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Patch Disease Symptoms Starting to Show in Dry Kentucky Bluegrass Lawns
Summer patch and necrotic ring spot are root-infecting diseases in Kentucky bluegrass lawns. Their symptoms are some of the most difficult to manage, much less control the disease. Although these diseases differ slightly in when they attack the roots, the pathogens generally are active in the late spring to early summer. Infection-impaired roots have a limited capacity to take up and transfer water to the rest of the plant. Symptoms will become visible with the first round of drought stress and appear as yellow-tan arcs, rings, or "frogeyes" in affected turf. Symptoms are most severe on lawns with 0.75" of thatch or more as most of the roots start growing in the thatch. Since thatch doesn't hold much water, these lawns are the first to show symptoms. These symptoms also tend to be more pronounced on sodded lawns that were poorly prepared (insufficient tillage) prior to sodding because roots cannot penetrate deeply into the soil and thus are susceptible to drought stress. A wet spring like we are in further exaggerates symptoms because of overall shallow rooting in water-logged soils. Symptoms usually take two to three years to surface after sodding and much longer than that on seeded lawns. Since water stress triggers the symptoms on turf with diseased roots, increasing irrigation frequency and decreasing amount applied each time will help minimize symptoms after they develop. Realize though that this only minimizes symptoms and does not encourage deeply rooted plants and long-term health of the plant. Any chemical controls attempted now should be done by professionals only. If patch disease are a perennial problem, overseeding with perennial ryegrass can help to mask the symptoms as long as other potential diseases like dollar spot, red, thread, and brown patch can be tolerated or managed. Much more information including long term cultural and chemical controls are included in the following publications:
BP-116: Necrotic Ring Spot at http://www.ces.purdue.edu/extmedia/BP/BP-116-W.pdf
Bp-115: Summer Patch at http://www.ces.purdue.edu/extmedia/BP/BP-115-W.pdf
Thatch is a tightly mingled mixture of partially decomposed roots, crowns, rhizomes, and/or stolons. It provides a great rooting medium because of the oxygen and pore space, but it holds very little water and thus turf rooted in thatch is very susceptible to drought stress.
Summer patch symptoms come in a variety of patterns including arcs, rings, and frog-eyes. Symptoms are most common after drought stress in lawns with compromised root systems (thatch, compacted soil, poorly prepared soil prior to sodding, over-watered, etc