Histopathology of Peronospora manshurica on soybean cultivars of varying degrees of resistance and on a nonhost

Abstract

Soybean leaves were inoculated with the downy mildew fungus Peronospora manshurica. Portions of the leaves were fixed, cleared, and stained for microscopic observation at intervals from 12 to 168 hr following inoculation. A susceptible cultivar inoculated at different ages, a resistant isogenic line, a lins heterozygous for resistance, a moderately resistant cultivar, and a nonhost were studied. Germination of conidia, germ tube growth, appressorium formation, development of penetration pegs, and hyphal growth for 36 hr were about the same on all categories of plants. After 36 hr hyphae grew within young leaves of the susceptible cultivar at the rate of about 59 pm/hr and formed about 57 haustoria/mm of hyphae. On mature leaves of the susceptible cultivar the fungus grew rapidly from 36 to 120 hr and then grew slowly until 144 hr when the final small lesion size was attained. On plants heterozygous for resist ance and on the moderately resistant cultivar, growth of the fungus hyphae was somewhat slower than on the susceptible cultivar. The number of haustoria/mm of hyphae was low on plants heterozygous for resistance, but high on the moderately resistant cultivar. On the resistant isogenic line growth was very slow after 36 hr and few haustoria were formed. On the nonhost, the common bean cultivar Tendercrop, growth of the fungus was very slow for 120 hr, but then, in a few infection sites, increased rapidly, before ceasing as lesions were delimited by veins; haustoria were numerous where lesions formed. Resistance generally appeared to V be related to or expressed as a slow hyphal growth rate between and hr after inoculation. The success of individual infections apparently was related to the ability of their hyphae to form haustoria.