Planting date, irrigation, maturity group, year, and environment effects on Phomopsis longicolla, seed germination, and seed health rating of soybean in the early soybean production system of the midsouthern USA

Abstract

Field studies were conducted in 1995–1997 and in 2001 in plots naturally infested with Phomopsis sp. at Stoneville, MS (33°26′N latitude) to determine the effects of irrigation, maturity group (MG), year and date of planting (DOP) on the incidence of Phomopsis longicolla (PL), seed germination, and seed health rating (SHR), a measure of diseased seed and germinated seedlings, in the early soybean [ Glycine max (L.) Merr.] production system (ESPS). Except for only an irrigation effect on SHR, there was a significant effect due to year, irrigation, MG, and DOP on the incidence of P. longicolla and seed germination. There was a negative relationship between PL and seed germination in 1995 ( r = - 0.77 , P ⩽ 0.05 ) , 1996 ( r = - 0.97 , P ⩽ 0.05 ) and 2001 ( r = - 0.87 , P ⩽ 0.05 ) . There was also a negative relationship between germination and SHR in 1996 ( r = - 0.98 , P ⩽ 0.05 ) , 1997 ( r = - 0.86 , P ⩽ 0.05 ) , and 2001 ( r = - 0.99 , P ⩽ 0.05 ) , but not in 1995. There was a positive relationship between PL and SHR for 1996 ( r = 0.97 , P ⩽ 0.05 ) , 1997, and 2001 ( r = 0.87 , P ⩽ 0.05 ) . Irrigated treatments in drier years (1995 and 1997) produced a higher level of PL than did nonirrigated treatments, suggesting that soil moisture could have increased the relative humidity in the canopy that provided favorable environment for PL. The incidence of PL was also associated with total rainfall and rainfall frequency for each year. It was higher in 1996 and 2001 than in 1995 and 1997. Germination was increased and PL and SHR were reduced in seed from drier years. The rainfall amount in August of 2001 and 1996 was 220 and 112 mm, and the number of rainy days in the same month was 14 and 11 days, respectively. Early planting of MG IV varieties resulted in higher level of PL and SHR and lower germination. Conducive moisture environment overrode the effects of irrigation. A significant difference between MG IV and V in 2001 was detected in SHR and germination than in PL. This indicates that SHR and germination may be used in concert when evaluating the effect of PL on seed quality. This research highlights the importance and significance of PL and its effect on SHR and seed germination. It also highlights that environmental conditions that are conducive are required to see significant treatment effects.