Summer patch just keeps growing and growing
One of the lawn problems that I am seeing more and more is summer patch. Why have I noticed more and more is because it is now a major problem in my own home lawn. It is what I like to call a monocyclic disease in that, like fairy ring, it tends to appear in the same area of the lawn year after year. What may start off as small patches tend to grow in size and severity over time. In other words, summer patch comes back each year a little stronger. Thus, the cultural practices described below are most effective preventatively.
Figure 1. Summer patch symptoms in a Kentucky bluegrass lawn (location: author's home lawn)
Summer patch caused by the pathogen Magnaporthe poae colonizes Kentucky bluegrass roots during spring and early summer, but symptoms do not show until later in the summer. Upon the arrival of hot dry weather of summer the root systems dieback and become dysfunctional. Symptoms appear as irregular shaped patches, rings or crescents when temperatures rise (85-95 F). Symptom expression is most severe when wet conditions are followed by dry or droughty conditions. Disease symptoms are chronic appearing in the same locations year after year. A disease sign is the presence of weeds growing in the center of the patches. The weed types include dandelion, crabgrass, and plantain – to name a few. 
Figure 2. Close-up of a summer patch symptom. The middle part of the patch is often colonized by a different grass species or weed.
Cultural management practices that help reduce the severity of summer patch include relieving soil compaction, providing adequate fertility, and planting where applicable resistant Kentucky bluegrass cultivars. Some cultivars of Kentucky bluegrass have shown some tolerance to summer patch, but the information is not complete for all the cultivars (consult ntep trials). In some instances, overseeding infected areas with perennial ryegrass is an effective method. Perennial ryegrass is more tolerant of summer patch than Kentucky bluegrass.
- Promoting good drainage and reducing soil compaction are practices that can reduce the severity of summer patch but are best accomplished in the autumn.
- From a growth perspective, a healthy vigorous (not over stimulated) growing turf is more likely to recover. Nitrogen applications during the summer should be moderate and if possible apply a slow release fertilizer instead of a quick release.
- Drought stress increases the severity of the patch disease, thus proper watering is important. Minimize drought or wilt injury through frequent (syringing) applications of water. The lack of a functional root system limits the amount of water taken up by the root system.
Fungicide applications reduce the severity of summer patch but do not eradicate the problem. Timing is critical in fungicide applications. The best program is to treat preventatively with an application made in spring when soil temperatures reach and remain above 65 F at the 2 to 3 inch level. A second application is made one month following the initial recommendation. Given the pathogen attacks, the roots the fungicide should be applied in 4 to 5 gallons of water per 1000 square feet and watered in following application.
Figure 3. Summer patch has continued to grow and expand yearly in this lawn.
About the author
Dr. Karl Danneberger is a professor of Turfgrass Science at The Ohio State University. Dr. Danneberger's contact information can be found here. You may also follow Dr. Danneberger on Twitter: