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Impact of Leaf Spot Diseases on Yield and Quality of Alfalfa in North Carolina. C. Lee Campbell, Associate Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. James A. Duthie, Graduate Assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 74:241-245. Accepted for publication 20 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0241.

Plants of alfalfa (Medicago sativa) grown in a test field in 1986 and 1987 were harvested five times each year. Leaf spot diseases on the plants were attributable to Leptosphaerulina briosiana, Stemphylium botryosum, Phoma medicaginis, and Cercospora medicaginis. The particular fungi present varied from harvest to harvest. Application of chlorothalonil (at rates equivalent to 0, 0.65, 1.3, 2.6, or 5.2 kg a.i./ha) to mature, healthy alfalfa plants in the greenhouse did not affect amount of dry matter accumulated during 4 wk of regrowth. In the field, mean disease severity (percentage of leaf area diseased) at all harvests in 1986 was consistently ≤5% in plots sprayed weekly with chlorothalonil (1.3 kg a.i./ha) but ranged from 5 to 14% in unsprayed plots. In 1987, final disease severity was consistently ≤3% in sprayed plots but ranged from 3 to 8% in unsprayed plots. Dry matter yield was 5.334.8% less in unsprayed plots than in sprayed plots at the five harvests in 1986 and 12.937.2% less in 1987. Annual differences in yield between sprayed and unsprayed plots were 18.6% (1.69 Mg/ha) in 1986 and 24.8% (2.64 Mg/ha) in 1987. Percentages of total digestible nutrients and acid detergent fiber did not differ in alfalfa harvested from treated or untreated plots. Mean adjusted crude protein did not differ with respect to fungicide treatment or harvest in 1986 but was greater in unsprayed plots (22.1%) than in plots sprayed with fungicide (20.6%) in 1987.