Gray leaf spot of St. Augustinegrass (pictured above) is a disease caused by fungal spores that are common during extended periods of hot, wet, and humid weather (which almost encompasses half the year in Gainesville, Florida lawns). Newly sodded or rapidly growing grass is more susceptible than well-established lawns. Although primarily a disease of St. Augustinegrass, it also attacks centipedegrass.
Melting-out leaf spot is a similar disease that attacks Zoysia grass (pictured below) in similar weather. As we have recently had rain for about the past week, and forecasted for rain over the past 10 days, these are the ideal scenarios for these 2 lawn diseases.
The fungus is most noticeable in the leaf blades as you can see in the photos. The infection produces gray or dirty-yellow spots with brown or purple borders. A water-soaked border will be seen during high moisture periods around actively growing spots. The general area around some spots may turn chlorotic (yellow), or much of the leaf blade may have a yellow cast.
The diseases are mostly a threat to the immediate visual appeal of the turf more than long-term health - although extended moisture conditions can lead to other fungal conditions that attack the long-term health of the turf.
The fungus thrives and the disease is most noticeable when air temperatures are 70–85 ºF, but it can reproduce and increase at cooler temperatures as well. For spores to germinate and infect the turf, they need 16 hours of free moisture on the leaf surface - which has been the case for most Gainesville lawns over the past week.
Tips for Leaf Spot Disease Management
- Avoid too much nitrogen lawn fertilization (slow release is okay, not quick-release) during these extremely rainy weeks.
- Set irrigation timers properly (early morning) so foliage doesn’t stay wet for extended periods.
- Repeated application of fungicides will be needed to control this disease effectively during warm, wet periods - but we