Phytophthora sojae Kaufm. and Gerd., Pythium irregulare Busiman, and Fusarium graminearum Schwabe [teleomorph: Gibberella zeae (Schwien.) Petch] are important pathogens of soybean [Glycine max (L.) Merr.] and are all capable of causing seed rot, damping‐off, and root rot. The objective of this study was to identify quantitative trait loci (QTL) for resistance to Py. irregulare and to refine previously mapped QTL for resistance to P. sojae and F. graminearum in a larger, more advanced ‘Conrad’ × ‘Sloan’ F9:11 recombinant inbred line population. The population was mapped with 1032 single nucleotide polymorphisms from the SoySNP6K BeadChip and 31 polymerase chain reaction–based molecular markers. Families were evaluated for resistance response to three isolates of P. sojae, one isolate of Py. irregulare, and one isolate of F. graminearum. A total of 10, 2, and 3 QTL and suggestive QTL were found that confer resistance to P. sojae, Py. irregulare, and F. graminearum, respectively. Individual QTL explained 2 to 13.6% of the phenotypic variance. Quantitative trait loci for resistance toward both Py. irregulare and F. graminearum colocalized on chromosome 19. This resistance was contributed by Sloan and was juxtaposed to a QTL for P. sojae with resistance contributed from Conrad. Alleles for resistance to different pathogens contributed from different parents in the same region; the number of unique QTL for each pathogen and the lack of correlation of resistance suggest that different mechanisms are involved in resistance toward P. sojae, Py. irregulare, and F. graminearum.

Key messageA major novel QTL was identified in a recombinant inbred line population derived from a cross of ‘Wyandot’ × PI 567301B forFusarium graminearum,a seed and seedling pathogen of soybean.AbstractFusarium graminearum is now recognized as a primary pathogen of soybean, causing root, seed rot and seedling damping-off in North America. In a preliminary screen, ‘Wyandot’ and PI 567301B were identified with medium and high levels of partial resistance to F. graminearum, respectively. The objective of this study was to characterise resistance towards F. graminearum using 184 recombinant inbred lines (RILs) derived from a cross of ‘Wyandot’ × PI 567301B. The parents and the RILs of the mapping population were evaluated for resistance towards F. graminearum using the rolled towel assay in a randomized incomplete block design. A genetic map was constructed from 2545 SNP markers and 2 SSR markers by composite interval mapping. One major and one minor QTL were identified on chromosomes 8 and 6, respectively, which explained 38.5 and 8.1 % of the phenotypic variance. The major QTL on chromosome 8 was mapped to a 300 kb size genomic region of the Williams 82 sequence. Annotation of this region indicates that there are 39 genes including the Rhg4 locus for soybean cyst nematode (SCN) resistance. Based on previous screens, PI 567301B is susceptible to SCN. Fine mapping of this locus will assist in cloning these candidate genes as well as identifying DNA markers flanking the QTL that can be used in marker-assisted breeding to develop cultivars with high levels of resistance to F. graminearum.

Tomatoes (Solanum lycopersicum L.) are a valuable horticultural crop that are grown and consumed worldwide. Optimal production is hindered by several factors, among which Verticillium dahliae, the cause of Verticillium wilt, is considered a major biological constraint in temperate production regions. V. dahliae is difficult to mitigate because it is a vascular pathogen, has a broad host range and worldwide distribution, and can persist in soil for years. Understanding pathogen virulence and genetic diversity, host resistance, and plant-pathogen interactions could ultimately inform the development of integrated strategies to manage the disease. In recent years, considerable research has focused on providing new insights into these processes, as well as the development and integration of environment-friendly management approaches. Here, we discuss the current knowledge on the race and population structure of V. dahliae, including pathogenicity factors, host genes, proteins, enzymes involved in defense, and the emergent management strategies and future research directions for managing Verticillium wilt in tomatoes.

Sorghum anthracnose (Colletotrichum sublineola) reduces grain yield up to 50% but suggested management tactics have not yet been developed for the mid-Atlantic United States, where warm, wet conditions favor disease. Under factorial arrangement, five fungicides plus a nontreated control and four application timings were compared for foliar anthracnose control, yield, and profitability of fungicide use in grain sorghum over eight site-years in Virginia and North Carolina. Anthracnose severity was rated at the hard dough stage, and grain yield was determined at harvest. Every percent increase in disease severity resulted in yield losses of 27 to 85 kg/ha. Pyraclostrobin and pyraclostrobin plus fluxapyroxad reduced anthracnose (P < 0.01), and three applications resulted in less disease and greater yield compared with single applications (P < 0.01). However, three applications exceed the labeled maximum application for the fungicides and are not economical. Among single applications, boot or flowering timings reduced disease, and flowering applications resulted in the overall greatest yield. Results suggest that when disease onset occurs at or prior to boot, a single application of pyraclostrobin-containing fungicide at or just prior to flowering reduces anthracnose, protects yield, and increases income. However, when disease is absent or severity is low prior to flowering, fungicide application may not be profitable.

No abstract

The Soybean Nested Association Mapping (SoyNAM) populations were developed from 40 parents and have been used to map genes underlying complex traits such as yield and disease resistance. Soil borne pathogens that affect soybean seed and seedlings result in significant losses due to reduced stands and costs associated with replanting. This study compared the response of these 40 SoyNAM parent genotypes to seed- and seedling-rot pathogens, Fusarium graminearum, Phytophthora sojae, Globisporangium ultimum var. ultimum, G. ultimum var. sporangiiferum, and G. irregulare groups 1, 2 and G. cryptoirregulare. None of the parental genotypes conferred high levels of resistance to F. graminearum nor G. ultimum var. sporangiiferum. Of the 40 parental genotypes, 15 were resistant to P. sojae OH1 (vir 7) indicating they contain Rps genes while the remaining (including the common parent IA 3023) do not have Rps genes. Based on inoculations with known P. sojae pathotypes, Rps1c was the most common followed by Rps1a as both Rps genes confer resistance to isolates OH4 (vir 1a, 1c, 7) and OH25 (vir 1a, 1b,1c, 1k, 7). Eight of the SoyNAM parents had higher levels of partial resistance to P. sojae than Conrad (cultivar with moderate resistance). There was moderate resistance to G. ultimum var ultimum among the 40 parents and to G. irregulare subpopulations among the 6 parents that were evaluated. The SoyNAM parental lines and populations are an excellent resource available for soybean breeders to advance the development of new cultivars with improved resistance to some soilborne pathogens.

No abstract