Effects of Month of Inoculation on Severity of Disease Caused byPhytophthoraspp. in Apple Root Crowns and Excised Shoots

Abstract

Effects of month of inoculation on severity of disease caused by Phytophthora spp. in apple roostock EMLA.106 were studied in excised shoots in vitro and in intact root crowns in an orchard. In both orchard and in vitro assays, 25 successive sets of host tissue were inoculated at monthly intervals with mycelial disks from V8 agar cultures of Phytophthora spp. Crown rot severity was measured in orchard trees as area of bark necrosis after 14 days of incubation, and canker severity was measured in excised shoots as length of bark necrosis after 5 days of incubation. In both assays, P. cactorum and P. cambivora caused very little necrosis in apple incubated during dormancy (December through February) and growth resumption (March). The amount of necrosis in root crowns and excised shoots was much greater following inoculations in late spring (May). Maximum necrosis and subsequent decline in disease development during incubation periods occurred I to 3 months later in orchard tree inoculations (August through October) than in excised shoot inoculations (May through August). Crown rot severity after 2 weeks of incubation was not a reliable predictor of disease severity after 13 months of incubation ; relatively mild crown rot caused by P. cactorum during and immediately following tree dormancy occasionally continued to develop, resulting in relatively severe crown rot on trees 13 months after inoculation. Under orchard conditions in California, apple root-stock EMLA.106 is apparently susceptible to development of relatively severe crown rot for a longer period of the year than would be expected from short-term indications of susceptibility in excised shoots in vitro.