Effect of planting date and soybean cultivar on development of sudden death syndrome, 2002/03

ZAMORANO, M A; PLOPER, L.D.; GONZÁLEZ, V; JALDO, H E; GÁLVEZ, M R; DÍAZ, C G

Biological And Cultural Tests For Control Of Plant Disease

The American Phytopathological Society

Lugar: St. Paul, MN, USA; Año: 2004 p. 1 - 1

0887-2236

Development of sudden death syndrome was evaluated on 6 soybean cultivars planted on 3 dates at La Invernada, Tucumán, Argentina.  The test was conducted in a field cropped to soybean and with high levels of the disease in previous years.  Cultivars within each date were arranged in a randomized complete block design with 3 replications.  Each plot consisted of four, 6-m rows, spaced 0.5 m apart.  Planting dates were 11 Dec 02, 20 Dec 02 and 7 Jan 03.  Evaluations were carried out, starting at growth stage V1, every 15 days until the appearance of SDS foliar symptoms, and weekly thereafter.  A Disease Index (DI) was calculated, based on the Maximum Incidence (MI) and its corresponding Severity (S), according to the following formula: DI= (MI x S)/5.  Incidence was determined as the percentage of diseased plants in the plot, and the Maximum Incidence was the highest incidence observed in each plot during the season.  Severity was estimated using a 0 to 5 scale, where 0= no foliar symptoms, 1= less than 10% of the foliar area with chlorosis, 2= 10-30% of the foliar area with chlorosis or less than 10% with necrosis, 3= 10-30% of the foliar area with necrosis, 4= 31-70% of the foliar area with necrosis, and 5= more than 70% of the foliar area with necrosis.  Rainfall between Nov and Apr amounted to 721.0 mm (average 763.8 mm); however, less than average precipitation occurred in Feb (30.0 mm, average 154.8 mm) and Mar (113.0 mm, average 169.4 mm).  All cultivars had the Maximum Incidence in R6.  For all cultivars, the highest Disease Index was observed in the plots with the earliest planting date, although there were significant differences among them.  Differences among cultivars were not as marked for the second and third planting dates.  Cultivars A 7636 RG (MG VII) and A 6401 RG (VI) showed consistently low Disease Index values, and there were no significant differences among the 3 dates.  Qaylla RR (VII), Munasqa RR (VIII), A 8100 RG (VIII), and AW 4902 RR (IV) showed the highest Disease Indices for the first date, with no significant differences among them.  AW 4902 RR had one of the highest Disease Index ratings at the first date, but low ones in the second and third dates.  No significant differences were observed between the Disease Index values of the first and second dates for A 8100 RG, although the value corresponding to the third date was significantly lower.  Disease Indices for Munasqa RR and Qaylla RR significantly decreased from the first to the third planting date.  These results indicate that less disease occurred as planting was delayed, and that the extent of this reduction was influenced by the soybean cultivar.