Field evaluation of maturity group V early soybean cultivars to Cercospora leaf blight, frogeye leaf spot, Septoria brown spot, southern blight, and target spot at Stoneville, MS (clay, irrigated), 2018

摘要

A field evaluation of 37 maturity group V early soybean cultivars to Cercospora leaf blight, frogeye leaf spot, Septoria leaf spot, southern blight, and target spot was performed in Stoneville, MS. Soybean cultivars were planted in a Sharkey clay soilon 8 May. The previous crop was rice. Plots consisted of four rows of soybean, spaced 30-in. apart, planted with 9 seed per foot of row and trimmed to a final length of 14.5 ft. The experimental design was a randomized complete block with three replications. Plots were furrow irrigated as needed. Field evaluations for the presence of foliar disease were conducted at approximately R6.5 using a 0 to 9 scale whereby 0=no disease and 9=excessive disease symptoms. Leaf surface area affected was evaluated for frogeye leaf spot based on a scale of 0 to 9 where 0 = no disease present and 9 = severe frogeye leaf spot. While Cercospora leaf blight was rated based on the presence of disease on leaves (0 to 5), petioles (6), and on pods and stems (7 to 9) based on disease intensity as assessed by coloration of plant material (leaves with distinct purple coloration and petioles, pods and main stems with purple to near black coloration). Septoria leaf spot and target spot were rated in a similar fashion based on the presence of the disease within the plant canopy so that 0 to 3 indicated the presence in the lower canopy, 4 to 5 in the middle canopy, 6 to 8 in the upper canopy, and 9 indicative of extensive defoliation throughout the entire plant canopy and the disease in the upper most canopy. Southern blight was evaluated using a 0 to 9 scale taking into account the appearance of the plants in the whole plot based on the presence of white mycelia and sclerotia (fungal signs) on the base of plants as well as additional observational symptoms that included mild interveinal chlorosis and premature wilting of plants with leaves remaining. Plots exhibiting no symptoms associated with southern blight were given a 0, plots whereby approximately half of the plot wereexhibiting the symptoms indicative of southern blight were given a 5, while plots exhibiting numerous plants with mycelia and sclerotia as well as severe early wilting (prior to R8) were given a 9. Plots were harvested on 8 Oct by harvesting the middle two rows of soybean. Yield was calculated by standardizing moisture to 13%. Prior to analysis, field disease evaluations were treated as ordinal data and therefore rank transformed using PROC RANK (Version 9.4, SAS Institute, Cary, NC). The resulting transformed data were subjected to analysis of variance in PROC GLM, and means were compared using Fisher's protected least significant difference (LSD) at the P=0.05 significance level. Means were back-transformed for the presentation of data. Yield data were analyzed using the SAS GLM procedure (Version 9.3, SAS Institute, Cary, NC). Means separation was conducted using Fisher's protected least significant difference at P=0.05.