The UMass Extension Plant Diagnostic Lab has recently seen four stress related, minor leaf spot and blight pathogens (Colletotrichum, Curvularia, Leptosphaerulina, and Ascochyta) on lawn specimens submitted. These fungi often appear when the turf is under stress from poor growing conditions such as excess thatch, soil compaction, mowing heights lower than recommended for the grass species present, high soil and air temperatures, or frequent light watering. They may also be secondary to other pathogens, especially root infecting fungi or nematodes that cause dysfunctional roots. Fungicide applications are seldom effective unless the underlying causes of stress are addressed.
All four fungi survive in the thatch and in infected plant debris and disease is more severe in areas of soil compaction, poor drainage, and/or heavy traffic. They infect through the tips of freshly mown grass. Anthracnose (Colletotrichum) is most serious on annual bluegrass (Poa annua) and bentgrass (Agrostis species), while Curvularia, Leptosphaerulina, and Ascochyta attack all species of turfgrass. Disease development is encouraged by light, frequent irrigation, overcast weather, and high relative humidity. Curvularia, Leptosphaerulina, and Ascochyta leaf spot and blights are favored by high nitrogen levels, while anthracnose is more serious when nitrogen levels are inadequate.
Steps to lessen the impact of these diseases include:
- Provide proper fertility based upon the results of a soil test. Sufficient levels of nitrogen, phosphorous, and potassium are required for robust turf growth and tolerance to disease.
- Address soil compaction with aeration. Perform aeration in late summer (best time) or early spring when the turf has the ability to recover from this potentially stressful practice.
- Reduce thatch levels if they are excessive as these fungi survive in the thatch. Excessive thatch causes the roots to grow mainly in the thatch layer where they are more exposed to the activity of both plant pathogenic fungi and insects.
- Avoid frequent, light irrigation which encourages the activity of these and other turf pathogens. Water deeply and infrequently. One inch of water per week is a good guideline.
- The optimum time to irrigate is early morning when the turf will dry rapidly and prolonged periods of leaf wetness are avoided. Avoid late afternoon and evening watering, especially when night temperatures are high.
- Reduce mowing frequency and mow at the height recommended for the grass species present.
- Avoid applying herbicides and installing sod during or just before an extended period of hot, humid weather.
- When leaf spots and blights are severe in most years, protective fungicide applications may be warranted. See our Turf Fungicide Charts for more information.
Submitted by: M. Bess Dicklow