An Overview of Aphanomyces Root Rot

Aphanomyces root rot is a significant disease in alfalfa production that can limit stand establishment in wet soil conditions and can be a chronic problem in established stands. Caused by the soilborne organism Aphanomyces euteiches, Aphanomyces root rot can be difficult to manage, especially since A. euteiches has a broad host range and can survive in soil for many years. There are multiple genetic variants of the causal organism, known as races, which also complicate disease management. Understanding and recognizing Aphanomyces root rot will aid in developing a successful management plan, especially in areas with a history of the disease.

Symptoms and Signs

Aphanomyces root rot typically results in stunted seedlings with chlorotic, purplish cotyledons and leaf yellowing (Figure 1). Roots will be poorly developed and discolored, turning from a water-soaked grayish brown to tan. Infected plants often have poor nodulation and in severe cases, dark brown lesions may be present on the taproot where lateral roots have died (Figure 2). Symptoms usually first appear in low areas of fields and after extended periods of rain.

Affected plants can be slow to resume growth after winter dormancy. Poor plant growth and reduced vigor can lead to gaps or patches in stand where weeds can establish and cause further yield loss and management problems. Loss of lateral roots and nodules result in stunting and yellowing of foliage due to nitrogen deficiency.

Disease Cycle

Aphanomyces root rot is caused by a soilborne organism known as an oomycete. The causal pathogen, A. euteiches, can survive for many years as thick-walled spores called oospores. These oospores can be found in soil or infected plant parts. In the spring, oospores can infect alfalfa seedlings directly, or they can produce a different fungal structure that produces “swimming” or motile spores called zoospores that can move in water through soil pore spaces to infect seedlings and roots (Figure 3).

Conditions that Favor Disease

Poorly drained, heavy, or compacted soils favor Aphanomyces root rot development. When soil is saturated, infection by A. euteiches can occur between temperatures of 60-80°F (16-28°C), although infection is more likely to occur when soil temperatures are between 70-80°F (24-28°C). Isolates of A. euteiches recovered from alfalfa can have a broad host range including other legume forages such as clover species, hairy vetch, peas, and snap and dry beans. Aphanomyces root rot of alfalfa can be especially serious in fields that have been cropped to peas.

Yield Losses and Impact

Aphanomyces root rot can reduce yield in both newly seeded and established alfalfa stands. When conditions are favorable after seeding a new stand, complete loss can occur. Reduced biomass may be more noticeable in established stands. The disease can reduce the longevity of a stand, resulting in the need for more frequent rotation to a less profitable forage crop.

Diagnosis

Aphanomyces root rot can be difficult to diagnose in the field and may require samples to be submitted to a diagnostic laboratory. Diseases and disorders that resemble Aphanomyces root rot are discussed below. There are also two known races, or genetic variants, of the pathogen that can cause disease on varieties with different genetic backgrounds and types of resistance. These races, race 1 and race 2, have been found across many alfalfa-producing regions in the U.S. and can be identified based on the alfalfa cultivars they are able to infect. Diagnostic tests are available in some University diagnostic laboratories that can distinguish race 1 from race 2.

It is important to know the race of A. euteiches in a given field, because race 2 can result in severe disease on varieties with resistance to race 1. Occasionally, Aphanomyces root rot can occur on alfalfa varieties with resistance to both race 1 and race 2. Research is underway to determine if additional races exist in different regions of the U.S., but additional races of A. euteiches have not been confirmed.

Aphanomyces Root Rot in Alfalfa

Diseases, Disorders, and Injury with Similar Symptoms

Diseases

Pythium and Phytophthora Root Rots

Pythium and Phytophthora root rots are also seedling diseases caused by oomycetes under wet soil conditions that result in stunted or dead seedlings with discolored roots. Affected plants can have yellow leaves and stunted growth. In some areas, Aphanomyces root rot and Phytophthora root rot occur together and can infect the same plant.

How to distinguish Pythium and Phytophthora root rots from Aphanomyces root rot: Pythium and Phytophthora can cause seedlings to wilt and die quickly after infection. Seedlings with Aphanomyces root rot may not die right away, and cotyledons will turn yellow and purple. Samples may need to be submitted to a diagnostic laboratory to distinguish among alfalfa root rots. On older plants, Phytophthora root rot results in dark rotted lesions that often girdle the root causing the end of the tap root to rot completely so that the roots have a “pencil point” appearance (Figures 4 and 5). Older plants with Aphanomyces root rot lack lateral and fibrous roots.

Disorders

Excess Soil Moisture or Water-Logged Seedlings

Water-logged soils or flooding can result in injury to seedlings, including poor root development, poor stands, and stunted seedlings.

How to distinguish water-logging injury from Aphanomyces root rot: It may be difficult to distinguish between water-logging and Aphanomyces root rot in the field. Samples may need to be submitted to a diagnostic laboratory to determine if the causal pathogen of Aphanomyces root rot is present in affected plants.

Nitrogen Deficiency

Plants with nitrogen deficiency have yellow leaves and can be stunted with poor nodulation.

How to distinguish nitrogen deficiency from Aphanomyces root rot: Plants with nitrogen deficiency will not have discolored roots or roots with brown lesions present.

Management

One of the most effective practices to reduce the impact of Aphanomyces root rot is to plant cultivars with race-specific resistance (Figure 6). Many commercial cultivars have resistance to one or both races of A. euteiches.

Where possible, improve soil drainage to reduce the length of time that soils remain saturated after rain events. Sub-surface tile can improve water drainage through the soil profile. Avoid field operations when soil is wet to reduce compaction.

Seed treatments with some fungicides may provide additional protection against Aphanomyces root rot when establishing alfalfa stands in high-risk areas, such as fields with poor drainage or a history of the disease. Although several seed treatments are available, it is important to note that the anti-oomycete active ingredients mefenoxam and metalaxyl, typically effective against Phytophthora and Pythium species, are not effective against A. euteiches. Seed treatments with the active ingredients pyraclostrobin or tolclofos-methyl have been shown to improve stand establishment when seeding a new alfalfa stand. Seed treatments only protect the seedlings for a short window (30 days at best after seeding), and do not provide long-term or multi-year protection. Contact your state Extension plant pathologist to get updated recommendations on seed treatments with efficacy against Aphanomyces root rot. Foliar fungicides are not effective against Aphanomyces root rot.

Acknowledgments

Authors

Kiersten Wise, University of Kentucky; Damon Smith, University of Wisconsin-Madison; and Deborah Samac, USDA-ARS, Research Plant Pathologist, St. Paul, MN.

Reviewers

Gary Bergstrom, Cornell University; Mary Burrows, Montana State University; Martin Chilvers, Michigan State University; Dean Malvick, University of Minnesota; Sam Markell, North Dakota State University; and Darcy Telenko, Purdue University.

Sponsors

The authors thank the National Alfalfa and Forage Alliance for their support.

Click the link below to access the CCA CEU quiz.

Overview of Aphanomyces Root Rot [CCA CEU Quiz]