Initiation of Induced Nonhost Resistance of Oat Leaves to Rust Infection

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Abstract
Cross-protection experiments were made by first inoculating the abaxial surfaces of primary leaves of oats (cultivar Shokan 1) with oat-noncompatible rust fungi (Puccinia coronata festucae, P. coronata folii, or P. graminis tritici) and later inoculating the adaxial surfaces with compatible race 203 of P. coronata avenae. The oat nonpathogens were heat-killed at various times after the 1st inoculation, and the subsequent inhibition of development of the challenge pathogen inside the leaf was used to assay induction of nonhost resistance. Elongation of intercellular hyphae and haustorium formation by race 203 were reduced when the 2nd inoculation was made 12 h or more, but not 8 h, after the 1st inoculation. When stomatal penetration by P. graminis tritici was controlled by exposing plants to light at various times after the 1st inoculation, reduction of development of challenge race 203 occurred only after the oat nonpathogen had penetrated and produced substomatal vesicles. The initiation of nonhost resistance was not related to postinoculation time or formation of appressoria over stomates, but it was dependent on the formation of substomatal vesicles.

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