See spot run
Wet weather combined with warm days and cool nights in the spring of 2000 provided good conditions for vigorous turf growth in the Northeast. Such conditions also favored certain diseases. One of the most prevalent was leaf spot/melting-out, or spring leaf spot. This disease was active on Kentucky bluegrass most of the spring. Favorable conditions for the disease persisted, and it reached the melting-out stage, which caused extensive damage to Kentucky bluegrass turf in many locations. No sooner did the disease begin to subside on Kentucky bluegrass in late spring, than it started to show up on perennial ryegrass.
Samples received at the turfgrass disease research laboratory at The Pennsylvania State University indicate that it is the most prevalent disease on Kentucky bluegrass and ryegrass turf so far this year in Pennsylvania. A similar situation exists in New Jersey, according to Richard Buckley, the diagnostician of the plant disease clinic at Rutgers State University.
This spring is not unique, however. Leaf spot/melting out is a disease with which turf managers frequently must cope. Let's take a look at how this disease develops and ways you can manage it.
Two phases of disease development Symptom development on Kentucky bluegrass and ryegrass is similar, despite the differences in timing. There are two phases of the disease: the leaf-spot phase and the melting-out phase.
* The leaf-spot phase causes thinned, off-color turf with a light reddish-brown tinge. In this phase, the disease develops first as necrotic specks or flecks on the blade, especially on older blades. Under favorable conditions, these expand rapidly and become reddish- or purplish-brown circular lesions (see photo, above). The lesions range upward in size from 2 millimeters and are usually oval in shape. A single lesion may cover the entire width of a grass blade. Irregularly shaped lesions may be prevalent on severely infected blades as the lesions coalesce. Lesions are often light brown in the center with dark- or purplish-brown margins; the center also can occasionally be gray. Complete or partial (tip) blighting of the blade results from disruption of vascular tissue as the lesions continue to expand.
* The melting-out phase is the more-advanced stage. This phase begins when mycelium (the fungal "body") infects and ultimately kills the crown, which can occur in two ways. First, the mycelium can grow and spread downward from the leaf lesions until it reaches the crown (see photo, below). Additionally, large numbers of spores are produced on infected blades. Water from rain or irrigation carries the spores to the crown areas, where they germinate and infect the crown tissue. Symptoms of the disease are most obvious in this phase, during which turf exhibits loss of density and dead pockets (see photo, page 18). In severe epidemics, large sections of turf may die.
Causal organisms Several fungi are involved in this disease, depending on the species of turfgrass. Previously, all of these fungi were considered to be species of Helminthosporium. Even now, this name is still used widely when referring to this disease. However, the most recent taxonomic work divides the fungi into three genera: Drechslera, Bipolaris and Exserohilum. Of these, Drechslera and Bipolaris are associated with leaf spot/melting out. Bipolaris sorokiniana has the widest host range (at least eight species). Drechslera spp. cause spring leaf spot in Kentucky bluegrass, brown blight in perennial and annual ryegrass and net-blotch in fescues.
Epidemiology Infection, growth and reproduction may require different environmental conditions, but this varies according to the fungal species. For example, infection by D. poae and initiation of the leaf-spot disease phase on Kentucky bluegrass is favored by prolonged rainy and drizzly periods with high humidity and cool temperatures in spring and fall. However, a different set of conditions is required for the disease to proceed to the melting-out phase. In the melting-out phase, warm and humid conditions favor the progression of the disease that eventually kills the entire plant. Thus, the disease in fall is normally not as severe as in spring.
By contrast, the conditions necessary for infection and initiation of the leaf-spot phase in ryegrasses by D. siccans are similar to those it needs for progression to crown and root rot in late spring and summer, and an additional leaf-spot phase in the fall. Net-blotch of fine-leaf and tall fescue, caused by D. dictyoides, also develops in spring and subsequently advances to the crown and root-rot phase without the need for warmer conditions (though it is generally not evident until warm and dry conditions of summer bring out the symptoms).
By contrast, B. sorokiniana thrives in warm temperatures. This fungus infects perennial rye and other grass hosts when temperatures reach the 70 degrees F. Disease severity increases with temperature. Blighting of the leaf blades occurs in the 80s, and the disease is most severe when temperatures reach the 90s and the leaves experience intermittent wetness.
Inoculum sources One of the major sources of the inoculum for newly established or overseeded turf is grass seed, especially Kentucky bluegrass. Grass clippings contaminated with infected blades also serve as a source of inoculum for primary and subsequent disease cycles in both newly established and existing turf. The fungi survive unfavorable conditions in inactive states in infected plant tissue and in debris.
The disease-causing fungi easily survive on non-living organic debris. They profusely produce spores in the early spring when the debris they've colonized is wetted and the temperature reaches 43 degrees F. Spore production increases with temperature, peaks at 55 to 65 degrees F and declines or stops at higher temperatures (see photo, page 24).
Conditions for discharge of spores from plant debris or infected blades differ from those required for spore production. Spore discharge is most efficient under dry conditions. Therefore, intermittent leaf wetness, rather than continuous wetness, contributes to successive disease cycles and epidemics. Wind, wind-blown rain, irrigation water, equipment and human activity all contribute to spore dispersal.
As spore production, spore germination and plant infection increases, leaf-spot disease symptoms eventually appear. With the advent of the warm and moist weather of late spring and summer, the disease enters the melting-out phase as crowns and roots are infected.
Disease management Turf managers can effectively manage this disease with a fungicide program and good cultural management. A preventive approach will not only aid in reduction of initial inoculum and delay the onset of disease, but also will improve the results you obtain from your fungicide program.
* Seed source. Check with your seed supplier for availability of disease-resistant varieties. Of course, don't forget to consider other important diseases in your locality. You can obtain information on turf cultivar performance in various locations by consulting NTEP evaluation results, available via the internet at www.ntep.org.
* Fertility. Excessive turf growth in spring and summer predisposes plants to infection by Drechslera and Bipolaris. High rates of quick-release nitrogen also have been reported to promote the disease. Therefore, you should split your applications of quick-release nitrogen, or using a slow-release fertilizer to help manage the disease.
* Mowing height. Increase the mowing height of turf during the cool, wet weather of spring and fall. This will make grass plants less prone to attack by leaf-spot fungi. Collect clippings when the disease is active.
* Thatch management. Prevent thatch from exceeding 0.5 inch in depth to reduce the inoculum during wet-dry cycles. Verticutting or core cultivation are good ways to control thatch.
* Irrigation. If possible, irrigation should take place during the middle of the day, so that the turf canopy will dry out as rapidly as possible.
* Herbicides. Phenoxy herbicides for broadleaf weed control and plant growth regulators aggravate leaf spot and melting out. If possible, avoid using these chemicals when the disease is active.
* Fungicides. Several fungicides effectively control leaf spot and melting out. See the table on page 24 for a list of chemicals registered for use on "Helminthosporium" diseases.
Dr. Wakar Uddin is assistant professor of plant pathology at the Pennsylvania State University (University Park, PA).
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