By Lee Miller, PhD
From the August 2024 Issue
The common denominator the whole turfgrass profession must deal with is the fickle nature of Nature. We’ve nearly all heard the statement at one time or another, “If you don’t like the weather in insert your location here just wait five minutes.” The inconvenience of rainfall and wild temperature swings makes scheduling our maintenance efforts challenging, but recent trends extend this to all pest control, including diseases.
The environment is perhaps the most vital cog in the disease triangle relating to plant health and disease control. We manage a large portion of the environment with our choices in mowing height, fertilization, irrigation, etc., while weather chooses the important temperature, humidity, and rainfall factors that drive plant growth and pathogen activity. Recent trends (noted below) have driven disease observations in the field and submissions of lawn samples into the Purdue diagnostic lab.
Intense Rainfall Events
In the central portion of the U.S., and unfortunately in Florida at the time of this writing, the days of nice long, steady rains have been replaced by more intense 0.5 – 1″ or more gully washers. Lawns, particularly in newer construction homes, are often established on heavy clay subsoil material with very little topsoil. The disadvantages of this suboptimal soil profile are plentiful, such as the lack of organic matter causing lower water holding capacity and increased risk of water runoff.
Clays also lack porosity, so even a slightly waterlogged clay soil has little to no leftover space for air, a requirement for deep and effective spring root growth. When the rain does shut off, the result is a quick “flash wilt,” turfgrass dormancy, and overall weaker plant. Compounding the matter, wet clays also compact readily from rainfall itself or even more through the traffic of a mower tire. On Kentucky bluegrass lawns, this compaction in the Spring can lead to increased severity of the soilborne disease summer patch. Extra care is required during Spring in scheduling maintenance events due to these more intense rainfall events.
On the disease front, we have noticed an increase in several foliar diseases on both Kentucky bluegrass and tall fescue. In the Spring, increased occurrence of damaging leaf spot/melting out has been observed on tall fescue as well as Kentucky bluegrass. During the hotter Summer months, Pythium blight has also been diagnosed more routinely on tall fescue. Both are often more severe on Kentucky bluegrass lawns than tall fescue, but, especially on irrigated lawns, tall fescue is also being significantly damaged.
If your clients have an in-ground irrigation system, teaching them how to use it wisely and effectively would be of great service to the lawn and environment, since many outside the profession view them as a “set it and forget it” device. This mindset never works, but less so with more sporadic, heavy rainfall events.
Prevalent Heat
Globally, last summer was the hottest on record and over 6,500 daily U.S. heat records fell in 2023. While the high-high temperatures make the headlines, in reality, the high-low temperatures drive disease activity since late night/early morning is when humidity and dew contributes to needed moisture and leaf wetness. For cool season grasses, heat over a sustained period is a metabolic ball and chain that restricts photosynthesis by inhibiting enzyme activity and increasing photorespiration. The result is a weaker plant that sheds leaves more quickly and is more susceptible to pathogen infection. Diseases, such as brown patch, Pythium blight, and some leaf spot diseases, thrive in the Summer heat, taking advantage of the weakened plant.
Length Of Season Differences
In Indiana and throughout much of the country, Spring, and the season as a whole, arrived and persisted three weeks ahead of schedule. In 2024, flowers bloomed earlier, mowers whirred earlier, pre-emergent went out earlier, and annually occurring diseases such as brown patch and dollar spot also burst on to the scene earlier. As shown above, the “season length” as defined by some in agronomy as the days between last Spring frost and first Fall frost has varied considerably over the last nine years, with a
range of 36-64 days difference at each Indiana city noted above. This large 1-2 month range is also reflected in the data for cities in Kentucky, Missouri, and Ohio.
The seasonal temperature variability among years means the calendar should become a looser guide for maintenance practices, including fungicide application. The current and forecasted weather status needs to be the defining guide, requiring management decisions made during the season rather than scheduled during the winter months. For planners, this is a tough pill to swallow and more difficult to manage from an operational perspective, particularly if a large number of properties need to be attended to. Fortunately, most lawns are not on a routine preventive fungicide program. Variability between years requires more careful attention to scouting and quick remediation earlier, later, and/or longer as the season dictates.
Disease prediction models which use temperature and relative humidity, such as the Smith-Kerns model for dollar spot, are useful for estimating when diseases may occur with seasonal weather patterns. These models have been integrated into several web-based applications such as the Growing Degree Day tracker from Michigan State University. The GDD tracker includes weather-based models for other pests including information for weed and insect control. The system will even send email alerts when the models are in range and trigger scouting or action. Web-based or social media reports from your regional university turfgrass extension specialist, such as this one from Purdue University, are also valuable in helping track the progress of the season and pest pressures.
In summary, more intense rainfall events and temperature variability are requiring decisions for plant health and disease management to adapt from a rigid schedule made while the snow is flying, towards more careful attention to the actual progression of the season. Scouting for disease and other pests has always been the undertaking of a successful turfgrass manager, but in the current climate attuning it to dynamic weather changes is a requirement.
Dr. Miller is an assistant professor of turfgrass pathology at Purdue University, where he directs research and extension programs focused towards disease control in turfgrasses. Lee’s current responsibilities include providing disease diagnosis and control recommendations for the lawn, golf, sports turf, and sod industries in Indiana and the surrounding region. His research activities center on the development of disease management strategies that require fewer inputs and result in sustainable, functional, and aesthetic turfgrass fields. Past and current research projects led by Dr. Miller include fungicide resistance detection in dollar spot populations, fairy ring biology and prevention with spring fungicide applications, spring dead spot management and cultural control, the effects of fertility and fungicide application methodologies on large patch control, and detection of Pythium spp. in golf course irrigation water. He can be reached at turfpath@purdue.edu.
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