Seedling Diseases of Alfalfa in California

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Abstract
Emergence of alfalfa in several field trials in the Central Valley of California ranged 30-65% of seeds planted. In the field and greenhouse, emergence of ''Lahonton'' was consistently lower than that of ''Moapa 69''. Percentages of emergence were between 75 and 85% of viable seeds, however, when either of these cultivars was planted in pasteurized field and artificial soils. In an untreated field soil, emergence was lower with both cultivars at a soil temperature regime with a daily cycle between 8.degree. and 17.degree. C than at a set of 4 constant soil temperature regimes (.+-.2.degree. C) with means of 16.5.degree., 20.0.degree., 25.5.degree., and 26.5.degree. C. Although differences between the 2 cultivars remained, the degrees of emergence for each cultivar were similar at the 4 temperatures; emergence was about 65 and 55% of viable seeds for ''Moapa 69'' and ''Lahonton'', respectively. Postemergence damping-off usually did not exceed 5% with either cultivar in the field or in greenhouse studies. Formation of multiple adventitious roots (forked-root disease) was frequently associated with fungal infection of seedling radicles. Plants with the forked-root disease were stunted but survived as multiple-taprooted mature plants in the field. Treatments of field soils with moderately narrow-spectrum fungicides (e.g., ethazole) frequently increased emergence to percentages equivalent to those found in the pasteurized soils. Pythium ultimum, Rhizoctonia solani and Fusarium roseum (= F. acuminatum and F. culmorum) were the most common fungi isolated from damped-off seedlings. In pathogenicity tests, all 4 of these soil-borne fungi caused preemergence and postemergence damping-off, whereas only P. ultimum incited a significant amount of the forked-root symptoms.