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Effect of Plant Age, Maturity Group, and the Environment on Disease Progress of Sudden Death Syndrome of Soybean. J. C. RUPE, Associate Professor, Department of Plant Patholog; University of Arkansas, Fayetteville 72701. E. E. GBUR, JR., Associate Professor, Agricultural Statistics Laboratory, University of Arkansas, Fayetteville 72701. Plant Dis. 79:139-143. Accepted for publication 14 October 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0139.

Disease progress of sudden death syndrome (SDS) of soybean, caused by Fusarium solani, was determined in a standard susceptible cultivar, Lee 74, planted uniformly throughout the test site and in 42 soybean cultivars representing maturity groups (MG) IV-VIII, 1986-1989. Disease severity was assessed weekly from the middle of July through the first of October using a visual rating scale of percent leaf area exhibiting foliar symptoms of SDS. The disease ratings were used to calculate the absolute infection rate (dy/dt) for each cultivar at each rating time and the area under the disease progress curve (AUDPC). In Lee 74, SDS progressed in two phases: an initial rapid increase in disease to about 35% leaf area affected and a slower increase after that point. This two-phase disease progress curve was fit by a segmented model using two exponential equations. The fits of these equations were highly significant (P< 0.0001) in all 3 yr. The equations were joined on 30 August 1986, 13 August 1987, and 17 August 1988, which corresponded to 74, 69, and 83 days after planting (DAP) in 1986, 1987, and 1988, respectively. These join points occurred at accumulated temperatures from planting of 925, 852, and 963 degree-days in 1986, 1987, and 1988, respectively, using 15 C as the base temperature. Disease progress in the 42 test cultivars was compared by determining the DAP of the first large increase in disease (dy/dt > 1%/day or, if lower than 1%/day, the highest dy/dt attained). DAP and dy/dt were significantly (P < 0.0001) related to AUDPC (R = 0.58 and R = 0.38, respectively). Cultivars susceptible to race 6 of the soybean cyst nematode (SCN), Heterodera glycines, the predominate race in the field, developed SDS 10 days earlier and had significantly higher levels of disease than cultivars resistant to race 6 of SCN. The earliest increases in SDS occurred between 71 and 77 DAP in 1986, 69 and 74 DAP in 1987, and 75 and 83 DAP in 1988 for most MGs. Accumulated heat for these periods was 910-942, 852-916, and 857 963 degree-days in 1986, 1987, and 1988, respectively. In 1987, three cultivars in MG VII and VIII had large increases 52 DAP after 608 degree-days. In general, however, disease increased at the same time across MGs, but at no specific reproductive stage.