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Photo by Neil Sperry, actually of Neil’s own grass. Texas turf expert Dr. James McAfee of the Texas AgriLife Extension suggested that he not apply nitrogen between mid-June and the end of August – that nitrogen would cause the outbreak to worsen quickly. Neil reports no major outbreaks in the ensuing 12 years, and his lawn hasn’t suffered from lack of mid-summer feedings.
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Gray Leaf Spot
Gray leaf spot: this recurring fungal disease is one of the most unwelcome guests that invade St. Augustinegrass lawns. Although the disease can affect perennial ryegrass and bermudagrass, its primary host is St. Augustinegrass.
This uninvited problem is the greatest nuisance during periods of high humidity and when temperatures are in the 80s and 90s. This means that from the end of May through early September, when days are overcast and very humid, gray leaf spot will be most active. Excessive fertilization and irrigation during these times will cause accelerated disease activity.
Individual leaf spots are brown- to ash-colored with purple to brown margins, which elongate and become oval to diamond shaped. Fully developed spots remain diamond shaped and have a depressed gray center with brown edges and adjacent tissues that are yellowish or chlorotic. Spots can also occur on the stolons (stems) and crowns of grass plants. At fruition, the spots are covered by fungal spores and have a velvety appearance. Advanced stages may include leaf blades that are manila or straw colored, and appear to be burned or scorched. The lawn may exhibit yellow splotches of turf throughout the affected area or may have an overall scorched appearance.
For the disease to become active, three things must be present simultaneously:
1. Host – Usually St. Augustinegrass plant
2. Pathogen – Spores for gray leaf spot disease
3. Environment – 80- to 90-degree temperatures, high humidity, and fertilization timing
When these three things come together, the development of gray leaf spot is inevitable. To relieve or correct the disease, one of the above factors will have to be modified or eliminated.
Removing the host would be a drastic measure and only a last resort, of course. In South Texas (from Austin south) homeowners can substitute purple-stem varieties of St. Augustinegrass (e.g., Floratam) that are more resistant to gray leaf spot than green-stem varieties. In areas north of Austin, however, purple-stemmed varieties should not be used, since cold temperatures will damage them.
Spores of most disease pathogens are ever-present in the air and/or soil and are just waiting to find a host in the right climate. Since spore elimination is out of our control, making changes to the host or the environment provides our only options. Managing more closely the water and fertilizer applications to times less conducive to disease development is a better strategy. Do not fertilize during hot, humid weather from mid-June through August. St. Augustinegrass can thrive and survive with a spring and fall feeding. Evening watering should be avoided, since it allows prolonged moisture on the leaf surface, enabling disease development. Irrigate, instead, during early morning hours to minimize the amount of time water is on the leaf blade. Re-program irrigation controllers to water turf zones no earlier than 4 a.m.
Once the pathogen has attacked the host and become a disease, there are some chemical controls. A partial list of chemicals labeled for gray leaf spot follows; there are other chemicals labeled for gray leaf spot, but they are not always available to homeowners through nurseries and garden centers.
• F-Stop, Eagle (myclobutanil)
• Banner (propiconazole)
• Mancozeb (mancozeb)
Knowing when gray leaf spot is most likely to occur and having material on hand to treat it at an early stage of development is helpful in minimizing potential plant loss. Before using any chemical control, examine the label to make sure the disease is still listed for that particular product. Labels are frequently amended for specific crops/plants and diseases. Read and follow all label directions.
Other suggestions for modifying the environment: improve airflow through the lawn area by arranging the landscape to utilize prevailing winds to keep turf leaf surface as dry as possible. Selectively prune trees for more favorable lighting and air circulation.
Implementing these solutions will help turn the unwelcome nightmare of gray leaf spot into a St. Augustinegrass lawn that’s a pleasant dream.
About the author: Mike Sutton is a horticulture graduate of Purdue University with more than 25 years’ experience with turf grasses in Texas.
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