Because of the dominance of alfalfa as a forage legume in New York State, disease management in alfalfa alone will be discussed. The same disease management strategies, however, can be applied to birdsfoot trefoil, clovers, and mixed legume-grass forage stands.
Diseases of alfalfa are common in our area, but as a rule they do not consistently cause severe losses within a single crop growth (cutting) period. Rather, they weaken plants over time, and along with other stress factors they contribute to reduced stand longevity. Infectious diseases contribute significantly to the death of alfalfa plants over the winter dormant period.
Management of alfalfa diseases in New York does not involve chemicals, except for limited use of fungicidal seed treatment, but relies instead on sound crop management and the use of varieties resistant to a few serious diseases. Diseases of importance include vascular wilts; root, crown, and stem rots; and foliar diseases. Check with your local Cornell Cooperative Extension office for recent fact sheets and other information available to aid in identification and control of forage legume diseases.
Vascular wilt diseases pose the greatest threat to alfalfa yields. These diseases result when the plant’s vascular (water-conducting) tissues are colonized by pathogens and become plugged. This prevents water and nutrients from moving into the shoots, and the plants wilt and eventually die. Varietal resistance is the single most important means of controlling vascular wilts. The bacterial wilt pathogen is endemic in New York soils, and all adapted varieties need to possess a high level of bacterial wilt resistance. Verticillium wilt occurred widely throughout New York State in the 1980s; it is advisable that any variety grown in New York possess at least a moderate level of Verticillium wilt resistance. Recent evidence indicates that Fusarium wilt may also be a problem in the state, but the need for resistance has not yet been established. Fortunately, most modern alfalfa varieties are at least moderately resistant to Fusarium wilt (Table 4.2.3).
ROOT, CROWN, AND STEM ROTS
Many pathogenic microorganisms cause disease in alfalfa by attacking the roots, crowns, or lower portions of the stem. These diseases interfere with water and nutrient uptake and/or translocation and thus result in stunting, yellowing, reduced dry matter accumulation, and premature plant death.
Phytophthora root rot causes significant losses during seedling establishment in wet, poorly drained soils. It can be controlled by a combination of strategies that includes selecting an appropriate site, planting resistant varieties, and following sound management practices.
The root and crown rot complex, which involves Fusarium fungi, is a debilitating and perhaps underestimated problem in alfalfa stands older than one year. Currently, sound management to minimize the effects of the disease is the only feasible control strategy.
Brown root rot was recently identified as a common contributor to poor overwinter stand survival following severe winters in New York, especially in northern counties. Research is currently being conducted to identify alfalfa cultivars that persist and yield well in soils infested with the brown root rot fungus.
Anthracnose is a fungal disease that attacks the lower stem and can girdle shoots; it also has a crown rot phase. Anthracnose can be severe in the warmer areas of eastern and southern New York, where it is controlled largely through the use of resistant varieties.
Sclerotinia crown and stem rot is becoming more prevalent in the state, but losses have been negligible. Disease incidence can be reduced by avoiding fall seeding, especially under minimum tillage. Where the disease develops, deep plowing of infected crop residues and long rotations between legume crops are necessary.
LEAF AND STEM BLIGHTS
Alfalfa foliage can be attacked by several fungal pathogens. Most foliar diseases are favored by wet weather and moderate temperatures. Severe leaf disease development reduces yield and nutritional quality of the forage and depletes the reserves of carbohydrate in the roots necessary for plant regrowth and dormant period survival. The effect of leaf diseases is minimized by maintaining a timely harvest schedule. Harvest should be delayed until a stage when root reserves have been replenished but should occur before full flower, when leaf spots tend to become more severe.
Foliar diseases of concern in New York include downy mildew, spring black stem and leaf spot, Leptosphaerulina leaf spot, common leaf spot, bacterial leaf spot, and Stemphyllium leaf spot.
The following disease management tactics help reduce losses from plant diseases when integrated into an overall forage management program.
Although protectant fungicide products are registered for use as seed treatments on small-seeded forage legumes, their general use in New York to prevent seed decay and seedling blights has not been supported by research findings.
The use of seed-applied metalaxyl (Allegiance) or mefenoxam (Apron XL) fungicides may be useful in the management of two fungal diseases in alfalfa, Pythium damping-off and Phytophthora root rot. Both diseases can cause serious problems in stand establishment for seedlings in wet soils.
Metalaxyl seed treatment affords some protection during the vulnerable early stages of plant development. Research has indicated that when metalaxyl seed treatment is used with alfalfa varieties possessing some resistance to Phytophthora root rot, its effectiveness against this disease is enhanced.
SELECTION OF DISEASE-RESISTANT VARIETIES
Forage varieties are heterogeneous populations of individual plants that vary within defined limits for many traits, including their reactions to diseases. Forage crops can tolerate a certain amount of symptom development and even loss of plants to disease before a significant yield reduction occurs. The response of alfalfa varieties to specific diseases varies from susceptible (less than 6 percent of plants having resistance) to highly resistant (greater than 50 percent of plants having resistance). The level of disease resistance required in a variety depends on the nature of the disease, the type of site where the forage is being grown, and the relative importance of various diseases in the local area. See Table 4.2.3 for disease ratings of alfalfa varieties sold in New York.
SOUND STAND MANAGEMENT
Stand management practices that limit the development and impact of diseases are basically the same as those recommended in the absence of serious disease problems. Any practice that reduces stress (biotic or abiotic) on the crop and promotes vigor helps extend the productive life of the stand. This becomes even more critical in the presence of serious disease organisms. The most critical management practices to consider include the following:
Site selection. Avoiding poorly drained soils can help reduce stand losses caused by Phytophthora root rot and other soilborne diseases.
Cropping sequence. Avoid planting forage legumes into fields recently cropped to legumes. Where diseases such as Verticillium wilt or Sclerotinia crown and stem rot have occurred, rotations of three or more years are advisable before replanting to forages.
Stand establishment. The use of establishment procedures recommended previously will result in a vigorous stand. Good seedbed preparation, weed control, adjustment of pH to 6.5 or above, and balanced fertilization are essential.
Sanitation procedures. To reduce the spread of pathogens between fields, harvest and perform other cultural procedures in young stands before doing so in older stands. Where infectious diseases such as anthracnose or Verticillium wilt are known to occur, remove debris from equipment before moving it to other fields.
Harvest schedules. A harvest schedule that allows for replenishment of root reserve carbohydrates is the most critical factor in stand management. Vigorous, nonstressed plants are best able to resist disease development. Harvesting before full bloom often reduces losses from leaf blights.