Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Yellow leaf blotch occurs worldwide in temperate climates. The disease is reported from countries in Asia, Australasia, Oceania, Europe, North America, Central America, the West Indies, and South America. In the northern Great Plains of North America, it is often the major leaf disease on alfalfa. In Wyoming, the disease is frequently severe on dryland alfalfa but rarely occurs in irrigated fields. Management practices and weather factors (such as rainfall, dew, and wind) markedly affect the development and persistence of the pathogen and the development of disease. Losses are in proportion to the number of leaves that fall, which may be as high as 40% at harvest or 80% at pod formation. Symptoms: Blotches are more abundant on leaves than on petioles and stems. Specific characteristics are nearly identical on various leaves of one plant but may differ considerably on leaves of other plants. Symptoms on leaves usually appear first as small chlorotic spots on the upper surface about 10 days after inoculation. In a resistant plant, the spots remain small and become light brown within 1 week. In a susceptible plant, the spots enlarge to form chlorotic streaks, which are restricted in width by the principal leaf veins, and then become yellow to orange blotches. The blotches become fan-shaped as they expand between principal veins toward the leaf edge, or they may become more or less circular. Several blotches may develop on a single leaflet. Numerous small, dark, epidermal openings (tears) develop on the upper leaf surface of blotched areas. Openings lead to subepidermal, wall-less locules, 100-200 um wide and up to 100 um deep, within which tiny conidia are produced and from which conidia may extrude in a whitish, fluid mass to the leaf surface. Conidia are not known to germinate, to infect the plant, or to function in any other way. Under moist conditions, blotches turn from light brown to dark brown to black and change from pseudostromatic to stromatic. Apothecial initials develop singly or in groups in association with locules and emerge on leaf surface as black stromatic bodies that ultimately yield apothecia. Affected leaflets curl as they dry. Causal Organism: The disease is caused by the teleomorphic state, Leptotrochila medicaginis. The nonpathogenic, anamorphic state is Sporonema phacidioides. Apothecia are black, 0.2-1.0 mm wide, round or elliptical, and are borne individually on broad, short stalks on both leaf surfaces. At or near maturity under moist conditions, the top of the apothecium opens to expose a light gray to yellowish brown hymenium of asci and paraphyses. Asci, which do not react with iodine, measure 55-75 X 7-10 um. They contain eight hyaline, ovoid, single-celled spores, 7.5-11 X 3-6 um. The paraphyses are slightly longer than the asci. Conidia are hyaline, one-celled, and 5-8 X 2 um. Disease Cycle and Epidemiology: Ascospores ejected from mature apothecia are the only known infective units. Under favorable moisture and temperature, apothecia may form within 2 to 3 weeks on leaves with well-developed, yellow blotches. In the Great Plains of North America with its occasional heavy rains, frequent high winds, and hot dry summers, apothecia occasionally form by late fall. However, they usually do not form until early spring in leaves on the ground. In regions with more frequent rains, high humidity, and little wind, mature apothecia may be produced throughout the summer on leaves on the ground and on leaves held on plants left for seed or because of delayed harvest. Mature apothecia on dry leaves may discharge viable ascospores after a year if rehydrated. Ascospores are discharged for distances up to 1.7 cm; 90% of them fall as single spores. Their release is favored by relative humidity above 97% and temperatures below 25 C. On overwintered leaves, ascospores may discharge until early July, after which ascospore inoculum must come from a summer generation of apothecia on spring-infected leaves. Apothecia formed in fall may discharge ascospores long before spring. Ascospores germinate at 2-31 C (optimally at 7.5-21 C). Their germ tubes may penetrate the host directly within 4 h at 20 C, 8 h at 12 C, and 24 h at 6 C; they fail to penetrate at 32 C. Relative humidity above 95% is required for ascospores to germinate and successfully penetrate the host. Management: Harvest early to reduce the buildup of infected leaves. Where disease is severe, remove or burn infected residue from the field. Crop rotation may reduce the negative impact of the disease on alfalfa. Cultivars with Medicago falcata germplasm tend to have some resistance.