Peronospora manshurica on a nonhost and on resistant, susceptible, and intermediate soybeans

Abstract

Soybeans with mature plant resistance and a nonhost, string bean, were inoculated with Peronospora manshurica. Soybeans resistant, susceptible, and intermediate in reaction were inoculated for comparison. Leaves were fixed, cleared, and stained, and whole mounts were made daily. Individual infection sites were examined, and hyphae were measured microscopically. Spore germination varied as a result of self-inhibitory factors. Where it occurred, entrance by the fungus appeared identical on each plant at 12 h. Hyphal growth became rapid in susceptible tissue 36 h after inoculation; growth slowed, starting at 72 h in mature tissue, and was uniformly slow in tissue intermediate in resistance. Hyphal growth after 36 h was essentially zero in resistant tissue, but paradoxically it was sometimes rapid on nonhost tissue. Haustoria occurred in all plants inoculated. The number of haustoria per lesion increased in proportion to the length of hyphae, except on resistant tissue, where the number of haustoria per millimetre of hyphae was very low.