Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Charcoal rot is reported occasionally on alfalfa in the U.S. and has also been found in Australia, Pakistan, Uganda, east Africa, and the former Soviet Union. The fungus causing the disease is widespread throughout tropical and subtropical countries. It causes disease on more than 500 crop and weed species including root rot on soybean and stalk rot on corn and sorghum. Although usually associated with tropical climates, the disease can develop in temperate areas when hot, dry conditions exist. Symptoms: Large, mature plants can be killed in one season. The stems remain erect but the stems, crown, and upper taproot often fragment when plants are dug due to destruction of the pith, leaving vascular strands beneath the epidermis. Infected tissues have a gray coloration due to the presence of numerous microsclerotia that resemble a sprinkling of finely powdered charcoal. Sclerotia are found in infected roots and stems and in decayed material on the ground surrounding dead plants. Mycelium and sclerotia of Sclerotium rolfsii are often found in plants with symptoms of charcoal rot, most likely as a secondary invader. Causal Organism: Macrophomina phaseolina is considered to be a weak pathogen, typically attacking plants stressed by drought and high temperature. Black, smooth, hard sclerotia (100 to 1,000 um in diameter) form in infected roots and stems. Dark-brown pycnidia with apical ostioles form on leaves and stems. Conidia are hyaline, ellipsoid to obovoid, 1-30 x 5-10 um. Strains isolated from white clover are pathogenic on alfalfa. Disease Cycle and Epidemiology: The disease cycle has not been investigated in alfalfa. Based on disease in other crops, it is expected that sclerotia persist in crop debris and soil under dry conditions and initiate infection of roots. Sclerotia are released from decaying plant material and are spread in infested soil and in crop debris. Although pycnidia are observed on inoculated alfalfa plants, the role of conidia in the disease cycle is unknown. The disease likely contributes to lack of persistence of alfalfa and white clover in the southeastern U.S. Management: No information on management of the disease is available. Reducing water stress and providing good soil fertility should minimize risk of the disease. Rotating to small grain crops for 2 years or corn for 3 years reduces potential inoculum because these crops are considered to be poor hosts for M. phaseolina.