Tar Spot on Maize: Impact of Soil Types and Environmental Conditions on the Survival of Phyllachora maydis in the Subtropical Climate of Florida.

Abstract

Tar spot, caused by Phyllachora maydis, is an established maize disease in the Midwest of the United States but remains an emerging concern in Florida. While this pathogen can overwinter on plant residue, its survival in Florida's subtropical environment is not well understood. This study evaluated how environmental factors affect the germination of P. maydis ascospores and stroma integrity. Symptomatic maize leaves were incubated under four conditions: Histosol soil (muck), Krome soil (rocky), 4 °C, and 23 °C. Extensive leaf decomposition occurred in both soil types, with most plant material degraded after eight weeks, while the stroma maintained its structure. Despite this, ascospore germination declined across all conditions. After eight weeks, ascospores incubated at 4 °C retained 25% viability, while those at 23 °C had the lowest germination (0.7%). Ascospores from leaves buried in soil exhibited low viability (1-6%), with no significant differences between soil types (p=0.9944). Weather analysis revealed that increased temperature reduced germination rates, while higher humidity enhanced them. These findings suggest that P. maydis displays limited survivability under Florida-like conditions, with germination rates declining over time. Therefore, cultural practices such as tillage, already employed by corn producers in Florida, may be effective in reducing sources of P. maydis inoculum.