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Charcoal rot of soybean, caused by Macrophomina phaseolina , is a disease of economic significance in the United States. Although there are soybean cultivars with moderate resistance, identifying and quantifying resistance is challenging. Existing assays are time consuming, and results are often highly variable. The objectives of this research were to (i) create a reproducible seed plate assay (SPA) for charcoal rot resistance and (ii) correlate field-based disease assessments with SPA results on diverse soybean accessions. To develop the SPA, surface-disinfected seeds from eight soybean genotypes (representing three susceptible and five resistant cultivars) were placed on water agar plates inoculated with M. phaseolina . After incubation at room temperature in darkness for 7 days, percent germination was determined for each cultivar relative to the germination on noninoculated plates. Results from SPA were in general agreement with published responses. None of the soybean genotypes showed complete resistance to M. phaseolina. For the second objective, charcoal rot resistance in 18 soybean accessions was assayed with SPA, and results were analyzed for correlation with field disease assessments from Stuttgart, AR, from 2011 to 2014 and from Rohwer, AR, in 2011 and 2012. SPA consistently categorized soybean genotype resistance compared with field disease assessment averages, and results were consistent with previously published resistance determinations. SPA was significantly correlated with percent height of internal stem discoloration (PHSD) at Stuttgart from 2011 to 2013 and in 2012 at Rohwer, with root and stem severity (RSS) at Rohwer in 2012, and with tap root colonization (CFU) at Stuttgart in 2012. SPA was significantly correlated to yield at Stuttgart in 2011, 2013, and 2014, and in 2011 and 2012 at Rohwer. Yield was not correlated to RSS, PHSD, or CFU at either location or in any year. Therefore, SPA is a reproducible and rapid assay for charcoal rot resistance in soybean and is significantly associated to field performance.

Gray leaf spot (GLS), caused by the sibling species Cercospora zeina or Cercospora zeae-maydis, is cited as one of the most important diseases threatening global maize production. C. zeina fails to produce cercosporin in vitro and, in most cases, causes large coalescing lesions during maize infection, a symptom generally absent from cercosporin-deficient mutants in other Cercospora spp. Here, we describe the C. zeina cercosporin toxin biosynthetic (CTB) gene cluster. The oxidoreductase gene CTB7 contained several insertions and deletions as compared with the C. zeae-maydis ortholog. We set out to determine whether complementing the defective CTB7 gene with the full-length gene from C. zeae-maydis could confer in vitro cercosporin production. C. zeina transformants containing C. zeae-maydis CTB7 were generated by Agrobacterium tumefaciens-mediated transformation and were evaluated for in vitro cercosporin production. When grown on nitrogen-limited medium in the light-conditions conducive to cercosporin production in other Cercospora spp.-one transformant accumulated a red pigment that was confirmed to be cercosporin by the KOH assay, thin-layer chromatography, and ultra performance liquid chromatography-quadrupole-time-of-flight mass spectrometry. Our results indicated that C. zeina has a defective CTB7, but all other necessary machinery required for synthesizing cercosporin-like molecules and, thus, C. zeina may produce a structural variant of cercosporin during maize infection.

The plant-pathogenic fungus Mycosphaerella graminicola (asexual stage: Septoria tritici) causes septoria tritici blotch, a disease that greatly reduces the yield and quality of wheat. This disease is economically important in most wheat-growing areas worldwide and threatens global food production. Control of the disease has been hampered by a limited understanding of the genetic and biochemical bases of pathogenicity, including mechanisms of infection and of resistance in the host. Unlike most other plant pathogens, M. graminicola has a long latent period during which it evades host defenses. Although this type of stealth pathogenicity occurs commonly in Mycosphaerella and other Dothideomycetes, the largest class of plant-pathogenic fungi, its genetic basis is not known. To address this problem, the genome of M. graminicola was sequenced completely. The finished genome contains 21 chromosomes, eight of which could be lost with no visible effect on the fungus and thus are dispensable. This eight-chromosome dispensome is dynamic in field and progeny isolates, is different from the core genome in gene and repeat content, and appears to have originated by ancient horizontal transfer from an unknown donor. Synteny plots of the M. graminicola chromosomes versus those of the only other sequenced Dothideomycete, Stagonospora nodorum, revealed conservation of gene content but not order or orientation, suggesting a high rate of intra-chromosomal rearrangement in one or both species. This observed "mesosynteny" is very different from synteny seen between other organisms. A surprising feature of the M. graminicola genome compared to other sequenced plant pathogens was that it contained very few genes for enzymes that break down plant cell walls, which was more similar to endophytes than to pathogens. The stealth pathogenesis of M. graminicola probably involves degradation of proteins rather than carbohydrates to evade host defenses during the biotrophic stage of infection and may have evolved from endophytic ancestors., Competing Interests: The authors have declared that no competing interests exist.