VIEW ARTICLE

Disease Detection and Losses

Development of Yield Loss Models in Relation to Reductions of Components of Soybean Infected with Phakopsora pachyrhizi. X. B. Yang, Department of Plant Pathology and Crop Physiology, Louisiana State University, Baton Rouge 70803, Present address: Department of Plant Pathology, University of Arkansas, Fayetteville 72701; A. T. Tschanz(2), W. M. Dowler(3), and T. C. Wang(4). (2)(4)Asian Vegetable Research and Development Center, Taiwan, (2)Present address: USDA-APHIS, 814 Federal Center Bldg., Hyattsville, MD 20782; (3)USDA Foreign Disease-Weed Science Research Unit, Fort Detrick, Frederick, MD 21701. Phytopathology 81:1420-1426. Accepted for publication 25 June 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/Phyto-81-1420.

Epidemics of soybean rust (Phakopsora pachyrhizi) were manipulated by applying protectant fungicides at weekly intervals to different plant growth stages. Effects of disease on plant growth and yield components were monitored at different growth stages. Reduction of shoot weights was different among the cultivars. The number of pods per plant measured at soybean growth stage R6 was reduced as much as 40% in the diseased plants, but the number of seeds per pod was not affected, indicating that disease affected the attainable yield by reducing pod set. From growth stages R6 to R7 (beginning maturity), percentage of pod abortion was high for severely diseased plants, although pods per plant already had been reduced at R6. Seed growth rate (grams per day) from R4 to R7 was reduced by 40–80% in diseased plants in 1986 and was negatively correlated with relative area under the disease progress curve. The time for diseased plants to growth from R4 to R7 was reduced by as many as 16 days compared with protected plants and was significantly correlated with the disease. A general yield loss model was modified to analyze the coefficients of empirical yield loss models. Slopes of simple linear yield loss models could be represented by the sum of reduction slopes of seed growth rate and seed growth period and were validated by actual data.