Fighting alfalfa diseases

ZUMBRO FALLS, Minn. — Whether it is planting the right variety of alfalfa, choosing a seed treatment or not, selecting a planting date, or determining the harvest window, choices impact the alfalfa maladies dairy farmers fight.

Dr. Debby Samac, who has 30 years of experience with the Agricultural Research Service researching alfalfa diseases for the U.S. Department of Agriculture, spoke on growing healthy alfalfa Aug. 5 at the Southeast Minnesota Forage Field Day hosted by Hyde Park Holsteins near Zumbro Falls.

Samac’s presentation included a discussion of several major alfalfa diseases and pathogens, and steps for growing healthy alfalfa in the face of these problems.

One of the first challenges of growing alfalfa is diagnosing diseases. Samac said producers need to consider whether the unhealthy alfalfa is a result of poor fertility, disease, insect feeding, or herbicide carryover. She said some corn herbicides can carry for 18 months. Symptoms of this problem can look like poor germination or curled or yellow leaves.

One of the main diseases Samac presented on was Aphanomyces Root Rot. Symptoms of Aphanomyces included thin, poor stands; yellow, dead seedlings on top of the soil; and adult plants with carrot-like roots.

Aphanomyces, common in the Midwest, is a pathogen that builds up in the soil. Tests can be run by plant disease clinics to determine if the pathogen is in the soil.

Aphanomyces can be activated in wet soil, which causes the spores to germinate. Germinated spores create a different type of spore that can swim through wet soil and infect an alfalfa root.

“It is fast; in 15 minutes, they get to the root (and) already cause damage,” Samac said.

There are several management tools for Aphanomyces. One tool is using a seed treatment such as Stamina.

The best management tool, however, is planting resistant cultivars, Samac said. There are multiple types of Aphanomyces, so producers should choose seed varieties resistant to Race 1 and Race 2 Aphanomyces. Samac said most alfalfa seed is resistant to Race 1, however, Race 2 resistance is a little less common.

Farmers can use the National Alfalfa and Forage Alliance booklet, which lists the resistance of many cultivars as a reference.

Other alfalfa diseases that producers must be mindful of are Pythium Seed Rot and Phytophthora Root Rot. These can also be addressed through seed treatment. Samac said Apron or Apron XL protects effectively against Phytophthora and partially against Pythium.

“These seed treatments only last about 30 days while the seedling is germinating,” Samac said. “After that, these plants are still going to be susceptible to these pathogens. So the real best way to protect them is through resistant varieties.”

Samac said overall investment in quality seed is important for healthy alfalfa.

“As a plant pathologist, I’ll tell you, good seed is a good investment,” Samac said. “Relative to other inputs, seed is cheap. … Brown bag seed … is a big gamble. You may get great seed, but you may get really bad seed.”

Some issues with brown bag seed, Samac said, include unknown disease resistance, potentially old or poorly germinating seed, and the potential of introducing weeds or diseases.

Another alfalfa disease Samac discussed that is especially an issue in cool months is bacterial stem blight. This disease causes widespread decline in the first harvest. Symptoms include yellowing between leaf veins and eventual leaf loss.

“It has this unique ability to accelerate frost damage,” Samac said. “Once you get some cold temperatures, you get a wound in the leaf, and the bacteria get inside, and then it starts disease. It starts rotting the leaves. It gets into the water-conducting vessels, into the vascular tissue, and gets all through the plant.”

The life cycle starts when the bacteria, which are very light, splash up from the soil or are brought by rain. Bacteria will then multiply on the foliage. If the plant goes through frost in the cool months while growing, the bacteria enter through frost damage areas, where they cause blight and stem lesions before getting into the vasculature of the plant. Once they have fully infected the plant, the bacteria are lofted back up into the atmosphere and travel somewhere else to infect other plants.

Growing healthy alfalfa, Samac said, starts with good soil drainage, which helps to alleviate pathogen issues that thrive in wet soils. Second, good, resistant cultivars are a must. Seed treatment of at least Apron is recommended. Samac said growers also should wait to plant alfalfa until soil temperatures are at least 50 degrees.

“(It is) really tempting to get out into the field pretty early, but if the soil is below 50 degrees, that’s just going to make it really favorable for the seed to rot,” she said.

Rotating alfalfa every three years is another tip Samac gave to keep alfalfa pathogens from building up in the soil. Finally, she said farmers should plan ahead on their crop rotation to avoid herbicide carryover.

Another, easier managed disease, Samac presented on is Foliar Leaf Spots. This disease has a life cycle of 25 days and can become a problem if alfalfa is not cut within 30 days.

“If you have a really heavily affected stand, you can harvest it early to minimize leaf loss,” Samac said.

Samac said there are good foliar fungicides for the disease, but these must be put on before symptoms appear. The pathogens persist in the crop debris, so the problem cannot be fully eliminated.

Samac wrapped up her presentation talking about something that many producers mistake for disease: root lenticels. Root lenticels are involved in gas exchange and are more likely to form in heavy, water-saturated soils.

“You see these really scary looking, white, fluffy things on the root … (but actually) it’s a natural part of the (plant),” Samac said.