Research on target spot of soybean in northwestern Argentina

PLOPER, L.D; GONZALEZ, V.; DE LISI, V.; REZNIKOV, S.; STEGMAYER, C.A.; HENRIQUEZ, D.D.; DEVANI, M.R.

Durban

Congreso; World Soybean Research Conference IX (WSRC IX); 2013

Protein Research Foundation

Target spot of soybean, caused by Corynespora cassiicola, is an endemic disease in nothwestern Argentina (NWA), a region located between Latitudes 22º and 29º South and Longitudes 63º and 68º West, which includes the subtropical provinces of Tucumán, Salta, Jujuy, Santiago del Estero, and Catamarca. Target spot was first detected in this region in 1990, although it was not until the late 1990s that its prevalence and severity started increasing, particularly in northern Salta. At present, it is considered one of the main components of the ?late season disease complex?. Occurrence of this group of diseases has gradually increased in most soybean-growing regions in Argentina since the mid 1990s, probably favored by successive years of continuous monocropping and no-tillage. Foliage-applied fungicides are used in most of the soybean area to manage this group of diseases. Annual disease surveys in NWA have revealed increased target spot levels during the last decade. In addition, data from a susceptible cultivar included in yield tests at different locations showed the gradual increase of target spot severity from the 2008-2009 season through 2010-2011. During this period there were environmental conditions that favored crop and disease development. This trend was altered in the 2011-2012 season, due to a severe drought that affected the region and resulted in significant disease and yield reductions. Genetic resistance is recommended to manage target spot. Since most commercial cultivars available in the region at present are susceptible to the disease, a major effort is devoted to the evaluation of newly released cultivars and advanced breeding lines for reaction to target spot. Additionally, field trials were conducted during the 2010-2011 and 2011-2012 seasons to assess the efficiency of active ingredients and the timing and number of fungicide applications for target spot chemical control. The trials were located on a farm near General Mosconi, in northern Salta. A total of 13 treatments were evaluated in each season. These included an untreated control and various active ingredients from the chemical groups strobilurins, triazoles, and bencimidazoles, alone or in combinations, which were applied at different moments (R3, R5, or R3+R5). Fungicides were evaluated at the rate recommended by the manufacturers. Treatments were arranged in a randomized complete block design with four replications. Plots consisted of four 6-m rows, spaced 0.5 m apart (12 m2). Disease and yield data were obtained from the two central rows. Fungicides were applied in water with a CO2 backpack sprayer, equipped with 3 hollow cone nozzles (Tee Jet model TXA 8001VX) on a handheld boom, calibrated to deliver 150 liters ha-1 at a pressure of 294 kPa. Target spot incidence and severity (measured as affected leaf area percentage) were evaluated on 30-leaflet samples. In 2010-2011, nearly all fungicide treatments reduced target spot severity significantly as compared with the untreated control (12.5%). Lower disease values were obtained with the double application (R3+R5) of a combination of bencimidazol + triazol (5.0%). No significant differences were detected in yield. In 2011-2012, only the treatment with a triazol applied at R5 (5.5%) showed significantly lower severity values than the control (22.5%). None of the fungicide treatments had significantly higher yields than the control.