Spring dead spot moves in mysterious ways, and that’s why Nathan Walker is studying it by introducing fluorescent protein genes into the fungus and using ultraviolet light to illuminate the proteins in the fungus as it infects grass roots. He has used a confocal laser microscope to see how the fungus interacts with the plant itself in a study funded by the USGA. Even so, some mysteries persist.
What is known, and seen every year by turfgrass managers and golf course superintendents in certain parts of the country, is that the disease will manifest itself in the spring as ugly, rounded, bleached-white dead spots in the grass. Bermudagrass is the primary and most common host of the fungal disease, which occurs across the U.S. in the transition zone where warm-season grasses go dormant in the winter.
“Under the right conditions, all varieties of bermudagrass will get the disease to some level,” says Walker, the turfgrass IPM scientist at Oklahoma State University. He has focused on the disease for years as part of his research into IPM programs for many pest insects, diseases and nematodes of turfgrass. His findings are not encouraging. The disease is tough to eradicate, though efforts can be taken to manage it.
Ophiosphaerella herpotricha is the primary fungal species in Oklahoma, while Ophiosphaerella korrae is predominant in other parts of the country, Walker says. The results are the same though. The fungi kill bermudagrass and are widespread in Oklahoma, being the number one disease of bermudagrass in the region. Based on its common occurance there, O. herpotricha is probably native to the Midwest, and if found elsewhere was probably moved on turfgrass equipment or plant material.
As spring soil temperatures warm up, but are still below 70 degrees, the over-wintering fungus becomes active as bermudagrass breaks dormancy. Fortunately, the soil-borne disease progresses fairly slowly if left on its own, and the bermudagrass will be able to recolonize dead areas in the heat of summer. Much of the fungal activity occurs in the fall, but spring is when the actual dead spots really shine.
It is especially prevalent in highly managed bermudagrass that where nitrogen fertilizer is applied late in the fall. One of the best ways to avoid the disease is to not fertilize late in the fall as temperatures drop off. Another is to not move equipment from infected areas to safe areas without sanitizing it, and any infected soil or dead plants carrying the disease must be disposed of properly. Another way to discourage the disease is to grow the healthiest grass possible, which can better resist the fungus or recover quickly once infested.
“Provide adequate irrigation, for example, to maintain healthy plants,” Walker says, because unlike foliar diseases no correlation exists between irrigation levels and spring dead spot disease severity. It can occur in different soil types and pH levels. One of the best ways of combating it is to establish new turfgrass plots or renovate with the more cold-tolerant bermudagrass varieties such as Riviera or Patriot that are bred for success in the transition zone, if possible. Those varieties can also be infected, but tend to be more resistant to the disease. True identification must be done in a lab, but an identifying trait of the fungus is that it leaves stolons of the infected bermudagrass dead and blackened.
Once an outbreak occurs, chemical controls may be necessary. Fungicides are available, though timing of applications is very important. “If fungicides are to be applied, what we generally recommend are fall applications,” Walker says. It usually requires two applications, one just before the soil temperature at 2 to 4 inches reaches 70 degrees and a second one four weeks later.
Walker has tested many chemicals on O. herpotricha over the years, and the most efficacious ones were in the Fungicide Resistance Action Group 3. He has gotten positive responses from two applications of the demethylation inhibitors (DMIs) such as propiconazole (Banner MAXX), myclobutanil (Eagle) and fenarimol (Rubigan). All of these are in liquid form, to be sprayed onto infested areas. They also must be irrigated into the soil, since the fungus is a soil-borne organism that lives down at root level.
Another option for the second application, four weeks after the first, is to change to a thiothanate-methyl product (Cleary S 3336). Recent results have demonstrated this to be an effective strategy, but more research may be needed to substantiate this approach. This is also a liquid to be sprayed on and watered into the soil. Both of these applications will probably require a boom sprayer, because outbreaks generally consist of several or many dead spots over a wide area. Applicators should make sure they cover the diseased areas as well as surrounding edges where the fungi may also be active.
Walker has also tested spring fungicide applications in different combinations with and separate from fall applications, and has found that one spring application followed by a second application in the fall will do the job just as well as two fall applications.
Accurate treatment of damaged areas over a period of years is necessary to achieve suppression of spring dead spot, and one of the most useful tools for this is the digital camera. Walker says the dead spots may not be highly visible in the fall since the bermudagrass can grow in over the summer. Documenting these spots with a camera in the spring allows the applicator to make precise chemical runs in the fall.
Incorporation of a good IPM program using cultural and chemical controls will provide the best management results long term. He cautions to watch for weeds, which may grow in the middle of the dead spots and require separate treatment.
Even with treatments available, Walker says the control of spring dead spot is a dicey proposition. With so many variables-different bermudagrass varieties, climate variations, management inputs and fungus species, for example-control may be more or less difficult on different facilities and in different regions of the country.
“It can be a frustrating disease for many,” he says, because control is difficult and the spots appear just when everyone wants their facilities to look their best. Eradication may or may not be possible, but management is feasible with persistance.
Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.