Alternaria alternata is a weak pathogen of maize, causing a progressive leaf blight under proper environmental conditions. Predisposition of the host by wounding or stress is necessary for infection. Small circular areas of leaf tissue were killed with a hot soldering iron, spores of A. alternata were placed on the wound, and inoculum and environmental requirements for lesion expansion were studied. The fungus would not cause lesions without a food base of some dead or moribund tissue. Inoculation of green tissue was successful, however, when it preceded tissue killing by several days and included areas contiguous to the area subsequently killed. The temperature optimum for lesion expansion was 20 C but lesion expansion occurred over a range of temperatures investigated (10-30 C). Dew periods of 10 hr or more/24 hr and 48 hr or more of total dew were required for lesion expansion. A continuous dry period of 24 hr or more essentially stopped further lesion expansion during subsequent dew periods. A. alternata was tolerant of osmotic potentials of -55 bars in vitro during more germination and spores germinated on leaves in ambient relative humidities as low as 85%. Lesion size increased directly with inoculum density and leaf age;Six maize inbred lines were tested for disease resistance and variability within the inbred lines for disease reaction was determined. Iowa derived lines were moderately susceptible while C103 and Oh43, two related lines, were moderately resistant. Variability within any inbred line was slight. In a limited host range study, an A. alternata isolate from maize induced progressive lesion expansion on maize and table beets after injured leaf tissue was inoculated;Culture filtrates from growth of the pathogen in liquid media were extracted, purified, and bioassayed for toxin production. An unidentified toxin activity (chlorosis) was demonstrated, but no toxin could be shown to play an active role in lesion expansion.