Death of maiden apple trees caused by Phytophthora syringae Kleb. and a comparison of the pathogen with P. cactorum (L. & C.) Schroet

Abstract

SUMMARY Phytophthora cactorum and the closely related (but only recently implicated) species, P. syringae, cause collar rot of mature apple trees, but infection of young trees is uncommon. In a Kent orchard, 279 maiden trees of Cox's Orange Pippin, Worcester Pearmain and Tydeman's Early Worcester were infected by P. syringae whilst heeled‐in prior to planting, and subsequently killed.These two paragynous antheridial species can readily be distinguished in agar culture. P. cactorum grows best at 25° C. and forms a diffuse, azonate colony with abundant reproductive structures; sporangia are uniformly obpyriform, markedly papillate, and occur in terminal clusters on the sporangiophores. P. syringae grows best at 20° C., very slightly at 25°C., and forms a compact, radially zonate colony with sparse reproductive structures; sporangia are irregular in size and shape, have a broadly flattened apex, and occur singly on the simple sporangiophores.Inoculation studies demonstrated that young trees are particularly susceptible to P. syringae during the period of low temperature and tree dormancy: the rate of lesion extension for P. syringae was inversely related to the mean accumulated air temperature while the reverse was true for P. cactoriim.Measures recommended for the protection of young trees during the pre‐planting period aim at preventing contact of scion variety stems with wet soil.