Resistance is (not) Futile

Penncross is the best-known creeping bentgrass, and historically this durable plant has been the standard by which all other varieties of creeping bentgrass have been measured. A staple of golf courses, Penncross is susceptible to many of the diseases afflicting turfgrass, including dollar spot, brown patch, pink snow mold and take-all patch. Superintendents and green keepers wage war with these destructive maladies day in and day out, but on this battlefront there have been positive developments in recent years. Newly cultivated bentgrasses have shown improved resistance to many traditional turfgrass afflictions, and for golf courses, this is very good news.

“The newer varieties of bentgrass tend to be more disease-resistant,” says David Willmott, assistant superintendent at Sharon Golf Club in Sharon Center, Ohio. “There are other advantages to these new breeds of grass, as well. They're smaller and grow tighter, both of which increase green speed and improve drought tolerance. Ironically, this advantage is also a disadvantage. You're dealing here with a smaller plant, and that means more plants per square inch producing more thatch. The plants grow upright instead of laterally, which makes it tougher for them to recover from mechanical injury.”

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Level and type of disease resistance vary from variety to variety. All of the grasses of the A&G series, for example, enjoy partial resistance to pink snow mold, while L-93 under certain conditions exhibits less dollar spot than Penncross. This is not to say that all the latest bentgrass varieties are more or even equally resistant to all diseases than Penncross. Some are, some aren't. The greatest virtue of the new bentgrass cultivars may be their tolerance of very low mowing heights, which translates into better-quality turf.

“No variety of bentgrass is completely disease-resistant,” says Dr. Paul Vincelli, extension professor for the department of plant pathology at the University of Kentucky (U.K.) at Lexington. “Some varieties do exhibit partial resistance, which is to say that they show less disease under uniform conditions than standard Penncross. Conversely, there are occasions when some creeping bent varieties are actually more disease-prone than Penncross. One example is Crenshaw, which has a weakness for dollar spot, and is therefore not recommended for golf course use here in Kentucky.”

DISEASE REACTIONS

The latest breeds of creeping bentgrass include A-1, A-2, A-4, G-1, G-6, L-93, Crenshaw, Backspin and Providence. Most, if not all, have proved to be practical alternatives to Penncross on golf courses in at least one respect: Their much-improved density and texture make them ideal for use on putting greens. Some also are appropriate for fairways and tees. Disease resistance is another matter, and testing in this critical area has brought mixed results. According to Vincelli, L-93 and Penncross demonstrated roughly equal susceptibility to dollar spot under most conditions. The results were about the same for brown patch, but against pink snow mold and microdochium patch, the results were a little more encouraging.

In the same tests, the A and G series proved equally susceptible to dollar spot as Penncross. The single exception was A-1, which occasionally proved significantly more resistant to the disease. A-4, meanwhile, sometimes showed more dollar-spot reactions in locales outside of Kentucky. Testing also showed A-4 far more susceptible to brown patch compared to Penncross, and A-1 and A-2 demonstrated this same weakness.

All the G series varieties, meanwhile, proved more resistant to pink snow mold and Microdochium patch than Penncross. G-6, in particular, showed a weakness to these diseases, as well as to take-all, to which it was as vulnerable as Penncross. The results were confirmed by tests in Kentucky and elsewhere. The other new bentgrasses also proved disease-prone to greater or lesser extents. Crenshaw, which was bred to thrive in summer swelter, showed a particular susceptibility to dollar spot, and Backspin demonstrated the same tendency.

“Crenshaw was developed under hot, dry conditions to withstand summer heat stress better than previous varieties,” wrote Vincelli. “Unfortunately, that variety was also selected under conditions of low dollar-spot pressure compared to what we experience in Kentucky. Consequently, Crenshaw is significantly more susceptible to this important disease than Penncross. This is a distinct disadvantage: Use of this variety under our conditions would mean increased need for fungicides (keep in mind that more fungicide is used to control dollar spot than any other turfgrass disease) as well as increased risk of fungicide resistance. There are other new varieties with unusually high susceptibility to dollar spot besides Crenshaw, including Backspin.

DISEASE CONTROL STRATEGIES

With the new, disease-resistant cultivars, golf course superintendents can limit fungicide use on greens, claims Jack Fry, Ph.D., professor of horticulture and turfgrass management at Kansas State University (Manhattan, Kan.). Using dollar-spot resistant cultivars allows for significantly more options on the part of superintendents, he says, which makes a curative strategy feasible rather than having to rely exclusively on a preventative program enacted every 14 to 21 days. But using a disease-susceptible cultivar often eliminates such options, he cautions, and the number of applications required will differ based on the resistance of the cultivar.

Fry's findings were based on a study he conducted with a team of colleagues between 1997 and 1999. The site of the experiment was a green constructed to USGA standards at Rocky Ford Turfgrass Research Center in Manhattan, Kan. Four cultivars were tested: Penncross, L-93, Crenshaw and Providence. Each was cultivated on a plot measuring 3 × 7 feet, and subjected to regular applications of fungicide in dosages recommended by the manufacturer. The researchers used different fungicide strategies on the cultivars, including a preventive schedule and curative schedule, according to Fry. Mowing and watering duplicated that of normal golf course conditions. The results indicated that curative fungicide applications suppressed dollar spot and brown patch. However, Fry says that the curative approach is not ideal for brown patch because by the time you can see symptoms of this disease, the damage has already occurred (resulting in 5 to 10 percent injury of turf). Therefore, a preventive fungicide schedule is preferable to avoid unsightly damage.

“Dollar spot injury increased gradually over time, whereas brown patch developed rapidly and often covered more than 10 percent of the plot area within 24 hours of initial symptoms,” Fry says. “Injury resulting from brown patch also persisted for several weeks after the initial infection because bentgrass recovered slowly during high summer temperatures. This work showed that superintendents who select disease resistant bentgrass cultivars have more options available to them as they develop disease control strategies. For example, L-93 exhibited good turf quality when a 28-day preventative fungicide program was employed, and also performed well when chlorothalonil was applied preventively at low rates every seven days or curatively at label rates when brown patch was not active.”

Fungicide is a common bentgrass disease-control strategy, Vincelli adds, especially for plants under high pressure from dollar spot. Dollar spot, which is caused by the fungus Sclerotinia homoeocarpa, is one of the most common afflictions of bentgrass. As it is also one of the most destructive, more fungicide is directed against it than any other common bent grass ailment. Dollar spot control may call for cultural practices as well, including the removal of leaf wetness in the morning to retard the growth of fungus. Early winter fertilization is a good idea as well, he continues, along with summer time “spoon-feeding” programs consisting of light applications of nitrogen instead of heavy ones.

“The results of Dr. Fry's study suggest that less fungicide may be required to control dollar spot on the more resistant cultivars, and that is a very encouraging result,” he says. “Similar studies should be done in the areas of the country suffering with the most dollar-spot pressure, but fundamentally Fry's research seems very sound. I would strongly advise interested parties to log on to the National Turf Grass Evaluation Program's Web site at www.ntep.org. The site contains many useful reports of similar studies, and how the various varieties of creeping bentgrass performed using different testing criteria in different areas of the country. The data on this site will prove very beneficial to anyone with a professional interest in turf, or who works with it everyday.”

D. Douglas Graham is a green industry freelance writer who lives in Manchester, Mo.

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