The summer of 2016 has been quite favorable for the development of an assortment of infectious diseases on residential turf. The primary driver of all of this disease activity is the weather. In the lower Midwest, the combination of elevated temperature and ample precipitation since the beginning August of has spawned outbreaks of brown patch, dollar spot, Pythium blight, leaf spot diseases, and the appearance of symptoms of summer patch and necrotic ring spot. Although infection is almost entirely a function of temperature and moisture, there are other factors (addressed below) that may hasten the decline of turf once disease becomes established.
Turf species: Not all turf species are susceptible to all diseases.
In general, Kentucky bluegrass (KBG) is less prone to brown patch , dollar spot, gray leaf spot, and Pythium blight—but very susceptible to root diseases such as necrotic ring spot and summer patch. Root pathogens are present in all soils, and infect turf as soil temperatures increase beyond 60F. With ample moisture, infection will continue through summer. Plants are killed outright in some instances, but in most cases, KBG can tolerate some infection. However, during periods of environmental stress, infection-impaired plants cannot survive, and characteristic dead patches or rings or frog-eyes appear.
After the droughty years of 2009-2012, many of us turned to turf type tall (TTF) fescue to better tolerate periods of dry weather. The summers of 2015 and 2016 have revealed the weakness of these grasses in terms of susceptibility to gray leaf spot and brown patch. They are not susceptible to root diseases-and that is an important reason for favoring this type of grass.
This was not a good year for perennial ryegrass (PRG)! Its only redeeming feature is that PRG is not susceptible to the root diseases. We are learning that even the so-called “gray leaf spot resistant” cultivars will suffer during prolonged periods of favorable weather.
Fertility: Nitrogen status will influence disease severity.
Some pathogens are favored by nitrogen levels that approach excess, others thrive when nitrogen levels are low. I usually recommend a little (0.2 lb/M) supplemental N in early summer, when the fall-applied reserves have been used. This helps keep plants growing through summer and helps recovery from red thread, dollar spot and actually can prevent serious rust outbreaks. However, this practice can be risky on PRG and TTF because brown patch is favored by lush growth promoted by nitrogen. Also, from my experience, supplemental N (0.25 – 0.50 lb/N) during summer can help mask the effects of summer patch and necrotic ring spot (on KBG only). Again this can be risky because some leaf spot diseases (NOT gray leaf spot) flourish when leaf and shoot growth are stimulated by N. Pythium blight, a disease that strikes when temperatures exceed 90F during the day and 70F during the night is also favored by high nitrogen nutrition. This year seems to have been one where we are “damned if we do and damned if we don’t” with regard to nitrogen fertility.
Irrigation: Timing and amount.
We cannot define the best time to irrigate in all different scenarios, but we do know the worst time—early evening hours (6:00 pm - 9:00 pm). Irrigation at this time adds to the period during which leaves are wet. An increase in leaf wetness by a few hours can mean the difference between a severe outbreak and the appearance of some mild symptoms. It is difficult to generalize about the amount of water to apply. However, from a disease prevention perspective, deep infrequent watering is better than light frequent irrigation.
Growth stage: Pitfalls of over-seeding during disease-favorable weather
We tend to think about repairing damaged stands of turf in late August through September. Seedlings and juvenile plants are most susceptible to infection. Normally, the risk of infection is acceptable because weather changes to a pattern that favors turf growth more than fungal growth. Not this year.
Fungicides: Timing is everything (almost)
Fungicides are valuable chemical tools to help avoid or mitigate turf damage. Granular products are available with multiple active ingredients to protect against a variety of diseases. However, they work best when applied preventatively, or before symptoms appear. Having said that, I am convinced that modern fungicides can be used to control dollar spot, brown patch, red thread, and rust--even after symptoms appear (but certainly long before the lawn is wiped out). Leaf spot diseases (including gray leaf spot) are best controlled before symptoms show. When symptoms become apparent to most observers, infection is way beyond the stage of a “mild” infection. Most of these fungi reside in the turf or thatch or soil, so the same areas of turf will be prone to outbreaks each year. The weather will determine if a severe outbreak materializes. This year the weather favored disease.
The most difficult “fungicide” decisions involve the root diseases on KBG. Fungicides can be effective, but they must be applied in late spring, often before there are any symptoms at all. (We know that fungicides are NOT effective against root diseases once the patches and/or rings are in full form). So, it is important to understand the disease history of the site before considering a fungicide option. Caution--because fungicides do not move down to the roots through the leaves inside the plant, fungicides must be washed off leaves (into thatch/soil) for any chance of success. Getting the right amount of chemical to the right place at the right time is not easy , and this difficulty further complicates decisions to use fungicides for controlling root diseases on residential turf.
Summary:
More than any time in the past 10 years, weather during the summer of 2016 has favored outbreaks of so many turf diseases. Mercifully, fall is approaching and we can begin the process of repairing damaged lawns. We should also take note of diseases that caused problems this year and the factors that influence their outbreaks. To be sure, when such weather patterns return in the future, the same disease threats will revisit our lawns!
For further information on turf diseases, visit our website at
https://ag.purdue.edu/btny/Extension/Pages/ExtPubs.aspx
or, use your favorite browser to search “Purdue Turfgrass Disease Profiles”.
Rick Latin, Ph.D.
Professor of Plant Pathology
Purdue University
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