Influence of planting date and genotype on late season diseases and yield of soybean in Tucumán, Argentina

PLOPER, L.D.; DÍAZ, C; GALVEZ, R; GONZÁLEZ, V; ZAMORANO, M A; JALDO, H; LÓPEZ, C

Foz do Iguazú

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

Embrapa SOJA y WSRC Continuing Committee

The occurrence of soybean diseases in northwestern Argentina has increased gradually over the last 15 years.  The late season disease complex, a combination of various diseases that becomes most noticeable in the mid to late reproductive stages, can cause significant yield losses.  The objective of this study was to evaluate the influence of planting date and soybean genotype on development of late season diseases (LSD) and 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 experimental design was a randomized complete block with 3 replications.  Plots consisted of four, 6-m rows, spaced 0.5 m apart.  Disease severity was evaluated in 10 plants per plot every 3 weeks, with the aid of a standard area diagram. Severity values were used to calculate the area under the LSD progress curves (AULSDPC). 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.  LSD progress curves started at 35 days after planting  (DAP) with severity values for the first planting date that ranged from 2% to 12% and 2% to 8%, for the 2000/01 and 2001/02 seasons, respectively, and reached maximum values at 75 DAP, with 25% to 42% and 16% to 42%, for the first and second growing seasons, respectively.  As planting was delayed the LSD progress curves were flatter and the epidemics started later.  For the fourth PD, severity values at 35 DAP ranged from 0.5% to 2% and 0% to 6% for the 2000/01 and 2001/02 seasons, respectively.  As in the first planting date, the maximum values were recorded at 75 DAP with values from 9% to 16% and 15% to 31%, for the first and second season, respectively.  Average reduction of AULSDPC between the first and last PD was 30% for 2000/01 and 63% for 2001/02. There was a significant interaction for AULSDPC between PD and genotype for both growing seasons. Yields decreased as planting was delayed. Average yield reductions between the first and last PD were 60% for 2000/01 and 36% for 2001/02.These results indicate that the risk of high LSD levels can be minimized by late plantings, although this results in significant yield reductions.