Relative Resistance of Eighteen Selections ofMalusspp. to Three Species ofPhytophthora

Eighteen selections from 13 species and hybrids of Malus (M. baccata. M. brevipes. M. coronaria. M. domestica, M. fusca, M. halliana, M. ioensis, M. × magdeburgensis, M. mandshurica, M. × platycarpa, M. prunifolia, M. sargentii, and Malus sp.) were evaluated in a greenhouse for resistance to root and crown rots caused by Phytophthora cactorum, P. cambivora, and P. cryptogea. Seven- to nine-week-old seedlings were transplanted into noninfested soil or soil artificially infested with P. cactorum, P. cryptogea, or P. cambivora. The soil was flooded once every 2 wk for 48 h, and plants were evaluated for severity of disease 3 mo after transplanting. Seedlings of domestic apple (M. domestica, (used as a standard for comparison in this study) were among the most susceptible of the Malus spp. to crown and root rot caused by P. cactorum. P. cryptogea, and P. cambivora. The relative resistance of other selections of Malus spp. varied according to species of Phytophthora and whether crown or root rot variables were used to assess resistance. Among 10 selections of Malus spp. that were evaluated along with two selections of M. domestica for resistance to P. cactorum, P. cryptogea, and P. cambivora, six selections were relatively resistant compared to M. domestica to root and crown rot caused by P. cactorum, but only M. halliana, M. × magdeburgensis, and M. sargentii were relatively resistant to root and crown rot caused by all three fungi. Species of Malus can vary greatly in resistance to species of Phytophthora, and, among Malus spp., assessments of relative resistance to P. cactorum are not necessarily extendible to other Phytophthora spp