Fusarium head blight severity and deoxynivalenol concentration in wheat in response to Gibberella zeae inoculum concentration.

The relationship between inoculum dose and resulting disease levels and deoxynivalenol (DON) accumulation in the Fusarium head blight (FHB) of wheat pathosystem was examined under controlled conditions. Greenhouse-grown spring wheat plants were inoculated at flowering with suspensions that varied in Gibberella zeae macroconidia concentration. The spikes were bagged for 72 h to promote infection and plants were then kept under ambient greenhouse conditions and disease allowed to develop. Spikes were rated at 15 days after inoculation for disease incidence and severity, removed from the greenhouse, and dried. DON concentration was determined in grain-only and whole-spike samples for each inoculation treatment. Regression analysis was used to evaluate the mathematical relationship between inoculum dose and the (i) disease metrics or (ii) DON concentration. Both disease incidence and severity were found to increase sharply in relation to inoculum concentration until an asymptote was reached. In both instances, a negative exponential function was found to best explain this relationship. By contrast, DON concentration in both grain-only and whole-spike tissues increased with additional inoculum. These relationships were best explained with linear functions for both sample types, although DON accumulation increased at a greater rate in whole-spike tissue. The functions were evaluated further using data collected from unrelated field studies and, although not particularly consistent, provided reasonably accurate predictions in growing seasons when the environment was only moderately favorable for FHB.

Family-based mapping of quantitative trait loci in plant breeding populations with resistance to Fusarium head blight in wheat as an illustration.

Traditional quantitative trait loci (QTL) mapping approaches are typically based on early or advanced generation analysis of bi-parental populations. A limitation associated with this methodology is the fact that mapping populations rarely give rise to new cultivars. Additionally, markers linked to the QTL of interest are often not immediately available for use in breeding and they may not be useful within diverse genetic backgrounds. Use of breeding populations for simultaneous QTL mapping, marker validation, marker assisted selection (MAS), and cultivar release has recently caught the attention of plant breeders to circumvent the weaknesses of conventional QTL mapping. The first objective of this study was to test the feasibility of using family-pedigree based QTL mapping techniques generally used with humans and animals within plant breeding populations (PBPs). The second objective was to evaluate two methods (linkage and association) to detect marker-QTL associations. The techniques described in this study were applied to map the well characterized QTL, Fhb1 for Fusarium head blight resistance in wheat (Triticum aestivum L.). The experimental populations consisted of 82 families and 793 individuals. The QTL was mapped using both linkage (variance component and pedigree-wide regression) and association (using quantitative transmission disequilibrium test, QTDT) approaches developed for extended family-pedigrees. Each approach successfully identified the known QTL location with a high probability value. Markers linked to the QTL explained 40-50% of the phenotypic variation. These results show the usefulness of a human genetics approach to detect QTL in PBPs and subsequent use in MAS.

Reaction of Elite Wheat Genotypes from the Northern Great Plains of North America to Septoria Diseases.

Stagonospora nodorum blotch (SNB), caused by Phaeosphaeria nodorum, and Septoria tritici blotch (STB), caused by Mycosphaerella graminicola, are the main pathogens of the Septoria disease complex of wheat (Triticum aestivum) in North America. This study was conducted to determine the disease reaction of 126 elite hard red spring, white, and durum wheat cultivars and advanced breeding lines collected from the northern Great Plains of the United States and Canada to SNB and STB. Seedlings of the 126 wheat genotypes were evaluated for resistance to SNB and STB under controlled environmental conditions. Moreover, these 126 wheat genotypes also were infiltrated with culture filtrate of P. nodorum isolate Sn2000. Based on disease reactions, three cultivars (McNeal, Dapps, and Oklee) and 12 advanced breeding lines (CA-901-580W, 97SO254-8-1, MN03291, MN03308, WA007925, MT0245, ND756, ND801, ND803, ND808, ND809, and ND811) adapted to the northern Great Plains were found to be resistant to both Septoria diseases and insensitive to the culture filtrate. Additionally, eight genetically diverse lines and cultivars, including two tetraploid wheat genotypes, were identified to be resistant to both Septoria diseases. These results suggest that the wheat genotypes contain a broad genetic base for resistance to the Septoria diseases in the northern Great Plains of the United States and Canada, and the resistant sources identified in this study may be utilized in wheat-breeding programs.

Evaluation of Elite Wheat Germ Plasm for Resistance to Tan Spot.

Tan spot, caused by Pyrenophora tritici-repentis, is a serious foliar disease of wheat (Triticum aestivum) in North America. Control of tan spot through management practices and fungicide application is possible; however, the use of resistant varieties is the most effective and economical means of controlling tan spot. This study was conducted to determine the disease reaction of 126 elite hard red spring, white, and durum wheat varieties and advanced breeding lines collected from the northern Great Plains of the United States and Canada to individual races/toxins of P. tritici-repentis. Seedling evaluation of the 126 genotypes was done under controlled environmental conditions with virulent races 2, 3, and 5 of P. tritici-repentis and toxins Ptr ToxA and Ptr ToxB. Based on disease reactions, two resistant varieties and two advanced breeding lines adapted to the northern Great Plains were found to be resistant to all the races and insensitive to the toxins tested. Additionally, six genetically diverse lines/varieties were identified to be resistant to tan spot; however, these sources may not be well adapted to the northern Great Plains. These results suggest that the wheat germ plasm contains a broad genetic base for resistance to the most prevalent races of P. tritici-repentis in North America, and the resistant sources identified in this study may be utilized in wheat breeding programs to develop tan spot resistant varieties.