Alligatorweed leaf spot is a disease of invasive Alternanthera philoxeroides (Alligatorweed) in the southern US, caused by Alternaria alternantherae. However, little is known about when or where this pathogen naturally occurs. To better understand this species’ life history, we examined temporal (every 2–3 weeks) and spatial (latitudinal) patterns of A. alternantherae occurrence at sites in Louisiana for 2 y. Pathogen presence reflected clear within-year temporal and spatial patterns. Overall, the percentage of leaves infected with A. alternantherae was low during spring each year (0–20% infected) but increased throughout summer (maximum of 50% infected), and plants in northern sites had lower frequency of infection relative to southern sites until later in the year (late summer/early fall) but only in 1 of the 2 years of our study. The mean proportion of leaves infected with A. alternantherae declined with latitude both years (P = 0.01) and variability increased with latitude (P = 0.04), a pattern suggestive of range limitation in northern areas. We estimate a northern distributional limit of 34°N for A. alternantherae in Louisiana, but Alligatorweed occurs farther north. Although we did not directly examine disease impacts to Alligatorweed during the study, they may be greatest in southern areas, where the pathogen is more common early and throughout the growing season, and thus may be less likely to provide control in northern infestations of the invasive Alligatorweed.