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

Abstract

Fifteen monocotyledonous species were separately inoculated with spores of Erysiphe graminis f. sp. hordei (a parasite of barley) and E. cichoracearum (a parasite of cucurbits) to learn cytologically what types of parasite failures and host-cell responses protect nonhosts from disease. Tissues were stained with acid fuchsin and aniline blue, 24 and 72 h after inoculation, and examined microscopically. For each combination of higher plant species and fungus, fungal development and host responses were recorded for 2022–4679 spores and 15–2206 germlings, using a binary pathway system to tabulate and statistically analyze results.Nonhosts showed a battery of defenses, each of which could stop a large part of an attacking powdery mildew population. These defenses were expressed sequentially during fungal differentiation, leading to a progressive attrition in the number of germlings able to continue. The defenses were expressed on or in the following host structures: (i) the leaf surface, which was often inhospitable to germination and differentiation of infection structures; (ii) the cuticle and cell wall, which the fungi frequently failed to penetrate; (iii) the papilla, which was frequently induced but rarely penetrated; and (iv) the first cell invaded, which had a high probability of collapsing hypersensitively in the relatively few instances in which the fungi successfully entered a cell.Erysiphe cichoracearum 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 unfavorable 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 (indiangrass, sorghum, big bluestem, and corn) than on plants which are susceptible to certain formae speciales of E. graminis (wheat, oats, and rye). Thus, within the Graminae, the amount of E. graminis development on nonhosts was negatively correlated with taxonomic distance between nonhost and appropriate host. However, the results with other combinations of fungus and nonhosts suggest that fungus development is usually controlled by resistance factors which are characteristic of the plant family and are unrelated to the taxonomic distance between nonhost and appropriate host.