Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Spring black stem is the most destructive alfalfa diseases in temperate regions of the U.S., Canada, Australia, and countries of Europe, Asia, and South America. The disease causes serious yield losses by reducing canopy dry matter and also decreases seed weight and crown and root mass. Forage quality is affected by a decrease in concentration of crude protein and an increase in the content of some anti-quality factors. The first cutting is usually the most damaged. Rotting of crowns and taproots can compromise stand survival. Symptoms: The most prominent symptoms of spring black stem and leaf spot occur in the canopy. The pathogen also causes disease in the crown, upper taproot, and secondary roots, but symptoms are difficult to observe in field-grown plants or distinguish from symptoms caused by other pathogens. Seedpods and seeds can be infected if the disease occurs during seed production. During early stages of infection, lower leaves develop small, dark brown to black spots (tarspots) typically without chlorosis. The lesions enlarge irregularly and coalesce covering larger areas, frequently accompanied by chlorosis. Petioles and stipules develop dark spots. Petioles can be girdled by the fungus causing leaves to wither. Defoliation usually starts at the base of the stem and progresses upward. Stem symptoms appear as small, dark discolorations that lengthen and combine, blackening most of the surface in severe infections. Lower portions of stems can become bleached or tanned. Young stems may be completely girdled and killed. Artificial infection of the crown results in a dry, black necrosis that extends externally and internally from the point of inoculation. Necrosis can advance to the base of stems and upper taproot. Inoculation of roots results in a dry, black necrosis followed by collapse of root tissues. In humid areas, infected pods become flat and discolored, and seeds are small and shriveled. Causal Organism: Phoma medicaginis (sensu lato) is attributed by most researchers to be the causal agent of spring black stem and leaf spot. In sensu stricto two varietal ranks, medicaginis and macrospora, have been recognized based on pycnidiospore size and number of septa mostly observable in vivo at low temperatures (6 C). At room temperatures (20-22 C) and in vitro, spore differences between the two varietal ranks are less clear. Pathogenicity on alfalfa may differ with var. macrospora reported to be strongly pathogenic on alfalfa. Relevance and distribution of the two varietal types in the U.S. remains to be determined. On potato-dextrose agar (PDA), young colonies are dark olive green with a white to pale cottony margin that becomes darker with a crusty, carbonaceous appearance when older. Growth rate in PDA is moderate, reaching 38 and 62 mm at 20 C, and 39 and 75 mm at 25 C, at 7 and 14 days, respectively. Two distinct branched septate hyphae may be present in the mycelium; 2-3 um wide hyaline hyphae, and light brown 5-6 um wide hyphae. Abundant pycnidia and spores are produced at 18-24 C. Pycnidia are brown to black, simple or complex, relatively thin-walled, and pseudoparenchymatous, globose to subglobose, with or without a distinct ostiole, ranging from 120-280 um in diameter. Often they are surrounded by dark and thick-walled hyphae and associated with matrix material. Conidiogenous cells are simple and ampulliform. Under humid conditions, conidia are released into a white to pink ooze. In general conidia are enteroblastic, hyaline, and smooth. Pycnidia and pycnidiospores form readily on infected leaves and stems placed in a moist chamber. The variety medicaginis appears to have conidia with sizes of 4.2 to 12.5 um length and 1.5–4 um wide, mostly aseptate although one-septa spores can be found in old cultures. P. medicaginis var. macrospora has overall larger spores from 5.8 up to 28 um in length and 1.4 to 6 um in width, and variable percentage (10 to 67%) of 1 to 3 septa. Some isolates of the fungus produce dendritic crystals of brefeldin A, especially in malt-agar cultures 2- to 3-weeks-old. Chlamydospores, when present, are usually produced in old cultures, mostly solitary, thick-walled, and 7-14 um in diameter. Molecular identification of the species (sensu lato) can be achieved by gene sequencing. In general, isolates of the fungus appear molecularly diverse. Disease Cycle and Epidemiology: In the field, the disease is prevalent during cool and wet periods. In spring, mature pycnidia develop on lesions of overwintered alfalfa residue and from lesions on the crown and upper taproot. Abundant pycnidiospores, wrapped in conical-shaped matrices, are released and disseminated in the presence of free water. Local dispersal is by water splash and dew while farther dissemination occurs by wind, insects, and human activities. In artificial inoculations on leaves, spores increase in size and germinate by 24 h after deposition on susceptible alfalfa leaves at 20-22 C. Germ tubes grow toward the surface of epidermal cells or their junctions and penetrate them mostly directly, with or without formation of appressorium-like structures. Initially, hyphae can grow subcuticularly and later colonize deeper tissues. The fungus primarily invades inter- and intracellular spaces and less frequently leaf veins. Collapse of epidermal cells and chlorosis is observed in susceptible plants by 3 days after inoculation. Pycnidia and pycnidiospores are produced after 6 to 8 days. Germination and penetration of stems occurs by 72 h after inoculation. The fungus invades stem tissues directly through stomata or wounds, colonizing and causing collapse of epidermal cells first and later moving toward chlorenchyma and cortical tissues. Lesions appear as sunken, dark areas that enlarge and merge as the fungus moves along and across stem tissues, mostly in the cortex. Conductive vessels are the last to be affected. Pycnidia may erupt though the epidermis. From the stem the fungus can move into internal crown tissues; however, external infections of crowns occur only through wounds. In upper taproots, the fungus infects the stele, woody tissues, secondary xylem, axial parenchyma, and fibers. Before invasion, hyphae may cover the root surface and penetrate directly or through wounds. Root injuries, caused by biotic and abiotic agents, enhance disease severity. Invasion of the root epidermal and cortical cells is primarily intercellular and causes cells to collapse. In later stages, hyphae can invade cortical cells and xylem vessels. Spring black stem and leaf spot is likely a polycyclic disease in the field. The fungus infects new growth if favorable environmental conditions occur, usually in spring and fall. It can survive as pycnidia and mycelia in dead and living tissue. It is a seed and soil-borne pathogen although survival in soil is apparently limited to 2 years without a host. The role of clamydospores in fungal survival is unclear. P. medicaginis var. medicaginis (sensu lato) is reported to be pathogenic on species of Medicago, Melilotus, Trifolium, and other Papilionoides. However, pathogenicity of isolates collected from M. truncatula is limited to this species and to M. sativa. Some non-legume hosts have been reported as well. Host range specificity may differ between the two varietal ranks. Management: Plant certified seed in rotation with non-host crops. The level of resistance in current varieties has been improved by traditional breeding methods, although no highly resistant varieties are available. Early harvest when the disease is prevalent tends to reduce leaf loss. Fungicide applications differ in efficacy for reducing disease intensity depending on environmental conditions.