A Model for Predicting Onset of Stagonospora nodorum Blotch in Winter Wheat Based on Preplanting and Weather Factors.

Stagonospora nodorum blotch (SNB) caused by Parastagonospora nodorum is a serious disease of wheat worldwide. In the United States, the disease is prevalent on winter wheat in many eastern states, and its management relies mainly on fungicide application after flag leaf emergence. Although SNB can occur prior to flag leaf emergence, the relationship between the time of disease onset and yield has not been determined. Such a relationship is useful in identifying a threshold to facilitate prediction of disease onset in the field. Disease occurred in 390 of 435 disease cases that were recorded across 11 counties in North Carolina from 2012 to 2014. Using cases with disease occurrence, the effect of disease onset on yield was analyzed to identify a disease onset threshold that related time of disease onset to yield. Regression analysis showed that disease onset explained 32% of the variation in yield (P < 0.0001) and from this relationship, day of year (DOY) 102 was identified as the disease onset threshold. Below-average yield occurred in 87% of the disease cases when disease onset occurred before DOY 102 but in only 28% of those cases when onset occurred on or after DOY 102. Subsequently, binary logistic regression models were developed to predict the occurrence and onset of SNB using preplanting factors and cumulative daily infection values (cDIV) starting 1 to 3 weeks prior to DOY 102. Logistic regression showed that previous crop, latitude, and cDIV accumulated 2 weeks prior to DOY 102 (cDIV.2) were significant (P < 0.0001) predictors of disease occurrence, and wheat residue, latitude, longitude, and cDIV.2 were significant (P < 0.0001) predictors of disease onset. The disease onset model had a correct classification rate of 0.94 and specificity and sensitivity rates >0.90. Performance of the disease onset model based on the area under the receiver operating characteristic curve (AUC), κ, and the true skill statistic (TSS) was excellent, with prediction accuracy values >0.88. Similarly, internal validation of the disease onset model based on AUC, κ, and TSS indicated good performance, with accuracy values >0.88. This disease onset prediction model could serve as a useful decision support tool to guide fungicide applications to manage SNB in wheat.

Quantifying the Effects of Wheat Residue on Severity of Stagonospora nodorum Blotch and Yield in Winter Wheat.

Stagonospora nodorum blotch (SNB), caused by the fungus Parastagonospora nodorum, is a major disease of wheat (Triticum aestivum). Residue from a previously infected wheat crop can be an important source of initial inoculum, but the effects of infected residue on disease severity and yield have not previously been quantified. Experiments were conducted in Raleigh and Salisbury, North Carolina, in 2012, 2013, and 2014 using the moderately susceptible winter wheat cultivar DG Shirley. In 2014, the highly susceptible cultivar DG 9012 was added to the experiment and the study was conducted at an additional site in Tyner, North Carolina. Four (2012) or six (2013 and 2014) wheat residue treatments were applied in the field in a randomized complete block design with five replicates. Treatments in 2012 were 0, 30, 60, and 90% residue coverage of the soil surface, while 10 and 20% residue treatments were added in 2013 and 2014. Across site-years, disease severity ranged from 0 to 50% and increased nonlinearly (P < 0.05) as residue level increased, with a rapid rise to an upper limit and showing little change in severity above 20 to 30% soil surface coverage. Residue coverage had a significant (P < 0.05) effect on disease severity in all site-years. The effect of residue coverage on yield was only significant (P < 0.05) for DG Shirley at Raleigh and Salisbury in 2012 and for DG 9012 at Salisbury in 2014. Similarly, residue coverage significantly (P < 0.05) affected thousand-kernel weight only of DG 9012 in 2014 at Raleigh and Salisbury. Our results showed that when wheat residue was sparse, small additions to residue density produced greater increases in SNB than when residue was abundant. SNB only led to effects on yield and test weight in the most disease-conducive environments, suggesting that the economic threshold for the disease may be higher than previously assumed and warrants review.

Postharvest Disease Development on Southern Highbush Blueberry Fruit in Relation to Berry Flesh Type and Harvest Method.

Postharvest decay, incited by various fungal pathogens, is a major concern in most blueberry production areas of the United States. Because the risk of infection is increased by fruit bruising, which in turn is increased by machine-harvesting, it has been difficult to harvest fruit from the early-maturing but soft-textured southern highbush blueberries (SHB) mechanically for the fresh market. This could change fundamentally with the recent development of SHB genotypes with crisp-textured ("crispy") berries, i.e., fruit with qualitatively firmer flesh and/or more resistant skin. Four replicate row sections of two or three SHB genotypes having crispy fruit and three with conventional fruit were either hand- or machine-harvested at a commercial blueberry farm in northern Florida in April 2009 and May 2010. Harvested fruit were sorted, packed, and placed in cold storage (2°C) for up to 3 weeks. Average counts of aerobic bacteria, total yeasts and molds, coliforms, and Escherichia coli on fruit samples before the cold storage period were below commercial tolerance levels in most cases. In both years, natural disease incidence after cold storage was lowest for hand-harvested crispy fruit and highest for machine-harvested conventional fruit. Interestingly, machine-harvested crispy fruit had the same or lower disease incidence as hand-harvested conventional fruit. Across all treatments, natural postharvest disease incidence was inversely related to fruit firmness, with firmness values >220 g/mm associated with low disease. In separate experiments, samples from the 0-day cold storage period were inoculated at the stem end with Alternaria alternata, Botrytis cinerea, or Colletotrichum acutatum, and disease incidence was assessed after 7 days in a cold room followed by 60 to 72 h at room temperature. In response to artificial inoculation, less disease developed on crispy berries. No significant effect of harvest method was observed, except for A. alternata inoculation in 2009, when hand-harvested fruit developed a lower level of disease than machine-harvested fruit. Taken together, this study suggests that mechanical harvesting of SHB cultivars with crisp-textured berries is feasible from a postharvest pathology perspective.