January 31, 2017
When I ask most farmers about disease management in alfalfa, first thoughts are usually about foliar fungicides, followed by why a stand winterkilled, and sometimes why the new seeding failed. That response is understandable, relative to what they hear with marketing of fungicide products, and see when it’s time to rotate an old stand or establish a new one.
However, as with many crops, disease management in alfalfa clearly starts with variety selection. The second line of defense is to minimize stress using appropriate management practices. These include adequate soil pH and fertility, proper stand establishment practices, timely harvest management during both in-season and fall, wheel traffic management, insect control, and possibly the use of foliar fungicides.
Variety selection decisions
Every year the National Alfalfa and Forage Alliance provides a publication listing most of the alfalfa varieties, rating their winter survival, disease resistance, insect resistance and other traits. The disease resistance information is on wilts and root rots. Both the Winter Survival and Fall Dormancy ratings are important factors to consider in selecting alfalfa varieties to fit your management goals. The NAFA publication is available at alfalfa.org/varietyratings.php.
Pay particular attention to ratings for root rots: phytophthora and aphanomyces. These soil-borne pathogens are common and long-lived in Iowa soils, with a higher prevalence in wetter, heavier, more poorly drained soils. Aphanomyces and phytophthora root rot may kill seedlings, reduce yield, decrease stand density and shorten stand life. Iowa State University routinely recommends selecting alfalfa varieties resistant to both of these diseases for planting in Iowa.
Study the yield trial results
Multiple-year yield trials from universities and private companies are excellent sources of information to complement the varietal characteristic ratings mentioned above. Unfortunately, fewer universities conduct alfalfa trials these days. Those with current variety trials include:
• University of Wisconsin
• University of Minnesota
• Michigan State University
• Ohio State University
• Penn State University
• Cornell University
Comparing multiple years of yield data is critical. Varieties that yield in the top 10% of local trials for the three to four years of the trial imply an adapted variety with excellent stress tolerance, rapid regrowth and winter survival. However, you should still pay attention to variety disease resistance and soil characteristics.
Many yield trials are only conducted on well-drained soils and may not challenge a disease susceptible variety in less-than-optimal soil conditions. Most university recommendations are to plant an alfalfa variety that has high resistance to all wilts and root rots, which are listed in the NAFA ratings. However, many varieties still lack high resistance to aphanomyces race 2, as assessment of the significance of this root rot disease is somewhat recent.
Establishing an alfalfa stand
To minimize disease problems with alfalfa establishment, consider the following recommendations.
Use fungicide treated seed when planting. Varieties resistant to some diseases such as phytophthora root rot do not express the resistance until later in the seedling stage, so the fungicide seed treatment mefenoxam (Apron) is commonly used to provide protection from germination through the early growth phase. This fungicide also provides protection against pythium, common in Iowa soils and of greater concern with higher soil moisture and cool temperatures. These early season seedling disease complexes are often referred to as “damping off.” However, mefenoxam does not provide early protection against aphanomyces. Recently a fungicide named Stamina, which protects seedlings against aphanomyces was labeled for use as an alfalfa seed treatment and is being used along with Apron. For organic production, the main alternative at this time is to delay planting until soils are warmer, or plant alfalfa in late summer. A future option has current research studying a natural mineral seed coating (called zeolite). This product is showing strong activity against seedling alfalfa pathogens.
Do not plant too early in spring. Alfalfa seed can germinate at temperatures in the 40 degree Fahrenheit, thus planting in April is common. However, without a fungicide seed treatment, planting should be delayed until soil temperatures are in the 50s and are increasing. Farmers using an oat nurse crop may be tempted to plant earlier, as oats can germinate at soil temperatures in the mid-to-upper 30s. Oats for grain production should be planted as early as possible and at a full seeding rate, but oats as a nurse crop in alfalfa establishment should be planted at about half of a full seeding rate and at a soil temperature best suited for alfalfa.
Always achieve the “Big 3” with planting operations: proper seeding depth; good seed-to-soil contact; and uniform seed distribution. Seed alfalfa at ¼ to ½ inch deep in loam soils, and ½ to 1 inch deep in sandy soils. Always achieve good seed-to-soil contact, whether with press wheels or a cultipacker. Rapid germination and emergence minimizes chances of having seedling disease issues.
Disease management and soil fertility
Nutrient deficiency is a non-infectious disease that can also stress plants and increase their vulnerability to infectious root, crown and leaf diseases. Good alfalfa management requires proper soil pH, and adequate amounts of phosphorus (P), potassium (K) and sulfur (S) fertilizer. Recommendations are provided in ISU Extension publication PM-1688, “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa,” and in publication CROP-3072, “Sulfur Management for Iowa Crop Production.”
Over the last decade ISU Extension education programs have practically beat-to-death the need for sulfur fertilizer in alfalfa production. However, I am still seeing fields that are sulfur deficient. The S soil test is not reliable for determining S fertilizer recommendations, but plant analysis testing works well. For recommendations, refer to the ISU publication “Sulfur Management for Iowa Crop Production.”
ISU does not provide recommendations for boron in “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa.” Boron (B) soil fertility trials have not achieved a consistent yield response to B fertilization of alfalfa, thus a reliable correlation of B soil test level to yield response from fertilization is not possible. The two most recently completed boron trials in Iowa (2014 and 2011) are available at ag.iastate.edu/sites/default/files/FoliarMicronutrients.pdf and farms.ag.iastate.edu/sites/default/files/EffectSulfurBoron.pdf.
Apparently, adequate soil boron levels existed in the 3% to 4% organic matter loam soils in which these trials were conducted. Alfalfa production on lighter, sandy soils may still require application of boron fertilizer.
Scouting to correctly identify diseases
Your crop scouting notes from previous seasons help identify the most prominent reoccurring diseases. For visual identification of alfalfa diseases, NAFA publishes the Alfalfa Analyst. The most common leaf diseases in Iowa are spring black stem, common leaf spot, lepto leaf spot, downy mildew, stemphylium leaf spot, summer black stem and bacterial leaf spot.
These first three leaf diseases are the most common that are found in northeast Iowa, followed by much lower occurrences of stemphylium leaf spot in summer and downy mildew in spring. While the NAFA publication Alfalfa Variety Ratings does not provide disease ratings for leaf diseases, individual seed companies may have information on some of their varieties.
A comprehensive alfalfa disease management program starts with variety selection and growers studying yield trial results. Other best management practices that minimize stress, strongly complement the program. Proper soil fertility and stand establishment, timely harvest management during the growing season and in the fall, controlled wheel traffic, insect pest management, selective use of foliar fungicides, and some luck to avoid significant weather events, are all interdependent in a successful disease management program. It is no accident that the same BMPs recommended for maximizing alfalfa production also minimize plant disease issues.
Lang is the ISU Extension field agronomist based at Decorah in northeast Iowa. Contact him at [email protected].
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