Resistance in Susceptible Maize toHelminthosporium carbonumRace 1 Induced by Prior Inoculation with Race 2

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Abstract
Inoculation of leaves of susceptible maize [Zea mays] genotypes with the nonpathogenic race 2 of Helminthosporium carbonum at least 10 hr before inoculation with the pathogenic race 1 prevented development of large lesions typical of the susceptible reaction. Appressorium formation, penetration, and hyphal growth by the pathogen were decreased. Addition of HC-toxin (the host-specific toxin produced by pathogenic race 1) to the race 1 challenge inoculum abolished the resistance induced by race 2. Inoculation with H. victoriae, H. turcicium, or Alternaria sp. at least 18 hr before inoculation with H. carbonum race 1 also induced resistance. The results suggest that a general resistance mechanism is activated upon contact of the maize leaf with a potential pathogen and that HC-toxin plays a role in pathogenesis by preventing or overcoming those events.