Effect of planting date and climatic factors on soybean sudden death syndrome response in cultivars of different maturities

Abstract

A serious disease affecting soybeans (Glycine max (L. ) Merr.) is sudden death syndrome (SDS). SDS is a relatively new disease, which has occurred in several states, causing great concern among farmers growing soybeans. This disease is caused by a soil-borne organism, Fusarium solani f. sp. Phaseoli strain A. wihich infects soybean roots. SDS affects mainly fields of high yield potential causing losses up to 80%. Leaf symptoms of SDS usually first appear after flowering as yellow spots between leaf veins, turning into necroisis. As disease progress leaves abscise and pods begin to drop causing yield losses. No effective control has been found for SDS, and varietal choice has been the only relief against it. Several reports have indicated that SDS is influenced by physical environmental factors. High soil moisture and cool temperatures during the reproductive period appear to encourage greater expression of disease while hot and dry conditions during this period seem to restrict disease development. It has previously been shown that varietal reaction to SDS is influenced by planting date though not in a consistent manner. The main purpose of this study was to evaluate the effect of planting dates on the expression of SDS in four varieties of different maturity groups. The study was conducted at Ridgway and Villa Ridge, two locations in Southern Illinois chosen based on their previous history of SDS infestation. Four cultivars from maturity group III to VI were used as main plot, and four planting dates spanning from mid May to late June were used as a subplot in a split plot design. Disease score as wel as growth stage were recorded weekly after disease appearance. Rainfall, minimum-maximum soil temperatures and soil moisture data were collected weekly form the beginning of the season. First disease appearance occurred as early as the beginning of pod set and more commonly during pod elongation and pod fill stages, with symptoms remaining minor before R4 stage (Full length pods). Delayed planting caused reduced (p‹ 0.001) disease incidence (DI) in both environments and later maturity groups were affected more than earlier maturity groups (P ‹ 0.05). However, delayed planting only reduced disease severity (DS) significantly (P ‹ 0.05) at Ridgway. At both location maturity groups differed significantly (P‹ 0.001) in DS. In some cases, there was also an interaction of planting date and maturity group which affected SDS expression (P‹0.05). These results from 1994 agree with the overall pattern of results of similar studies conducted from 1987 to 1993, although results from 1990 showed no difference among planting dates. Delay in planting caused reduced yield at Villa Ridge but not at Ridgway. No effect of planting date was observed in seed weight at Villa Ridge, but a slight increase of seed weight at Ridgway with later plantings was observed. Graphical analysis revealed that most pronounced periods of disease increase were related to periods of high soil moisture. Overall disease was much higher at Villa Ridge where soil moisture remained high throughout the season than at Ridgway where soil miosture was low between early July and the beginning of September. The effect of soil temperature was not clear but it appeared that periods of rapid increase of disease were related to periods of lower soil temperatures. According to this and other studies, farmers can reduce the amount of SDS by avoiding SDS- prone fields earlier than necessary. However, the yield reduction often associated with delayed planting must be balanced against the benefit of reducing the SDS. Farmers should minimize SDS risk by planting several varieties which have done best in SDS trials and which encompass a range of maturity. Further research should examine more critically the effect of environmental factors by improving the assessment of soil and air temperature as well as soil moisture. It would be informative to compare soil moistures and other environmental factors not only to leaf symptoms but also to the time of root infection and progression of root and stem symptoms. Súbita A serious disease affecting soybeans (Glycine max (L. ) Merr.) is sudden death syndrome (SDS). SDS is a relatively new disease, which has occurred in several states, causing great concern among farmers growing soybeans. This disease is caused by a soil-borne organism, Fusarium solani f. sp. Phaseoli strain A. wihich infects soybean roots. SDS affects mainly fields of high yield potential causing losses up to 80%. Leaf symptoms of SDS usually first appear after flowering as yellow spots between leaf veins, turning into necroisis. As disease progress leaves abscise and pods begin to drop causing yield losses. No effective control has been found for SDS, and varietal choice has been the only relief against it. Several reports have indicated that SDS is influenced by physical environmental factors. High soil moisture and cool temperatures during the reproductive period appear to encourage greater expression of disease while hot and dry conditions during this period seem to restrict disease development. It has previously been shown that varietal reaction to SDS is influenced by planting date though not in a consistent manner. The main purpose of this study was to evaluate the effect of planting dates on the expression of SDS in four varieties of different maturity groups. The study was conducted at Ridgway and Villa Ridge, two locations in Southern Illinois chosen based on their previous history of SDS infestation. Four cultivars from maturity group III to VI were used as main plot, and four planting dates spanning from mid May to late June were used as a subplot in a split plot design. Disease score as wel as growth stage were recorded weekly after disease appearance. Rainfall, minimum-maximum soil temperatures and soil moisture data were collected weekly form the beginning of the season. First disease appearance occurred as early as the beginning of pod set and more commonly during pod elongation and pod fill stages, with symptoms remaining minor before R4 stage (Full length pods). Delayed planting caused reduced (p‹ 0.001) disease incidence (DI) in both environments and later maturity groups were affected more than earlier maturity groups (P ‹ 0.05). However, delayed planting only reduced disease severity (DS) significantly (P ‹ 0.05) at Ridgway. At both location maturity groups differed significantly (P‹ 0.001) in DS. In some cases, there was also an interaction of planting date and maturity group which affected SDS expression (P‹0.05). These results from 1994 agree with the overall pattern of results of similar studies conducted from 1987 to 1993, although results from 1990 showed no difference among planting dates. Delay in planting caused reduced yield at Villa Ridge but not at Ridgway. No effect of planting date was observed in seed weight at Villa Ridge, but a slight increase of seed weight at Ridgway with later plantings was observed. Graphical analysis revealed that most pronounced periods of disease increase were related to periods of high soil moisture. Overall disease was much higher at Villa Ridge where soil moisture remained high throughout the season than at Ridgway where soil miosture was low between early July and the beginning of September. The effect of soil temperature was not clear but it appeared that periods of rapid increase of disease were related to periods of lower soil temperatures. According to this and other studies, farmers can reduce the amount of SDS by avoiding SDS- prone fields earlier than necessary. However, the yield reduction often associated with delayed planting must be balanced against the benefit of reducing the SDS. Farmers should minimize SDS risk by planting several varieties which have done best in SDS trials and which encompass a range of maturity. Further research should examine more critically the effect of environmental factors by improving the assessment of soil and air temperature as well as soil moisture. It would be informative to compare soil moistures and other environmental factors not only to leaf symptoms but also to the time of root infection and progression of root and stem