Leaf spot and melting-out are the common names given to a large number of lawn diseases caused by the same family of fungi. Helminthosporium (now that's a mouthful!) contains a large number of individual disease-causing organisms. What's similar in the grass diseases of this family is that they give your lawn a real one-two punch by creating the leaf spot phase in the spring and fall but changing gears into a sheath and root rot phase during the heat of summer.
The lawn disease fungi survive the winter as spores (fungus seeds) and as mycelium (fungus strands) in and on diseased turf tissue. When spring temperatures reach 55 to 60 degrees and there is a lot of moisture from rain or dew, the fungus begins to grow and spreads its spores by wind and water. As the weather warms, the disease moves into the second phase of melting out.
During the leaf spot phase of this lawn disease (spring or fall), individual grass blades will have large numbers of lesions (or infection marks) that appear a little like a cigarette burn, a dark circle with a tan spot in the center. During this part of the leaf spot lawn disease, the grass takes on a brown under cast making it appear underfed or dried out. Even though leaf spot does not do significant damage to the turf during this phase, it sets the stage for the much more serious melting-out phase. The melting-out phase begins during hot, dry weather and causes large irregular areas to appear dried out. There may not be any distinctly visible lawn disease symptoms during the melting out phase, as the damaged areas may appear very similar to the effects of dry weather or insect injury.
As with all lawn diseases, how the turf is watered, fed and mowed makes a huge difference. It’s best to be sure the grass does not stay continuously wet or become excessively dry. Water no more than once per week and soak the soil to a depth of 6''. Water in the morning so the grass will not remain wet for long periods. Open the area around the grass crowns (where the plant goes through the thatch and into the soil) with core aeration. Regular lawn fertilizing and aeration can go far towards disease prevention. Collecting the clippings during the spring and fall (leaf spot stage) will help reduce the spread of disease as well. Lawn treatment services, if needed, should begin early. Spring and early summer applications of fungicide can be effective, but little can be done to reverse lawn disease once the melting-out phase begins.
Whenever you have a question on leaf spot (or any other lawn disease) contact your neighborhood Spring Green. We've got the answers!