Dealing with anthracnose basal rot can feel complicated, especially when combating resistance issues.But careful cultural practices and a varied approach to fungicides can control the disease.
Anthracnose, also known as Colletotrichum cereale, focuses on weakened or stressed turfgrasses, says Dr. Mike Agnew, Syngenta’s Northeast field technical manager. Poa annua and creeping bentgrass are at particular risk for infection, as it attacks the crowns of both plants.
The first stages of anthracnose are easy to miss, say Todd Hicks, The Ohio State University’s turfgrass pathology coordinator, and Joe Rimelspach, turf pathology extension specialist. Several factors that cause stress on turf can mimic similar symptoms, making them harder to spot before anthracnose progresses.
As the disease progresses and the crowns are damaged, symptoms of decline become more apparent. Turf starts to lose color with chlorosis, and the leaves may yellow, or even have an orange color starting at the tip until the entire plant is affected, Hicks says. There may be a general thinning of Poa annua or small spots and patches showing symptoms, as well as larger irregular areas covering sections of a green. “On creeping bentgrass, there is usually less yellowing and orange color, and more of a thinning of the stand and a brown appearance,” he says. “Infected plants have a blackening of the base and necrotic tissue.”
On Poa annua, look at the lower stem and crown for blackening and discoloration, and for bentgrass, look for discoloration and necrosis at the base of the plant and stolon, Hicks adds.
The reddish-brown discoloration and thinning will show in the summer to early fall, Agnew says. The fungus becomes dormant in cold weather, but “severely affected plants may turn yellow during mild winters or winter thaws.”
A quick diagnostic look at the stolon with a hand lens is one reliable way to determine whether turf is under attack by anthracnose or just heavily stressed, says Dr. Chuck Silcox, AMVAC Environmental Products product development manager. “One of the key diagnostic features that anthracnose produces is a spore structure called acervuli,” Silcox says. “It’s small, kind of globular and black. If you see that structure, then you know you’re dealing with anthracnose.”
While not always an option, one effective way to get ahead of anthracnose is to encourage the adoption of bentgrass putting surfaces because the turf type is more tolerant of the disease than many Poa biotypes, says Hicks.
Aggressive cultural practices should already be in place long before the disease strikes to lessen its impact, especially if the superintendent has gone rounds with anthracnose on the course before. Any course using highly susceptible grass, facing frequent weather patterns conducive to anthracnose or handling intensive maintenance of greens to meet golfer expectations should be wary, Hicks says.
Mowing heights should be maintained to at least .125 inches, and single or double rolling can be used to keep up green speeds, says Dr. Zac Reicher, Bayer Green Solutions team specialist. Use frequent sand topdressing that’s light enough to be watered in or brushed in with one pass.
Rutgers University research shows regular topdressing is an effective tactic, Silcox says. Originally, topdressing was thought to contribute to the disease, but “it was just the opposite,” he adds.
“It’s thought that the sand protects some of the vulnerable parts of the plant,” Silcox says. “It’s functionally like raising your mowing height, because you’re keeping the growing point of the plant covered. Frequent topdressing every two weeks is good management.”
Another option to manage ball speed instead of a low height of cut is double-cutting greens along with a rolling program, says Dr. Brian Aynardi, PBI-Gordon’s Northeast research scientist.
Even slight increases in mowing height can greatly reduce the severity of the disease, according to Rutgers’ guidelines. Using solid rollers as compared to grooved rollers at the same bench height setting could also be helpful, and rolling every other day can decrease disease severity.
Anthracnose preys on weakened turf dealing with excessively wet conditions or wilt stress. Manage excess moisture, running irrigation at 60 to 80 percent evapotranspiration rate to prevent moisture stress to already shallow-rooted Poa annua, Reicher says. Plant growth regulators can be used to control seedheads, manage growth and preserve carbohydrates for use later in the summer to limit anthracnose, he adds.
Research indicates PGRs are safe to use around turf affected by anthracnose, Silcox says. It even improves the turf’s ability to resist the disease by making it more low-mow tolerant.
Nutrition is also a key part of the maintenance approach, starting with regular doses of nitrogen through the summer to keep levels up, Silcox says. That comes out to about .1 to .2 pounds of a quick-release nitrogen about every two weeks.
Starting a soluble nitrogen program in April or May at about .4 to .8 pounds can build up nitrogen in turf as it heads into summer, reducing the severity of later anthracnose. Granular nitrogen fertilization should focus on the early months of the season at rates of 1 to 3 pounds to slow down anthracnose.
Potassium levels should also be monitored, and soluble potassium applications maintained at a 1:1 or 2:1 nitorgen (N) to potassium (K) molar adjusted-ratio every 14 days, according to Rutgers’ research.
“Maintain greens with a sound fertility program for healthy turfgrass. Soil tests may be helpful in determining details for the program,” Hicks says. “Do not under-fertilize the greens to achieve speed, but fertilize for turf health. In most cases, a combination of granular applications and applying light rates of soluble fertilizers to improve plant health is most successful.”
When an infection does happen, start by trying to relieve whatever could be causing stress to the plant to give it the best chance to fight back, Agnew says. “Anthracnose is a stress-related disease,” he adds. “Alleviating plant stresses will help with recovery. Therefore, make a slight change in mowing height and avoid severe wilt and excessive wet conditions.”
Use walk-behind greens mowers and divert traffic away from affected areas by moving cups frequently, Hicks says.
“All those management techniques you can utilize essentially sets your fungicide program up for success,” Silcox says.
Beating fungicide resistance
A superintendent’s fungicide program should start about a month before they expect symptoms to show up, with many researchers usually aiming between May 14 and Memorial Day, Silcox says.
“Since disease outbreaks can occur at different times of the year even within a small geographical area, keep records of when and under what conditions the disease occurs,” Hicks says. “Use that information as a guide for application timing.”
Another way to gauge when applications should begin is by soil temperatures, Reicher says. When 2-inch soil temperatures average 65 degrees for five consecutive days, it’s time to start fungicide applications.
Make certain fungicides are labeled for the control of anthracnose, and keep the pathogen’s location in mind. Since it’s located inside the crown, and many fungicides are only systemic upward or local penetrants, the fungicides must be placed at the base of the plant, Hicks says.
Aim for a fungicide application program of about 14-day intervals, and go in planning to rotate active ingredients regularly to cut down on resistance, Silcox says. “The challenge, then, is picking active ingredients that in your region have proven efficacy against anthracnose,” he adds. “Then what you want to do is use as many of those as you can in a program.”
Rutgers’research shows QoIs, DMIs, chlorothalonil, benzimidazole, iprodione, phosphonate, polyoxin-D and phenylpyrrole fungicides all as effective against anthracnose, though some might be more effective than others in different regions.
The goal is to not use any two active ingredients in a row, says Silcox, though it’s a good strategy to pair up two active ingredients together in each application to get the strongest effect. When rotating fungicides, it’s important to choose products that use different Fungicide Resistance Action Committee codes, as well as modes of action and use of phosphites, Aynardi says.
Even if fungicide resistance isn’t a problem in a particular region, that makes it even more important to get started on a multiple-approach program, Silcox says. “The counterpoint I make is that if you don’t have resistance, then you need to start using other products now,” he adds. “Not that you can’t use them while they’re still working, but don’t just continue to rely on them. We know what history is going to tell us: If you keep putting pressure on the pathogen population, you’re going to have resistance.”
It’s recommended to tank mix a multi-site fungicide while applying a single-site fungicide, Agnew says. Look at the active ingredients being paired, and match fungicides to get the most out of each application. If a superintendent is applying for anthracnose, they might as well also get coverage on other diseases, Silcox says. Plan for six to eight applications until anthracnose conditions are less favorable with shortening day lengths and less heat intensity. But continue to keep an eye on problem areas through the season.