A New Disease of Rough Bluegrass, Annual Bluegrass

and Perennial Ryegrass Identified in South Carolina
 

Bruce Martin, Ph.D

Plant Pathology and Physiology

Clemson University

      In the winter and spring of 2001, several golf courses in South Carolina experienced a devastating attack of their rough bluegrass overseeded bermudagrass putting greens. Superintendents indicated a patch disease that was not responding to normal fungicide applications was affecting their turf overseedings. Samples that were submitted for diagnosis indicated that the causal agent was not a common pathogen, explaining in part why fungicide applications had been unsuccessful. Unfortunately, the disease has been diagnosed in the fall of 2001 in overseedings of rough bluegrass on 12 different golf courses as of this writing. In some cases, the disease has also attacked perennial ryegrass on fairways, roughs or tees.

Symptoms of the Disease
 

     The disease followed a similar pattern of occurrence on the golf courses. Initial symptoms are small, yellowed spots in the rough bluegrass. Bermudagrass may or may not be dormant, but the bermudagrass does not appear to be affected. When the disease is active, these spots enlarge rapidly to patches up to a foot in diameter, but overall symptoms indicated variable sizes of affected areas. Spots and patches are roughly circular in appearance, with a chlorotic appearance. As disease progresses, rough bluegrass seedlings die altogether. Disease appeared to start in some areas in higher, drier areas of greens, affecting greens with good air circulation and exposure. However, once initiated, disease moves to lower and wetter portions of greens. Symptoms have indicated that mowers can “track” the disease when active. Margins of patches appear water-soaked (similar to an active infection of Fusarium patch), with the affected margin several inches in width (see Figure 1). Foliar mycelium is never present; in fact, mycelia are not visible microscopically either.

     Fungicide applications made to combat the unknown disease curatively, have mostly not been successful. Materials that have been applied include iprodione (as Chipco 26GT), mefanoxam (Subdue Maxx), ethazole (Koban, Terrazole), azoxystrobin (Heritage), chlorothalonil, mancozeb, propamocarb (Banol), and Aliette or Chipco Signature.

A Tentative Identification
 

     Dr. Larry Stowell, of PACE consulting in California, had been dealing with this disease on annual bluegrass putting greens and on rough bluegrass as a bermudagrass green overseeding. The suspected pathogen is a primitive fungus most closely resembling a Chytridiomycete fungus. These organisms are sometimes referred to as ‘chytrids’. Chytrids are abundant in the environment, and are associated with plants, animals, soil and water. Over 1,000 species have been described, and a few are known plant pathogens. Among the plant pathogens are Synchytrium endobioticum, the cause of potato black wart disease, and Physoderma zeae, the cause of brown spot disease of corn. In fact, a disease called Physoderma leaf spot and leaf streak, has been reported on turf (see Compendium of Turfgrass Diseases, American Phytopathological Society Press). However, the organism associated with this unknown turf disease does not fit the descriptions of causal agents of any known chytrid disease in turf.  

       Samples submitted to my laboratory are consistent with samples received by Dr. Stowell in California from annual bluegrass. Tissues are water-soaked in appearance. Leaf preparations examined under a powerful microscope indicate that leaf epidermal and mesophyll cells have abundant oval structures, that are colorless (hence hard to see) (Figure 2). These structures resemble sporangia formed by some chytrid fungi. In one case, a sample was incubated in my lab, and another spore stage formed apparently from maturing sporangia, into a rounded spore with a thickened cell wall. I have tentatively termed these spores “resting sporangia” until a more definite identification is made (Figure 3).

        Zoospores are formed as sporangia germinate in chytrid fungi. Although zoospores have not been observed with the structures associated with this disease, they are very ephemeral in nature. Zoospores are also formed by Pythium fungi, and are the tiny swimming spores these fungi utilize to disperse in films of water.

Host Range
 

       To date, the disease has caused severe damage to annual bluegrass putting greens, rough bluegrass overseedings, and in some cases, even perennial ryegrass. In a preliminary overseeding trial, I determined that rough bluegrass (3 varieties as single entries or blends) was severely affected in the trial. Also, Colonial and Velvet bentgrasses were affected, but a Chewings fescue and a slender creeping fescue were not affected. These data are very preliminary and subsequent trials are needed to confirm susceptibility and to develop best overseeding grasses to use where this disease has been a problem.

Predisposition by High Soil Salinity?
 

     In all cases diagnosed to date in SC, golf courses affected had poor irrigation water quality, with high levels of bicarbonate (raising pH to high levels) as well as high sodium concentrations. Also, and perhaps related, the disease has occurred in the late fall of 2000 and spring and fall of 2001, which have been exceedingly dry seasons. In many of the cases that have occurred in western states, high soil salts have also been present. The combination of highly saline irrigation water, and lack of leaching rainfalls leads to high salt levels in soil. However, it has been observed that not all golf courses with high soil salts has developed this disease, so judgement as to the importance of these observations will await scientific testing.

Chemical Control
 

     Curative fungicide applications made by golf course superintendents have been largely unsuccessful. In a preliminary replicated curative fungicide experiment, I found that typical fungicides effective for Pythium have not worked, including Subdue Maxx, Banol, Koban, Aliette Signature, and Heritage. In that trial, I also included the broad spectrum fungicides mancozeb, chlorothalonil, and iprodione, which also did not promote healing. In a separate trial, healing was promoted by Compass, and the experimental fungicide, Insignia. Interestingly, both Compass and Insignia (strobilurin fungicides) very effectively move within leaves (actually moving from one leaf surface to another). However, addtional experiments must be conducted to confirm these preliminary results. Some superintendents in the western states have had success with preventive applications of mancozeb, at the high rates and intervals for Pythium. These applications, according to Dr. Stowell, must be applied prior to symptom expression.

            We intend to continue to study this disease in all phases, including alternative grasses for overseeding, chemical control, role of environment (weather, salts, etc.), and the biological aspects of the fungus (spore production, spore release, survival, etc.). These data will be critical for management decisions.

First printed in the Carolinas Green, January-February 2002

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