One day your cool-season lawn is green. Then you have a hot, humid night and the next day the grass shows some yellow patches. The day after, the yellow turns to brown. What's going on? While seemingly overnight yellowing on can be a symptom of a number of different lawn diseases, this particular culprit is most likely a lawn fungus called brown patch. Here's how you can tell.

Favorable Conditions for Brown Patch

Brown patch thrives when it is hot and humid, and can affect your lawn in late spring, summer, or early fall, depending on what type of cool-season grass you have, where you live, and recent weather conditions. Prolonged wetness from dew, rain, nighttime or evening watering, or poor drainage—or any other activity that keeps grass blades wet during weather in which temperatures are above 80 degrees F during the day and 65 degrees F at night—create ideal conditions for developing this serious fungal disease. In addition, grass that has been over-stimulated with nitrogen fertilizer just before or during hot, humid weather tends to be more susceptible to brown patch.

Cool-Season Grass Types Commonly Affected by Brown Patch

• Tall fescue
• Perennial ryegrass
• Bentgrass

How to Identify Brown Patch

If your lawn has begun to yellow rapidly, take a closer look at the size and shape of the affected areas. With brown patch, they are likely to be roughly circular, though a bit irregular. Patch sizes can range from a few inches to several feet in diameter. If brown patch has been active for a while, the spots might also look like patches of good grass with rings of dead or thin grass around them.

Besides yellow, do you notice any other colors in the patchy areas? Brown patch patches can be darker purple or burgundy on the outside, for example. Other signs include irregular tan spots bordered by a darker outline on grass blades, brown and shriveled blades, rotting at the base of the blades, darker blades that look water-soaked, and possibly white, cobweb-like growth around the blades (typically found in early morning, before the dew dries).

How to Maintain Your Lawn to Help Reduce Brown Patch

A healthy lawn is a stronger lawn and will be better able to withstand fungal diseases like brown patch. Follow these guidelines:

• Plant disease-resistant varieties and seed mixtures. For example, Scotts® Turf Builder® Grass Seed Heat-Tolerant Blue® Mix for Tall Fescue Lawns combines improved turf-type tall fescue varieties with a heat-tolerant Kentucky bluegrass to help reduce the impact of brown patch on your lawn.
• Reduce the amount of time the grass blades are wet. When needed, water during the early morning hours. Don't water between 7:00 PM and 3:00 AM, as the blades will stay wet overnight.
• Feed the lawn regularly with Scotts® Turf Builder® Lawn Food. Be sure to follow label directions, as overfed lawns are more susceptible to brown patch.
• Unless it is a controlled-release product, don't apply nitrogen fertilizer when hot, humid weather is expected (particularly in late spring or summer).

Since wet conditions help promote brown patch, make sure your lawn has good drainage and air flow, and consider pruning overgrown trees to prevent too much shade and increase air movement for faster drying. It may also be helpful to aerate and dethatch the lawn every 1 to 3 years to allow air, water, and nutrients to move more freely into the soil.

How to Control Brown Patch

Watch the weather forecast for the kind of weather that is conducive to brown patch (see above) and treat your lawn for brown patch at the first sign of it. Apply a  broad-spectrum control and prevention product like Scotts® DiseaseEX™ Lawn Fungicide. This is a quick, effective fix that lasts up to 4 weeks and also helps prevent the spread of the disease to the rest of your lawn. Be sure to follow label directions.

How to Repair Lawn Spots Damaged by Brown Patch

Once brown patch has been treated and is under control, it's time to fix up any bare spots left by the disease. A simple way to do this is to use an three-in-one patch and repair mix like Scotts® EZ Seed®. Formulated to absorb water to keep the seed moist and protected, it has everything you need to patch and repair bare spots. Just be sure to choose the formula designed for your grass type, follow the directions on the bottle, and apply during early spring or fall for best results.

To thicken up lawns that have had large areas thinned out by brown patch, overseed your entire lawn during the late summer or early fall with Scotts® Turf Builder® Thick'R Lawn™ Sun & Shade or Scotts® Turf Builder® Thick'R Lawn™ Tall Fescue Mix (depending on your lawn type). Both formulas contain a combination of seed, controlled-release fertilizer, and soil improvers.

With a little care and Scotts by your side, your problems with brown patch will be a thing of the past.