleb root and crown tissues to colonization by Phytophthora spp. may increase during periods of reduced oxygen availability which develop during persistent flooding.