Evidence of Selection for Fungicide Resistance in Zymoseptoria tritici Populations on Wheat in Western Oregon.

Plant pathogens pose a major challenge to maintaining food security in many parts of the world. Where major plant pathogens are fungal, fungicide resistance can often thwart regional control efforts. Zymoseptoria tritici, causal agent of Septoria tritici blotch, is a major fungal pathogen of wheat that has evolved resistance to chemical control products in four fungicide classes in Europe. Compared with Europe, however, fungicide use has been less and studies of fungicide resistance have been infrequent in North American Z. tritici populations. Here, we confirm first reports of Z. tritici fungicide resistance evolution in western Oregon through analysis of the effects of spray applications of propiconazole and an azoxystrobin + propiconazole mixture during a single growing season. Frequencies of strobilurin-resistant isolates, quantified as proportions of G143A mutants, were significantly higher in azoxystrobin-sprayed plots compared with plots with no azoxystrobin treatment at two different locations and were significantly higher in plots of a moderately resistant cultivar than in plots of a susceptible cultivar. Thus, it appears that western Oregon Z. tritici populations have the potential to evolve levels of strobilurin resistance similar to those observed in Europe. Although the concentration of propiconazole required to reduce pathogen growth by 50% values were numerically greater for isolates collected from plots receiving propiconazole than in control plots, this effect was not significant (P > 0.05).

Spring Wheat Tolerance and Resistance to Heterodera avenae in the Pacific Northwest.

The cereal cyst nematode Heterodera avenae reduces wheat yields in the Pacific Northwest. Previous evaluations of cultivar resistance had been in controlled environments. Cultivar tolerance had not been evaluated. Seven spring wheat trials were conducted in naturally infested fields in three states over 2 years. A split-plot design was used for all trials. Five trials evaluated both tolerance and resistance in 1.8-by-9-m plots treated or not treated with nematicides. Two trials evaluated resistance in 1-m head rows where each wheat entry was paired with an adjacent row of a susceptible cultivar. Cultivars with the Cre1 resistance gene ('Ouyen' and 'Chara') reduced the postharvest density of H. avenae under field conditions, confirming Cre1 parents as useful for germplasm development. Ouyen was resistant but it was also intolerant, producing significantly lower grain yield in controls than in plots treated with nematicides. Susceptible cultivars varied in tolerance. Undefined resistance was identified in one commercial cultivar ('WB-Rockland) and four breeding lines (UC1711, SO900163, SY-B041418, and SY-97621-05). This research was the first systematic field demonstration of potential benefits to be derived through development and deployment of cultivars with resistance plus tolerance to cereal cyst nematode in North America.

Optimizing the statistical estimation of the parameters of the Farquhar-von Caemmerer-Berry model of photosynthesis.

The model of Farquhar, von Caemmerer and Berry is the standard in relating photosynthetic carbon assimilation and concentration of intercellular CO(2). The techniques used in collecting the data from which its parameters are estimated have been the object of extensive optimization, but the statistical aspects of estimation have not received the same attention. The model segments assimilation into three regions, each modeled by a distinct function. Three parameters of the model, namely the maximum rate of Rubisco carboxylation (V(c max)), the rate of electron transport (J), and nonphotorespiratory CO(2) evolution (R(d)), are customarily estimated from gas exchange data through separate fitting of the component functions corresponding to the first two segments. This disjunct approach is problematic in requiring preliminary arbitrary subsetting of data into sets believed to correspond to each region. It is shown how multiple segments can be estimated simultaneously, using the entire data set, without predetermination of transitions by the investigator. Investigation of the number of parameters that can be estimated in the two-segment model suggests that, under some conditions, it is possible to estimate four or even five parameters, but that only V(c max), J, and R(d), have good statistical properties. Practical difficulties and their solutions are reviewed, and software programs are provided.