Author
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Castell-Miller, Claudia |
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Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter Publication Acceptance Date: 12/5/2012 Publication Date: N/A Citation: N/A Interpretive Summary: 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 |
