Effect of late season diseases on healthy leaf area index and healthy leaf area duration of different soybean genotypes in Tucumán, Argentina

DÍAZ, C; PLOPER, L.D.; GONZÁLEZ, V; GÁLVEZ, M R; ZAMORANO, M A; LEDESMA, F; LÓPEZ, C; RAMALLO, J C

Foz do Iguassú, Paraná, Brasi

Conferencia; VII World Soybean Research Conference (VII WSRC).; 2004

Embrapa SOJA y WSRC Continuing Committee

The effect of late season diseases (LSD) on different soybean genotypes with 4 different planting dates (PD) was assessed by an integrated variable, healthy leaf area duration (HAD), and two discrete variables, LSD severity and healthy leaf area index (HLAI), in relation to yield.  Field tests were conducted in San Agustin, Tucumán, Argentina. Twelve genotypes (Maturity groups VI to XI) were evaluated in the 2000/01 growing season and 15 genotypes were evaluated in the 2001/02 season, with four planting dates (PD) in each season.  The first two PD were also compared with fungicide treated plots.  The experimental design was a randomized complete block with 3 replications.  Plots consisted of four, 6-m rows, spaced 0.5 m apart.  Brown spot (Septoria glycines), leaf blight (Cercospora kikuchii), bacterial blight (Pseudomonas savastanoi pv. glycinea), downy mildew (Peronospora manshurica) and frogeye leaf spot (Cercospora sojina) were the main LSD during the first season, while brown spot, bacterial blight, downy mildew and target spot (Corynespora cassiicola) predominated during the following season.  Growing season, PD, and maturity group affected HLAI progress curves; in addition, interactions among growing season, PD and genotype were observed for HAD.  The HLAI values for 2000/01 were higher than those for 2001/02.  The HLAI progress curves in late plantings were flatter than in early plantings.  For fungicide treated plots, long maturity group genotypes showed higher HLAI values (highest values were 4.9 for FAM 940 in 2000/01 and 4.4 for IB 8212/22 in 2001/02) than short maturity group (4.6 for A 6445 RG and 2.0 for A 6401 RG for 2000/01 and 2001/02, respectively).  Reduction of soybean growth was determined using HAD reduction percentages.  In 2000/01, growth reduction percentages varied between 17 and 36 for the first PD and between 10 and 36 % for the second PD. In 2001/02, reduction rates varied between 17 and 49 % and from 17 to 51 % for the first and second PD, respectively. Associated with these reductions in HLAI and HAD, yield decreased between 4 and 38 %.  These results show that the late season disease complex significantly affected the HLAI and HAD parameters, which was reflected in important yield reductions. Thus, appropriate management strategies should be adopted to diminish the impact on soybean of this disease complex