An Overview of Anthracnose in Alfalfa

Introduction

Anthracnose is a serious disease of alfalfa, causing a crown and stem rot that reduces forage yield and stand longevity in susceptible cultivars. The disease occurs throughout most of the U.S. where alfalfa is grown, but is most damaging in humid regions of the eastern, Midwest, and southern U.S. The crown rot phase causes significant losses in California and Arizona where alfalfa is grown under irrigation. Anthracnose is caused by several fungal species, but is most commonly associated with Colletotrichum trifolii in the U.S. and C. destructivum in Canada. This disease results in distinctive symptoms on the stems and crowns of alfalfa plants. Five distinct genetic variants (called races) of C. trifolii have been identified that infect the crop. This is important to know because the races present in an area can affect disease management decisions. This publication provides information on anthracnose identification, disease development, and current management recommendations for farmers. 

Symptoms and Signs

The most important diagnostic symptoms of anthracnose are tan to brown, oval or diamond-shaped lesions on lower stems, which can elongate over time (Figure 1). Lesions have dark borders, and dark, round fungal fruiting structures (acervuli) with black “hair-like” growths (setae) present in the centers. These fungal structures are visible in lesions with magnification (Figure 2) and can be plentiful, giving the lesions a dirty appearance. Lesions can merge and girdle the stem resulting in wilted or dead shoots with a “shepherd’s crook” appearance (Figure 3). Stems with “shepherd’s crooking” retain bleached, dead leaves and are often scattered throughout affected fields. The fungus can spread internally from stem lesions into the plant's crown. Crown infections cause dead, bleached stems without stem lesions. Crown rot caused by C. trifolii may appear as blue to black discoloration in the woody crown tissue (Figure 4). Lesions are not common on leaves.

Disease Cycle 

The causal fungi of anthracnose survive in infected alfalfa residue and crowns. Rainfall, irrigation, and/or heavy dew disperse fungal spores from lesions to surrounding stems or leaves, where new infection occurs. Under optimal temperatures (64-86°F; 18-30°C) and moisture, new spores can be released every 7 to 10 days. In southern regions of the U.S., C. trifolii can remain active throughout the year. Colletotrichum trifolii can survive as a contaminant on harvested seed. However, the impact of seedborne C. trifolii on new infection cycles is not well understood.

Conditions that Favor Disease Development 

Anthracnose is sometimes referred to as “summer decline” because the disease is favored by warm (64-86°F; 18-30 °C), wet, and humid weather conditions. Stressed alfalfa stands are more prone to anthracnose crown rot. Excess water, poor fertility, insufficient weed management, improper cutting schedules, drought, and winter injury can weaken plants and increase susceptibility to this disease. Stands affected by anthracnose crown rot may not fully recover in spring. 

Impact and Distribution of Races

Anthracnose is a serious disease of alfalfa because the crown rot phase is often lethal, and affected fields with susceptible varieties decline quickly, often within two years. However, due to the widespread use of race 1 resistant cultivars, anthracnose symptoms have been observed sporadically in recent years. Of the five confirmed races of C. trifolli, race 1 is the primary race, and is widespread across the U.S. and Canada. Race 2 has been observed in Wisconsin, after being mostly confined to the mid-Atlantic states for many years. Race 3 and Race 4 have been confirmed in Oklahoma and Ohio, respectively, but have not been confirmed in other states. Race 5 has been confirmed in Wisconsin and Minnesota. 

Diagnosis

There are key characteristics that can aid in field diagnosis of anthracnose, including the bleached “shepherd’s crook,” the distinct fungal structures in stem lesions, and the blue to black discoloration in plant crowns. However, there are several crown rots of alfalfa, including Rhizoctonia root, stem, and crown rot (caused by Rhizoctonia solani), and co-infection of alfalfa crowns by multiple fungi and fungal-like organisms is possible (Figure 6). Submitting samples to a plant disease diagnostic laboratory is the best way to ensure a proper diagnosis.

Disease Management 

Planting anthracnose resistant alfalfa varieties is the most effective way to manage this disease. Most anthracnose-resistant varieties have resistance to both the stem and crown rot phases of the disease. Consult extension specialists in your state or province to determine which races might be prevalent and select varieties that are resistant to the races in your area. The National Alfalfa and Forage Alliance reports anthracnose (race 1) resistance ratings in the annual Alfalfa Variety Ratings publication. Due to the complex inheritance of disease resistance in alfalfa, highly resistant cultivars may contain a small proportion of susceptible plants.

Practice sanitation by cutting younger stands first, and clean harvest equipment before moving to non-affected fields. Frequent cutting can reduce pathogen spread, since infected stem tissue is removed when cutting. Infected plant material is not toxic to livestock and can be safely fed. Maintain fertility and manage weeds to avoid unnecessary crop stress. If crown rot causes stand decline, rotate away from alfalfa and clover for 2 to 3 years. 

Foliar fungicides are available to manage anthracnose. However, their efficacy is mostly unknown across major alfalfa production areas. 

Find Out More

Other publications in the Alfalfa Disease Management series are available on the Crop Protection Network website (cropprotectionnetwork.org).

Acknowledgements

Authors 

Kiersten Wise, University of Kentucky; Deborah Samac, USDA-ARS; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin-Madison; Darcy Telenko, Purdue University; and Richard Wade Webster, North Dakota State University.

Reviewers 

Kelsey Andersen Onofre, Kansas State University; Gary Bergstrom, Cornell University; Martin Chilvers, Michigan State University; Maira Duffeck, Oklahoma State University; Dean Malvick, University of Minnesota; and Jean Williams-Woodward, University of Wyoming.

This work is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2022-70006-38001).