No abstract

Field studies were conducted on a Crowley silt loam under weed-free conditions to determine herbicidal tolerance of ‘Centennial’ soybeans. Early season phytotoxicity and mature plant height reductions were caused by imazaquin at 140 g ai/ha applied preemergence (PRE) followed by imazaquin applied postemergence (POE) at both 140 (labeled rate) and 280 g/ha and for metribuzin plus chlorimuron at 360 plus 60 g ai/ha PRE followed by chlorimuron POE at 18 g/ha (double rate). Soybean yields, averaged across 3 yr, were reduced only for metribuzin plus chlorimuron PRE followed by chlorimuron POE at both 9 and 18 g/ha (10 and 15%, respectively).

Variation in seasonal rainfall accumulation directly affects soybean [Glycine max(L.) Merr.] yields. In southwest Louisiana, where soybeans are primarily grown in rotation with rice (Oryza sativaL.), moisture shortages frequently occur due to the presence of a traffic‐induced hardpan 15 to 20 cm below the soil surface, which restricts root development. Field studies were conducted in southwest Louisiana on a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualf) soil during 1981–1983 to study response of solid‐seeded ‘Dare’ (Maturity Group V) and ‘Ransom’ (Maturity Group VII) soybeans to flood irrigation. Water was applied at R2 bloom (B), R5– R6 pod fill (PF), B + PF, and full season (FS) as needed based on tensiometer readings (60 to 65 cbar). A nonirrigated control was included for comparison. In 1981 rainfall was adequate prior to bloom, but deficient when plants entered the reproductive stage. In 1982 and 1983, rainfall was generally deficient prior to bloom but adequate during the reproductive stage. Yield responses to the irrigation treatments varied among years and were directly related to rainfall distribution. Averaged over years, Dare yields were 161 kg ha−1higher (P=0.05) than the control where water was applied at B + PF and Ransom yields were 168 and 323 kg ha−1higher (P=0.05) than the control where water was applied at B + PF and FS, respectively. Seed/pod for Ransom and 100‐seed weight for Ransom and Dare averaged over years were significantly higher than the control only where water was applied FS. Pods/plant, seed/plant, plant height at maturity, lower pod height, and plant density were not affected by the irrigation treatments. Results indicate that in years where moisture is limited prior to bloom, early application of water can be beneficial, provided it is applied as needed throughout the remainder of the growing season. In years where moisture is adequate in early season prior to bloom, water should be applied during the reproductive stage if water shortage occurs.

Rhizoctonia foliar blight (RFB) is caused by the fungus Rhizoctonia solani Kühn anastomosis group 1, intraspecific groups IA and IB. The disease affects more than half of the soybean [Glycine max (L.) Merr.] acreage in Louisiana. Although less‐susceptible cultivars are recommended, reliable estimates of cultivar susceptibility have been difficult to obtain because of several factors, including uneven distribution of RFB in fields, differences in plant developmental stage at time of infection, and the requirement for free moisture in RFD development. The objective of this study was to screen commercial soybean cultivars grown throughout the southern USA, as well as advanced breeding genotypes, for field reaction to RFB in south Louisiana. Ninety‐one soybean genotypes, representing Maturity Groups IV, V, VI, and VII, were evaluated at two locations in 1993 and 1994. Disease ratings were made with a severity scale that included pods as well as foliage. A range of reactions to RFB was observed among cultivars, but most were severely damaged. Significant negative correlations between RFB severity and yield occurred among cultivars in all Maturity Groups. Results identified several cultivars, notably Buckshot 66 and Pioneer variety 9593, with promising levels of resistance.