Frank Wong, Ph.D., is part of the Bayer Green Solutions Team.
Summer is here and hot days and warm nights can bring on Pythium as a major threat to cool season turfgrasses. Of the various Pythium diseases, Pythium blight and Pythium root rot are of the highest concern under high temperatures.
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Although we often classify Pythium as a ‘fungus’ it’s not a true fungus in the sense that it is actually in a different kingdom (Chromista) and, biologically, shares more in common with algae than true fungi that cause other diseases like brown patch and dollar spot (kingdom Fungi). That is why most fungicides that control Pythium do not usually give control of other diseases.
Furthermore, Pythium belongs to the Oomycota, which are called “water molds” for a reason: Pythium and other oomycetes need a lot of water to grow and reproduce effectively. That is why a common factor in Pythium disease development is the presence of water or high moisture, e.g., wet, water-saturated soils or long periods of high humidity or leaf wetness. It is also why we see turf damage caused by Pythium diseases track with water movement, e.g., drainage areas, mowing patterns, etc.
Pythium Blight of Cool Season Turf
Among the Pythium diseases, Pythium blight (a.k.a. ‘greasy spot’ or ‘cottony blight’) is the most straightforward to understand and control. Pythium aphanidermatum, and a few other species that cause blight, are most active under hot, wet conditions. Pythium blight is most feared because it can cause widespread plant death in 24-48 hours with little chance of recovery.
 Pythium blight damaging a creeping bentgrass tee during the overly wet and hot summer of 2010. The visual symptoms are small circular patches that have a distinctive orange color (the patches have coalesced to affect a large area). Photo: D. Settle, Ph.D., Bayer CropScience |
High night time temperatures and moisture favor the development of Pythium blight, and there are a few ways to predict outbreaks (some more scientific than others).
Forrest W. Nutter, Ph.D., developed a model in 1983 as a research assistant at Penn State which predicts Pythium blight is likely to occur when all of these conditions are met:
(1) minimum temperatures are above 68°F
(2) maximum temperatures are above 86°F
(3) the period of more than 90 percent relative humidity is greater than or equal to 14 hours
A rough (but useful) predictor of Pythium blight has been the 150 rule: when minimum daytime temperature plus maximum relative humidity is greater than 150, you are at risk for Pythium blight.
Personally, I like the country wisdom of NC State’s Professor Emeritus Leon T. Lucas who I will politely paraphrase as saying, “when it’s so hot and humid that your boxer shorts are stuck to your backside first thing in the morning, it’s time to treat for Pythium.” In any case, the gist of it is that hot plus wet equals Pythium blight, which equals rapid plant loss. So treat preventively to avoid this.
Pythium Root Rot
OK, now Pythium root rot is a little more complicated to understand. Why? Because it’s a complex interaction of root health and Pythium activity in the soil with summer stresses. In wet soils, there are a wide range of Pythium species that can infect roots, with some causing more damage than others. Combine this with the slowing or stopping of plant root growth under high soil temperatures and you get plants that are losing functional roots due to both environment and pathogen infection. In many cases, Pythium species isolated from roots are considered ‘weak pathogens,’ but even these can be significant if you have weak root systems. Under summer stress, plants with poor root systems are likely to fail, and prospects for recovery can be poor during hot weather.
Pythium root rot requires a more complicated approach for management. Establishing healthy roots and maintaining them is just as important as preventive fungicide applications that reduce the Pythium populations in the soil. A golf course superintendent does this through adequate spring fertilizer inputs, soil structure improvement, aerification and water management.
 Pythium root rot is harder to recognize. Affected turf may start out as chlorotic and thinned. Root mass is decreased, but may or may not have a rotted appearance. Microscopic examination is often the best way to confirm the presence of Pythium root rot. Photo: D. Settle, Ph.D., Bayer CropScience |
Bayer Solutions for Summer Pythium Diseases
Bayer has two products that provide two different avenues for Pythium control. Both should be integrated into preventive summer cultural and fungicide programs to ensure the best management of Pythium diseases. Chipco Signature has fosetyl-Al which is a potent preventive fungicide that protects against multiple Pythium species and also induces plant defenses against other diseases. The StressGard™ Formulation Technology in Chipco Signature also improves root health and plant vigor, which leads to more robust plants that can withstand more stress.
Banol contains the carbamate fungicide propamocarb that is effective as both a preventive and curative treatment for Pythium blight and root rot.
Resistance to Chipco Signature and Banol has never been documented, so they not only provide reliable control of Pythium diseases, but also provide robust rotation partners for other Pythium treatments where resistance development has happened or is a concern.
Always read and follow label instructions. More detailed information on Pythium diseases and their management can be found in the Bayer publication: Pythium Disease Solutions.
