Co-Authored with Dr. Maria Tomaso-Peterson.
I recently made a trip back to Mississippi and had an opportunity to catch up with my first major professor, Dr. Maria Tomaso-Peterson. It was great to discuss some of the new things she was seeing in turf disease diagnostics. One pathogen that has been steadily increasing in her diagnostic work is foliar pythium blight on ultradwarf bermudagrass cultivars (Fig 1). Isn’t this just the way it works? Many golf courses in the Southeast have converted to ultradwarf bermudagrasses to improve playing conditions and limit some of the stresses on the putting greens, and now a new pest comes to the forefront that is causing problems on these ultradwarf bermudagrasses.
I wanted to write about this subject for a couple of reasons (1) it is a new host for a pathogen we are all familiar with and (2) the leaf symptoms are very similar to Bipolaris leaf spot on the ultradwarf bermudagrasses, which may result in misdiagnosis. The leaf symptoms caused by Pythium spp. on ultradwarf bermudagrasses are well pronounced dark purple to black leaf lesions (Fig 2). A couple of techniques can be used to differentiate Bipolaris leaf spot symptoms from Pythium blight. First, look for those initial tiny, pinpoint lesions that result from Bipolaris spp. Infection (Fig 3). Second, look for symptoms on the youngest leaf material emerging from the sheath (Fig 2). Bipolaris spp. primarily affect older leaves located near the base of the plant. Pythium blight on ultradwarf bermudagrass cultivars exhibit similar dark, pronounced leaf lesions on the youngest leaf material as it is emerging. A hand lens or microscope would likely be necessary for field diagnosis, but visually inspecting the symptomology and location of lesion development could provide the correct diagnosis to better select an appropriate cultural strategy or fungicide application for managing the disease. If a compound microscope is available, a slide of symptomatic leaf tissue could be viewed for heavy oospore production. Turfgrass symptoms initially have a greasy black appearance that turn necrotic once the leaves become dry (Fig 1). Necrotic symptoms persist if the disease occurs during fall transition. Pythium blight symptoms associated with a spring epidemic will diminish as growth conditions become favorable.
Samples exhibiting these symptoms have already been arriving at the turf diagnostic lab at Mississippi State University this year. Many of these samples have come from the gulf coast region in the southeastern US. However, this disease is becoming more prevalent in most of the locations ultradwarf bermudagrass putting greens are grown. The environmental conditions that seem to favor this disease include moderate temperatures (60s to mid-80s) and high humidity brought on by frequent rainfall events. The occurrence of Pythium blight corresponds to the transition of bermudagrass in and out of dormancy. High nitrogen fertility during these periods of physiological stress may predispose the plants to infection. Newly established ultradwarf bermudagrass putting greens appear to be more susceptible to Pythium blight during the first two to three years.
Thanks to Dr. Maria Tomaso-Peterson for assistance with developing this blog post and providing pictures of symptoms included.