New Developments in Managing Alfalfa Diseases and Salt Tolerance

• Mar 01, 2018

Alfalfa diseases such as aphanomyces root rot and anthracnose, as well as heavily saline soils, are common in different parts of the U.S. Each can negatively impact alfalfa growth, as well as return on investment and yield potential for farmers.

Researchers have made great progress in devising new genetic traits and fungicides to help manage these troublesome threats. Here is a quick snapshot of where the industry is in terms of research and what is currently in the pipeline.

Aphanomyces Root Rot
Aphanomyces root rot is a soilborne disease that causes stunting, reduced nodulation and poor root development in seedlings. It is commonly found in soils that are saturated, poorly drained, compacted or have limited water dispersal.

Aphanomyces race 1 is found across the northern half of the U.S. Fewer surveys have been done for race 2, but it also appears to have a similar distribution and may be more common than race 1. Topography doesn’t figure prominently into where the disease sets up, as aphanomyces is found in alfalfa grown on hillsides and in low-lying areas, and in both heavy soils and silt loams.

Race 1 aphanomyces is both well-controlled by resistant varieties of alfalfa seed, and there is a growing number of varieties with high resistance to race 2 aphanomyces currently available. Planting varieties with this type of resistance is a good insurance policy for alfalfa plants, whether at establishment or on established stands. If you are a farmer in Wisconsin, Minnesota or Iowa, where soils tend to be heavier, you probably know that you are going to be dealing with aphanomyces at some point.

And because aphanomyces is an evolving disease, new races are still being discovered. Each discovery puts into motion the painstaking process of breeding for resistance.

“There are probably many races of aphanomyces out in nature – at least that is what my research, as well as that of researchers at the University of Wisconsin, seems to point to,” says Deborah Samac, Ph.D., USDA-ARS in St. Paul, Minnesota.

Having multirace resistance to aphanomyces can help you, throughout the life of the stand, to achieve optimal yield potential and persistence. If you walk into an alfalfa field and half the plants are gone, it’s not hard to do the math to figure out if it was worth the investment to purchase a variety with good resistance.

Additionally, some alfalfa varieties come with a fungicide applied to the seed for aphanomyces. This buys you some time, even if you are planting in areas with heavy aphanomyces pressure. That is, a fungicide could provide resistance for 45 to 60 days after planting. That gives plants the opportunity to emerge and be healthy and vigorous, but after that period of time, you’ll rely heavily on the in-seed resistance that’s already in the plant.

Anthracnose
A severe stem and crown disease that causes defoliation, anthracnose is primarily found in eastern portions of the Midwest, and in the East and South.

The majority of alfalfa seed currently on the market has high resistance ratings for race 1 anthracnose. Race 2 anthracnose has been found in isolated areas of the U.S., including on the East Coast and in Wisconsin. Researchers have also identified additional races of anthracnose and are fine-tuning resistance mechanisms.

“A few years ago, new races of anthracnose seemed to be appearing in the Midwest,” Samac says. “I isolated the pathogen from the samples I received, and they were, in fact, novel races different from race 1 and race 2.” Today, work is underway to develop resistance to these novel races, she says.

Most alfalfa seed is sold as having high resistance to race 1 anthracnose. More information about identification of new anthracnose races is anticipated in early 2018.

Fortunately, farmers can use certain foliar-applied fungicides to help manage anthracnose. Many alfalfa seed varieties offer treated seed containing a fungicide, which helps with alfalfa root diseases such as pythium and phytophthora, but not anthracnose. As with any foliar fungicide, application timing is critical. Your agronomist will be able to advise you on the best time to spray for anthracnose.

Salt Tolerance
Soil salinity issues continue to increase across the U.S., and more farmers are seeking salt-tolerant alfalfa varieties to lessen the impact of excessive salt on alfalfa production.

Standard greenhouse tests that check for salinity levels in the soil include a one-week, salt germination test that measures alfalfa seed development under moderately high salt conditions and a second greenhouse test lasting six months that measures forage production potential under saline conditions.

These tests are short in duration and lack the interaction with stresses typical of saline soils. Plus, they are done in a greenhouse, not in the field.

To better understand the complexity of alfalfa growth and development in saline soils and to validate the effectiveness of the greenhouse salt test, our company has identified field testing and selection locations that typify this combination of stresses. Field locations currently include southeast Washington, California and Kansas. 

Greenhouse testing and in-field testing each have their place in finding better ways to grow alfalfa in soils with high salinity. It is necessary to have both to ensure exceptional on-farm performance.