Defense patterns in nonhost higher plant species against two powdery mildew fungi. I. Monocotyledonous species.

Fifteen monocotyledonous species were separately inoculated with spores of Erysiphe graminis f. sp. hordei (a parasite of barley) and E. cichoracearum (a parasite of curcurbits). Erysiphe cichoraceaurm germinated poorly and rarely developed beyond the appressorial stage on any monocotyledonous nonhost, suggesting that monocotyledonous leaf surfaces are unsuited for growth and differentiation of this fungus. Erysiphe graminis rarely developed beyond the appressorial stage on members of the Iridaceae and Liliaceae, suggesting that leaf surface environments in these plant families contain substances unfavourable for powdery mildew development. On species within the Graminae, E. graminis f. sp. hordei germinated and produced appressoria at lower rates on plants thought to be resistant to all powdery mildews than on plants which are susceptible to certain formae speciales of E. graminis. Thus, within the Graminae, the amount of E. graminis development on nonhosts was negatively correlated with taxonomic distance between nonhost and appropriate host. Fungus development, however, is usually controlled by resistance factors which are characteristics of the plant family and are unrelated to the taxonomic distance between nonhost and appropriate host.-from Authors