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Gray Leaf Spot Fungus
Randy McLaughlin has a PhD. in Plant Pathology from the University of Wisconsin, has published many research articles and has practiced his craft at Texas A&M, Rutgers University and the USDA-Agricultural Research Service.
Fungal Infection of Plants Sometimes Causes Leaf Spots
Plants are not immune to disease; they get attacked by fungi, bacteria, viruses and more. Those who grow St. Augustine grass may notice a gray leaf spot, which is caused by a fungus scientifically known as Pyricularia grisea. There is another fungus, Stemphyllium solani, that causes a gray leaf spot on tomato. As indicated in the above photo, several different plants, when they become infected by fungi will develop leaf spots that are gray in color. The species of fungus causing the leaf spot is usually a different in different plant species.
Leaf spots of plants can be controlled by a variety of measures including resistant varieties, cultural methods and chemicals. One should always consider the first two methods before resorting to chemicals. Below, I show the variety of fungi that can cause this plant disorder and discuss some general control principles and the availability of resistant cultivars and hybrids for preventing gray leaf spot fungus.
Names of Fungi Causing Gray Spot in Different Plants
Scientific Name of Fungus
Ryegrass, tall fescue
Cypress, arborvitae and juniper
Gray Leaf Spot Fungus Diagnosis on Turfgrass
Different Spore Morphology of Gray Leaf Spot-Causing Fungi
Conditions Causing Plant Infection
Leaf-infecting fungi need moisture in order to cause infection and a susceptible host plant. If the leaf surface is wet for a prolonged period, say several hours, this provides the amount of time needed for a spore to germinate and enter the leaf tissue. If there is a period of prolonged rain and warm temperatures, these conditions foster infection if there are pathogen spores near susceptible plants. This also goes for watering late during the day, because there is not enough time for the leaves to dry off during the night.
Irrigation. Irrigation practices can be optimized to prevent infection. With grasses, it is better to water in the morning to mid afternoon to allow for the leaf surfaces to dry off. With plants like tomatoes, ornamentals like photinia and blueberry it is best to provide water near the ground; overhead irrigation, on the other hand, should be avoided for these plants.
Nutrition. Optimum nutrition also helps prevent infection. With some plants, excess or poor levels of nitrogen can increase susceptibility. This is particularly true for susceptible grass species. In this case, it is better to use a slow-release, balanced fertilizer have equal parts of nitrogen, phosporus and pottassium (N-P-K).
Plant debris. If you have had this disease in a previous season in an area where you plan to plant again, it is best to avoid planting in the same area. This is because the spores can survive in the plant debris near the soil surface. If you have neighbors that have had a problem and there is still plant debris present, this can also be a spore source to cause infection of your plants. Alternatively, tilling the soil to thoroughly cover the debris or debris removal helps remove this source of spores.
Sunlight. Sunlight improves the vigor of full-sun requiring plants like tomatoes and grasses. Plants in this category which receive shade during a significant part of the day will be more susceptible to leaf spot fungal infection. Sunlight also helps dry off the leaf surfaces to help prevent adequate time for spore germination and infection.
The Plant Disease Triangle
Plant Resistant Varieties to Avoid Gray Leaf Spot Fungus
In some cases resistant varieties exist or have been developed so you can avoid having leaf spot problems on your plants. Here are some resistant varieties that exist for different plants:
- St. Augustinegrass - FX-10 and Bitterblu
- Tomato - Early Bingo, Early Heatwave, Early Sunmaster, Early Merced, Santiago, Small Fry, Cherry Grande - (look for "St" on the seed label)
- Ryegrass or Tall Fescue - Use Kentucky bluegrass, bentgrass or fine fescues as a replacement.
- Corn - no hybrids have immunity to this disease, but there are lines put out by different companies that show some resistance. Contact your extension agent and/or the seed company to understand the best hybrid choice for your situation.
- Blueberry - most rabbiteye and highbush cultivars are affected by gray spot disease. Bladen and Reveille seem to work well without additional fungicide sprays, while the following cultivars need some additional sprays to prevent infection: Jersey, Murphy and Croatan.
- Photinia - There are no resistant cultivars available.