Anthracnose- Why is it so Difficult to Control?

Anthracnose affects high maintenance turf such as golf course putting greens. It used to be a summer stress disease primarily affecting foliage and would respond well to most fungicides when accompanied by cultural management. Now it frequently attacks crowns as well as leaves and is often unresponsive to fungicide applications. In addition, anthracnose is now commonly seen throughout the growing season - even in cool weather. What has changed?

Golf course turf management has changed dramatically in the past few decades. Golfers demand faster greens, so superintendents mow them very low, a practice that causes significant turf stress. The increased stress on turfgrass has also resulted in the common occurrence of the basal rot or crown rot stage of the disease. Once the fungus has infected the growing point of the plant, recovery is much slower, if it occurs at all. Although they do not increase anthracnose severity, plant growth regulators applied to keep turf short can also inhibit its ability to outgrow damage.

More intensive management schedules and an uptick in rounds of play result in turf stress due to higher traffic. This traffic can contribute to the spread of anthracnose as well because Colletotrichum cereale, the fungus that causes anthracnose, produces abundant sticky spores that can be easily carried to new areas on machinery and shoes. The disease is exacerbated by heat and drought stress, both of which have become more severe in some areas over the past two decades. Lastly, in addition to increased turf stress, fungicide resistant populations of C. cereale are now widespread.

Good cultural practices are imperative for the prevention and mitigation of anthracnose; in some cases, cultural management alone may be sufficient for this purpose. These cultural practices include:

• Increase mowing height during disease outbreaks and periods of heat stress. Increasing mowing height by as little as 0.1” can help decrease disease severity. Lightweight rolling or double cutting may be employed to help maintain green speed.
• Skip the clean-up pass during disease outbreaks and periods of heat stress.
• Aerate regularly - core aeration in spring and fall, spiking or hydroject during periods of heat stress.
• Manage thatch.
• Improve drainage where needed.
• Improve air movement through pruning of trees and landscaping.
• Minimize leaf wetness by early watering and whipping of dew.
• Syringe turf in the heat of the day with care to continue into the late afternoon when needed.
• Wash mowers thoroughly after use in areas where disease is active.
• Modify traffic patterns and move cups frequently.
• Maintain sufficient nitrogen and potassium levels.
• Maintain adequate irrigation for good turf growth. Overwatering and underwatering can both increase disease severity.
• A good spring and summer topdressing program helps decrease disease severity. (For some time, conventional wisdom maintained that topdressing increased turf susceptibility to anthracnose by causing small wounds; however, research has shown that this is not the case.)
• The disease is often troublesome on greens that are too small, poorly drained, compacted, and/or have poor air circulation. Extensive modification or rebuilding may be considered for chronically problematic greens.

When chemical control is required, it is best to combine a multi-site protectant such as chlorothalonil with a systemic/penetrant fungicide. Early applications while the disease is still in the foliar (leaf) stage are most effective. Strains of C. cereale resistant to benzimidazoles, QoIs (strobilurins), and/or DMIs are now common in some locations. Fungicide rotation is critical to prevent the development and spread of resistant populations. For a listing of fungicides currently labeled to manage anthracnose, please refer to the Disease Management chapter of UMass Extension's Professional Guide for IPM in Turf for Massachusetts: https://ag.umass.edu/turf/professional-turf-ipm-guide

Anthracnose is not always easy to diagnose. If you are unsure about the presence of this disease, please consult the UMass Extension Plant Diagnostic Lab for confirmation. Please see https://ag.umass.edu/services/plant-diagnostics-laboratory for more information.