Effect of Inoculation with Selected Isolates of Stagonospora nodorum on Field Evaluations of Host Resistance in Winter Wheat

Evaluation of a Model for Predicting Onset of Septoria nodorum Blotch in Winter Wheat

Prediction models that aid growers in making decisions on timing of fungicide application are important components of integrated management programs for several foliar diseases of wheat. The risk of Septoria nodorum blotch (caused by Parastagonospora nodorum) onset in winter wheat has been reported to be influenced by location, amount of wheat residue in the field, and cumulative daily infection values 2 weeks prior to day of year (DOY) 102. A model previously developed based on these predictor variables was evaluated for its ability to predict disease onset under field conditions. An experiment was conducted at three locations in North Carolina in 2018, 2019, and 2020, where plots were either treated with >20% wheat residue or received no residue treatment. Plots were monitored for disease symptoms, and disease onset was defined to have occurred when mean disease incidence in a plot was 50%. Of the 298 disease cases recorded, disease onset occurred early (i.e., prior to DOY 102) in 257 cases, while onset was late (i.e., on or after DOY 102) in 41 cases. Model accuracy based on correct classification ranged from 0.67 to 0.95, with a mean of 0.87 across the study period. Similarly, sensitivity rates of the model ranged from 0.88 to 1.0 with a mean of 0.98 across all years. However, the model had low specificity, with a mean rate of 0.15 across the study period. Overall, there was no significant difference in the frequency of observed and predicted cases in the study (χ² = 0.50, P = 0.7788, df = 2). Time to disease onset was significantly correlated with grain yield and explained 26% of variation in yield (P < 0.0001). Results indicated that the disease onset model performs well in predicting early disease onset but requires further evaluation and improvement, particularly in the Piedmont, where it over-predicted early onset in 2 successive years.

Mapping of SnTox3-Snn3 as a major determinant of field susceptibility to Septoria nodorum leaf blotch in the SHA3/CBRD × Naxos population

The effect of the SnTox3-Snn3 interaction was documented for the first time under natural infection at the adult plant stage in the field. Co-segregating SNP markers were identified. Parastagonospora nodorum is a necrotrophic pathogen of wheat, causing Septoria nodorum blotch (SNB) affecting both the leaf and glume. P. nodorum is the major leaf blotch pathogen on spring wheat in Norway. Resistance to the disease is quantitative, but several host-specific interactions between necrotrophic effectors (NEs) and host sensitivity (Snn) genes have been identified, playing a major role at the seedling stage. However, the effect of these interactions in the field under natural infection has not been investigated. In the present study, we saturated the genetic map of the recombinant inbred (RI) population SHA3/CBRD × Naxos using the Illumina 90 K SNP chip. The population had previously been evaluated for segregation of SNB susceptibility in field trials. Here, we infiltrated the population with the purified NEs SnToxA, SnTox1 and SnTox3, and mapped the Snn3 locus on 5BS based on sensitivity segregation and SNP marker data. We also conducted inoculation and culture filtrate (CF) infiltration experiments on the population with four selected P. nodorum isolates from Norway and North America. Remapping of quantitative trait loci (QTL) for field resistance showed that the SnTox3-Snn3 interaction could explain 24% of the phenotypic variation in the field, and more than 51% of the variation in seedling inoculations. To our knowledge, this is the first time the effect of this interaction has been documented at the adult plant stage under natural infection in the field.

Quantitative trait loci for seedling and adult plant resistance to Stagonospora nodorum in wheat

Stagonospora nodorum blotch (SNB) caused by Stagonospora nodorum is a severe disease of wheat (Triticum aestivum) in many areas of the world. S. nodorum affects both seedling and adult plants causing necrosis of leaf and glume tissue, inhibiting photosynthetic capabilities, and reducing grain yield. The aims of this study were to evaluate disease response of 280 doubled haploid (DH) individuals derived from a cross between resistant (6HRWSN125) and susceptible (WAWHT2074) genotypes, compare quantitative trait loci (QTL) for seedling and adult plant resistance in two consecutive years, and assess the contribution of QTL on grain weight. Flag leaves and glumes of individuals from the DH population were inoculated with mixed isolates of S. nodorum at similar maturity time to provide accurate disease evaluation independent of morphological traits and identify true resistance for QTL analysis. Fungicide protected and inoculated plots were used to measure relative grain weight (RGW) as a yield-related trait under pathogen infection. The lack of similar QTL and little or no correlation in disease scores indicate different genes control seedling and adult plant disease and independent genes control flag leaf and glume resistance. This study consistently identified a QTL on chromosome 2DL for flag leaf resistance (QSnl.daw-2D) and 4BL for glume resistance (QSng.daw-4B) from the resistant parent, 6HRWSN125, explaining 4 to 19% of the phenotypic variation at each locus. A total of 5 QTL for RGW were consistently detected, where two were in the same marker interval for QSnl.daw-2D and QSng.daw-4B indicating the contribution of these QTL to yield related traits. Therefore, RGW measurement in QTL analysis could be used as a reliable indicator of grain yield affected by S. nodorum infection.

Artificial Inoculation of Wheat for Selecting Resistance to Stagonospora Nodorum Blotch

In the eastern United States, natural epidemics of Stagonospora nodorum blotch (SNB) are not consistently severe enough to facilitate substantial progress in breeding moderately resistant cultivars of soft red winter wheat. We compared three artificial inoculation methods to natural inoculum in a field experiment involving seven wheat (Triticum aestivum) cultivars with varying levels of SNB resistance. Artificial inoculation methods were: Phaeosphaeria nodorum conidia applied by atomization to three- to four-leaf wheat in early winter, P. nodorum conidia applied by atomization at boot stage in late spring, and P. nodorum-infected wheat straw applied in early winter. The experiment was conducted at Kinston and Plymouth, NC, in 2003-2004, 2004-2005, and 2005-2006, and all treatments had three replicates. Percent diseased canopy was assessed and comparisons were made using disease severity at a single date (early to soft dough stage) and area under the disease progress curve (AUDPC). The relative resistance level of cultivars was consistent across sites, years, and inoculum methods, although the rankings of moderately susceptible and susceptible cultivars were sometimes switched. On average, late spores and straw caused significantly more disease than early spores or natural inoculum (P ≤ 0.05). Biplot analysis indicated that all artificial methods had a higher mean capacity to discriminate among cultivars than did natural inoculum (P ≤ 0.05). On average, artificial inoculation increased the capacity of environments to separate wheat cultivars by SNB resistance.

Frequency of Phaeosphaeria nodorum, the Sexual Stage of Stagonospora nodorum, on Winter Wheat in North Carolina

ABSTRACT Ascocarps of Phaeosphaeria nodorum, which causes Stagonospora nodorum blotch (SNB) of wheat, have not been found by others in the eastern United States despite extensive searches. We sampled tissues from living wheat plants or wheat debris in Kinston, NC, each month except June from May to October 2003. Additional wheat samples were gathered in Kinston, Salisbury, and Plymouth, NC, in 2004 and 2005. For the 3 years, in all, 2,781 fruiting bodies were dissected from the wheat tissues and examined microscopically. Fruiting bodies were tallied as P. nodorum pycnidia or ascocarps, "unknown" (not containing spores, potentially P. nodorum or other fungi), or "other fungi." In the 2003 sample, asco-carps of P. nodorum were present each month after May at a frequency of 0.8 to 5.4%, and comprised a significantly higher percentage of fruiting bodies from wheat spikes than of those from lower stems and leaves. Ascocarps also were found at frequencies <10% in some wheat debris samples from 2004 and 2005. Analysis of the nucleotide sequences of internally transcribed spacer regions of 18 genetically distinct North Carolina isolates from 2003 suggested that all were P. nodorum, not the morphologically similar P. avenaria f. sp. triticea. Neither the 18 isolates from 2003 nor a set of 77 isolates derived from 2004 Kinston leaf samples gave reason to suspect a mating-type imbalance in the larger P. nodorum population (P >/= 0.4). We conclude that, in North Carolina, sexual reproduction plays a role in initiation of SNB epidemics and the creation of adaptively useful genetic variability, although its relative importance in structuring this population is uncertain.