D. V. Phillips

Performance of frogeye leaf spot-resistant and -susceptible near-isolines of soybean

Frogeye leaf spot (FLS) caused by Cercospora sojina Hara is a disease of soybean (Glycine max (L.) Merr.) that causes significant seed yield losses in warm, humid environments of southeastern United States. The Rcs3 gene in soybean has been reported to condition resistance to all known races of C. sojina. The objectives of this study were to determine the effectiveness of Rcs3 in limiting seed yield loss due to FLS and to compare the seed yield of the resistant and susceptible near-isolines (NILs) in the absence of significant FLS disease. Four pairs of NILs-Colquitt/Colquitt-Rcs3, Gordon/Gordon-Rcs3, Thomas/Thomas-Rcs3, and Wright/Wright-Rcs 3-were evaluated in 23 field experiments in Alabama, Florida, Georgia, Louisiana, Mississippi, and South Carolina during 1992 to 1994. The amount of damage to susceptible soybean caused by FLS was dependent on the specific environment. All four of the Rcs3 NILs were resistant to the prevalent races of FLS in all environments. In the absence of significant FLS disease, each of the Rcs3 NILs was at least equal to the respective susceptible line in its seed yield. In the presence of FLS infestation, the susceptible lines suffered significant seed yield loss (up to 31%) compared to their Rcs3 NILs. The effect of FLS on seed yield was dependent on cumulative disease severity over the growing season. Thus, the area under disease progress curve was more useful than percent of leaf area infected at the end of the growing season (R7 stage of development) in explaining the seed yield loss due to FLS.

Effect of crop rotation on take-all of wheat in double-cropping systems

Take-all of wheat (Triticum aestivum), caused by Gaeumannomyces graminis var. tritici, became a serious problem with the widespread adoption of wheat:soybean double-cropping and minimum tillage farming systems in the southeastern United States during the past 30 years. A long-term crop rotation study was initiated in 1994 with 12 double-cropping sequences incorporating wheat, rye, or canola as the fall-planted crop and soybean or grain pearl millet as the summer crop. Cotton and fallow were included in some summer rotations during the last 2 years of the study. The purpose was to identify sustainable alternatives to the continuous wheat:soybean system that would provide acceptable management of take-all. G. graminis var. tritici cultured on autoclaved oats was incorporated into soil prior to planting the first seasons crop. Take-all was severe in rotations with continuous wheat each year. Pearl millet was compatible with the cropping system but did not affect incidence or severity of take-all in a following wheat crop. Soybean or pearl millet had little effect on yield loss due to take-all in a subsequent wheat crop. A 1-year rotation with canola significantly reduced take-all incidence and severity. At the end of the second and third seasons, in those rotations where wheat followed 1 year of canola, wheat grain yield was the same as that in control plots that had little or no take-all. Two consecutive years of canola did not suppress take-all or improve wheat yields any more than a single year of canola between wheat crops. Seedling assays for take-all incidence and severity in growth chambers were conducted using soil collected twice each year near the end of each crops growing season. Results were similar to those observed in the field. However, canola in the rotation had a greater effect in suppressing disease severity than disease incidence. Canola can be a valuable rotational crop for management of take-all in wheat in the southeastern United States.

Genetic diversity of Cercospora sojina revealed by amplified fragment length polymorphism markers

Abstract Cercospora sojina Hara, the causal agent of frogeye leaf spot of soybean (Glycine max (L.) Merr.), causes yield reductions worldwide. Although the phenotypic diversity (physiological races) of this pathogen has been assessed through its ability to affect soybean lines with different resistant genes (Rcs genes), little is known about the pathogens genetic diversity. In order to better understand the genetic diversity that exists with C. sojina, a historical collection of 62 C. sojina isolates from Brazil (10 isolates), China (7 isolates), Nigeria (1 isolate), and United States (44 isolates) was used for genetic diversity analysis with amplified fragment length polymorphism (AFLP) markers. The average genetic similarity of the isolates was 0.56 on a scale between 0 and 1, indicating a high degree of genetic diversity within the species. Cluster analysis resulted in two major clusters and seven sub-clusters. Two isolates collected from Georgia were clustered together, and two isolates from China were clustered together. Besides these four isolates, no clear separation of isolates based on origin was found. Our results provide evidence that substantial genetic diversity exists within the species of C. sojina and that selection for broad-spectrum host-resistance should be targeted in soybean breeding programmes.