By Angela Orshinsky, Ph.D.
Current management practices
Dollar spot levels can be reduced by maintaining an adequate level of nitrogen in turf stands, removing morning dew, promoting air-flow, and choosing more resistant species and cultivars of turfgrasses. However, under severe disease pressure fungicides are necessary to keep dollar spot levels in check. On many golf courses, fungicides are applied bi-weekly from spring to fall. This high level of fungicide exposure has lead to the development resistance by the fungus to a variety of chemical classes. The high prevalence of fungicide resistance in dollar spot fungus populations has created challenges in maintaining disease-free turf.
Research on alternative cultural and biological control strategies has not yet provided fruitful results. Some reasons for the failure of biological controls in the turfgrass system may include the complex microenvironment on turf as well as a lack of understanding about the biology of dollar spot disease. Trying to develop integrated biological and cultural control strategies without knowing the biology of dollar spot disease is like driving through an unfamiliar city without a map. We can only make educated guesses about what might work. Without knowing more about dollar spot disease biology and how the fungus causes disease, we cannot explain the inconsistencies of biological and cultural management tactics that have been tried or assess their real value. So the question is: What do we need to know in order to develop more efficacious recommendations?
Figure: The disease triangle for dollar spot. There are still many fundamental questions about the biology of dollar spot disease that still exist even after nearly 90 years of research on the disease.
We live in a great time for science. Advances in genetic research have made it relatively affordable to get large datasets in a short period of time. At The Ohio State University turfgrass pathology program, we have made use of a technology called massively parallel sequencing to obtain millions of pieces of genetic sequence data from both the dollar spot fungus and a creeping bentgrass host. By putting the sequences together, we have put together a snapshot of what is happening in both the dollar spot fungus and in the creeping bentgrass host four days after inoculating the grass. We have been able to identify genes in the plant and the pathogen that are active during disease. So what have we learned?
Our research has demonstrated that S. homoeocarpa produces a large number of enzymes that break down grass cell walls, enzymes that are important for detoxifying plant defense compounds, and transporters that may play a role in fungicide resistance. The creeping bentgrass responses at four days after infection suggest that creeping bentgrass is likely using a defense strategy called the Induced Systemic Response.
It may be hard to imagine how one could possibly apply this treasure trove of molecular data into useful solutions for dollar spot management, but some new ideas have emerged from this project. For example, it may be possible to clone or use standard breeding programs to increase naturally-occurring xylanase inhibitors found in related plant species in creeping bentgrass. This may increase the resistance of the grass species to dollar spot disease. The information will also help researchers to understand the function of plant defense activators. Plant defense activators are much like vaccinations. When a plant defense activator is sprayed onto a plant, the plant immune system is activated, helping the plant to establish defenses before pathogen attack. By monitoring the strategies used by creeping bentgrass to defend itself against pathogens, it will be easier to determine which defense activators will work best, and better methods of applying them. For example, we will determine the cultivars that will work best with defense activators and the best timing to apply these chemicals relative to disease-promoting environmental conditions.
New technology is not cheap
More and more turf pathology laboratories are turning to high-tech options as it is often the most efficient means of answering some of the most fundamental questions about the causes of and solutions to plant disease. Many of these technologies have been used effectively to find disease management solutions in other cropping systems such as soybeans, corn, wheat and various fruit and vegetable crops. Unfortunately, these technologies are not cheap, and specially trained scientists are required to both perform the experiments and analyze the data. If turf pathologists are to make significant breakthroughs in the management of turf diseases, it will be important to have significantly more support. By working together, academia, industry and government cooperators may be able to use many of these new technologies to answer some of the most basic questions we still have about the biology of dollar spot disease. Only by finding answers to these fundamental questions can we develop more consistent and effective integrated management solutions.
A): Dollar spot on a Kentucky bluegrass lawn. (B): A typical hourglass-shaped lesion on fairway-cut creeping bentgrass. (C) mycelium growing from dead, diseased creeping bentgrass to infect fresh, green leaves. (Photo credits. A. Joe Rimelspach, B. and C. Angela Orshinsky).This article will be appearing in the green industry newsletter of Michigan, The Landsculpture.