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Stalk Rot of Corn: Mechanism of Predisposition by an Early Season Water Stress. R. W. Schneider, Assistant professor, Department of Plant Pathology, Cooperative Extension, University of California, Berkeley 94720; W. E. Pendery, farm advisor, Cooperative Extension, University of California, Berkeley 94720. Phytopathology 73:863-871. Accepted for publication 8 December 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-863.

The incidences of corn stalk rot (probably caused most often by Fusarium moniliforme) at the end of the season in field-grown plants exposed to a mild water stress during the pretassel, post pollination, or grain-filling stages of development were 60.3, 25.3, and 7.7%, respectively. The nonstressed control had 24.7% disease. There were no visible symptoms of water deficit during the treatment period. In a separate experiment, infection and systemic colonization of roots by F. moniliforme increased significantly following a mild early season water stress compared to nonstressed plants. Diurnal trends in plant water potential (φp) and leaf diffusive resistance (r1) of previously stressed infected (SI) plants were altered with respect to nonstressed infected (NI) plants and stressed and nonstressed healthy plants. At 3 days after the irrigation that terminated the stress, φp in the SI plants was as much as 5 bars lower and rl was significantly higher than those of plants that had received the other three treatments for several hours during the day. These trends were amplified at 8 days after the irrigation. By 15 days, when all plants were experiencing water deficits, diurnal trends in φp and rl in the SI and NI plants were similar. Calculations based on rates of water uptake and φp when soil was at field capacity, indicated that resistance to water flow between roots and leaves was approximately doubled in the SI plants compared to that in plants that had received the other three treatments. Thus, under relatively high evaporative demand, predisposed plants behaved like chronically stressed plants even when adequate soil water was available later in the season; roots proliferated at greater soil depth and stalk and root senescence occurred during the reproductive growth stage. Because F. moniliforme is a pathogen of senescing tissues, we conclude that predisposition by a mild early season water deficit permanently increases the likelihood of chronic water stress during periods of relatively high evaporative demand or limited soil water availability, which leads to earlier senescence and increased susceptibility to the stalk rot pathogens. Cultural practices and breeding strategies for disease control are discussed in light of the proposed mechanism.