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Leaf Spot and Melting-Out Diseases of TurfgrassNed
Tisserat
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SymptomsDrechslera leaf spot and melting out was once considered the most important spring and fall disease of Kentucky bluegrass. The introduction and widespread use of resistant bluegrass cultivars in recent years has lessened the impact of this disease. Still, some leaf spot can be found on most home lawns in the spring, and it may damage newly seeded lawns and certain susceptible cultivars of Kentucky bluegrass. Extended periods of cool, wet spring weather also may trigger leaf spot epidemics on tall fescue. |
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Figure 1. Drechslera leaf spot on Kentucky bluegrass. |
Figure 2. Drechslera leaf spot on tall fescue. (Photo courstesy of B. Bockus) |
| Drechslera leaf spot develops early April through late
May in Kansas during cool, wet weather. Affected plants initially
develop small elliptical purple leaf spots. The spots eventually turn
light gray or tan but remain bordered by a dark brown to purple margin.
Leaf tissue surrounding the spots may turn yellow. The leaf spot phase
of the disease usually does not damage the plant significantly. However,
during continuous cool, wet conditions the fungus invades the leaf
sheath and crown. Coalescing purple lesions girdle the sheath. The
fungus also may invade the crown, rhizomes, and roots. As daytime
temperatures increase, leaves on crown-infected plants begin to turn
light green or yellow, similar to nitrogen deficient turf.
Eventually these plants die and turn brown or straw colored. This is
referred to as the melting-out phase of the disease. Severe melting-out
can result in irregular patches of dead turf. Damaged lawns often appear
"thin" or uneven and tend to have weed problems. Symptoms on
tall fescue are similar to those on bluegrass.
Bipolaris leaf spot and melting out is similar in appearance to Drechslera leaf spot, but it is more common in mid- to late-summer during hot weather. This disease rarely causes significant turfgrass injury. |
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Figure 3. Bipolaris leaf spot on Kentucky bluegrass |
Figure 4. Microscopic view of spore of Bipolaris sorokiniana on bluegrass |
ConditionsLeaf spot fungi overwinter in the thatch layer or in small lesions on leaf blades. Drechslera leaf spot is favored by long periods of leaf wetness and moderate temperatures. The disease is inhibited by hot weather and rarely appears during the summer or early fall. In contrast, Bipolaris leaf spot is favored by high temperatures and normally develops in late summer. ControlThe most effective means of controlling leaf spot and melting out is to plant resistant cultivars. Most of the newer varieties of Kentucky bluegrass have moderate to high levels of resistance to this disease. When seeding, use a blend of three or more of these cultivars. Avoid planting cheaper, 'common' types of bluegrass such as 'Park', 'South Dakota', and 'Kenblue'. Several cultural practices will reduce the severity of leaf spot. Avoid excessive nitrogen fertilization in spring that favors lush growth, but do not "starve" the lawn of nitrogen during the spring. A well-balanced fertilization program will reduce the severity of the disease (see KSU Horticultural Fact sheets on cool-season turfgrass fertilization). There is some evidence that frequent watering to keep the thatch moist will help reduce sporulation by the fungus. Nevertheless, do not apply irrigation in late afternoon or night because this will keep leaves wet for long periods and will favor infection. If frequent watering is attempted, it should be done at mid-day. Use higher mowing heights (> 2 to 2 1/2 inches) in late spring or during dry weather. Thatch reduction will also help reduce disease severity. Several fungicides are available for the control of leaf spot and melting out. Application of fungicides depends somewhat on disease severity and the susceptibility of the turfgrass cultivar. If leaf spot has occurred in previous years but has not caused significant injury, then fungicides are not warranted. Nevertheless, chemical protection may be required on highly susceptible cultivars. Fungicides are protective in nature and should be applied as soon as symptoms are observed. The disease is very difficult to control once the melting-out and crown rot phase has begun. Fungicides recommended for melting-out control are listed in the table. |
| Fungicide | Efficacy | Application intervals | Examples of trade names |
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| azoxystrobin |
excellent |
14-21 days |
Heritage |
| chlorothalonil |
excellent |
14 days |
Chlorostar, Daconil, Echo, Manicure, Thalonil, others |
| iprodione |
excellent |
14 days |
Chipco 26019, Fungicide X |
| mancozeb |
good |
10-14 days |
Dithane, Fore, Foremec, Mancozeb, Protect |
| myclobutanil |
good |
14 days |
Eagle |
| propiconazole |
good |
14 days |
Banner |
| trifloxystrobin |
excellent |
14 days |
Compass |
| vinclozolin |
good |
14 days |
Curalan, Touche, Vorlan |
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Web updated 9/01/06 |
