Diseases - Alfalfa

Part 2, Section 6: Forages Pest Management

Forages Pest Management

DISEASES

Alfalfa

The following management practices will help minimize disease losses in alfalfa. Most of these recommendations apply to other legumes as well.

• Use the best-adapted, disease-resistant varieties.
• Do not plant alfalfa in poorly drained fields. Red clover, birdsfoot trefoil, or a grass species is a better choice. In fields that are moderately to poorly drained, Phytophthora- and Aphanomyces-resistant varieties should be used along with fungicide treated seed and/or Ridomil Gold (mefenoxam) or MetaStar (metalaxyl) soil treatment.
• Use a cereal, corn, or grass crop for at least 2 years in rotation with alfalfa.
• Keep soil pH, phosphorus, and potassium at optimal levels for crop growth.
• Because they interact with diseases, control leafhoppers.
• Clean all plant debris from equipment before storing it for the winter; some pathogens that do not survive well in the field survive well on equipment under shelter.
• Mow youngest stands first. This reduces the spread of pathogens by machinery from older, more diseased stands into healthier, younger stands. If this is not feasible, a thorough cleaning with a pressure washer between older stands and newer stands should help reduce pathogen spread.
  
• Mow after the dew has dried, because many pathogens are spread easily in water films.
• When stands are hit hard by leaf spot diseases, mow a few days earlier than usual to retain more leaves and reduce inoculum levels in the field.
• Maintain a cutting schedule that ensures the recharging of root carbohydrates both during the growing season and before fall dormancy.
• To minimize Sclerotinia stem and crown rot outbreaks, avoid late-summer plantings of alfalfa by conservation-tillage methods.

Wilt diseases can cause severe stand losses in Pennsylvania. Bacterial, Fusarium, and Verticillium wilts occur statewide, Fusarium wilt is worse in southern counties and Verticillium wilt worse in northern counties. Resistance to these wilts is available in current varieties and is necessary for maximum production.

Anthracnose is a fungal disease that occurs statewide and is particularly severe in southeastern Pennsylvania. This fungus often cannot overwinter in the field, but it does so in infected plant debris on equipment in storage. Therefore, cleaning equipment before storing it for the winter helps delay the introduction of this pathogen into young seedlings in the spring. Resistant varieties are available and should be used state-wide.

Phytophthora root rot can devastate young stands of alfalfa and can cause serious plant loss in older stands. This fungal pathogen lives in the soil, and soils saturated with water for three or more days can trigger a disease outbreak. In perennially wet sites, the use of alternative crops is recommended. The use of resistant varieties, seed treatment with Apron fungicide, and soil treatment with Ridomil Gold (mefenoxam) or MetaStar 2E (metalaxyl) fungicide are effective control measures.

Crown and root rot complex, caused by Fusarium spp. plus other fungi and bacteria, is common in alfalfa. Resistant varieties are not available; therefore, growers must use best proper management practices to minimize stress on the plants, which slows down the rate of root rot development. Root-rotting pathogens can align with root-feeding insects, and root deterioration progresses with increasing stand age.

Aphanomyces root rot is a disease that causes establishment problems in some areas. To date, only trace amounts of the disease have been reported in Pennsylvania, although both strains of the pathogen are known to widely exist in Pennsylvania soils. This disease is likely to occur under the same wet soil conditions as does Phytophthora, so it is possible that losses caused by this fungus have been occurring but were attributed to Phytophthora. The fungicide treatments available against Pythium spp. and Phytophthora spp. are not effective against Aphanomyces spp.; however, resistance to two races of the fungus is available in some newer varieties.

Foliar diseases are common in Pennsylvania throughout most of the growing season and can cause significant quality and yield loss through defoliation. When foliar diseases are severe, early mowing helps in leaf retention and reduces inoculum in the field. Some of the current varieties have improved levels of resistance, but all become diseased if favorable moisture levels and temperatures prevail.

Spring and summer blackstem occur on alfalfa in Pennsylvania, with spring blackstem usually more severe. Leaves, petioles, and stems are attacked, with the spring blackstem fungus also causing crown and root rot. As with other foliar diseases, early harvesting of severely diseased stands can increase leaf retention and reduce inoculum in the field. A few current varieties have improved levels of resistance, but all become severely diseased if extended moist periods prevail.

Nematodes generally do not cause serious problems on forages in Pennsylvania, as long as rotations with corn, cereal, or grass crops are used.

Sclerotinia crown and stem blight can cause seeding failures. Late summer or early fall seedings using conservation tillage favor disease development. The infective stage for this disease usually occurs in October, and fall-seeded plants are very susceptible at this time. Because conservation tillage does not bury the survival structures of the fungus, disease severity in no-till plantings is often more severe than in conventionally tilled seedbeds. Infected seedlings often survive until spring when the plants die, and entire stands may be lost. Spring plantings, because of the plants’ increased maturity in the fall, are not as likely to be devastated. Resistant varieties are not available.

Virus diseases are not considered serious on alfalfa in Pennsylvania. Viruses may be present, however, without causing obvious symptoms, and it is possible that viruses contribute to premature stand decline. No virus-resistant varieties are available.