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A theoretical probability distribution conveys more information than the mean in summarizing data. We investigated the ability of two discrete probability distributions, binomial and β-binomial, to describe the incidence (proportion) of seed-borne fungi among seed lots. The fit of the distributions to 185 data sets was assessed by either χ2 analysis or a dithered Kolmogorov-Smirnov goodness-of-fit test. The data sets represented a range of fungi, crops, and geographic regions. The binomial distribution was an adequate fit to only 36% of the data sets, whereas the β-binomial distribution adequately fit 85% of the data sets (P > 0.05). The β-binomial was a better fit than the binomial in 72% of data sets (P < 0.01) based on the likelihood-ratio test, indicating that there was greater variability in seed infection than expected for a binomial (i.e., random) distribution. For a subset of 25 data sets on wheat-seed infection by Fusarium graminearum Schwabe, a binary power law analysis indicated that heterogeneity of seed infection (summarized by the θ parameter of the β-binomial) was a function of mean incidence. Therefore, in most instances, the β-binomial captures the observed heterogeneity in the incidence of seed-borne fungi.

Fungicide utilization is expected to increase on winter wheat (Triticum aestivumL.) in response to registration of fungicides that control powdery mildew (Erysiphe graminisDC) and Septoria nodorum blotch [Phaeosphaeria nodorum(Müller)]. Field experiments were conducted in New York in 1985 and 1986 on a Honeoye silt loam (fine‐loamy, mixed, mesic Glossoboric Hapludalf) to examine the influence of N [67 kg ha−1at Zadoks Growth Stage (GS) 25, and split‐application of 67 + 67 kg ha−1at GS 25 and GS 31], and timing of fungicide application (0.28 and 1.8 kg ha−1a.i. of triadimefon [1‐(4‐cholorphenoxy)‐3,3‐dimethyl‐1‐(1H‐1,2,4‐triazol‐1‐yl(‐2‐butanone] and mancozeb [zinc ion and manganese ethylene bisdithiocarbamate], respectively, at GS 31 or GS 55) on disease severity, CO2exchange rate (CER) of the flag leaf, yield components, and grain yield. Moderately dry and warm conditions resulted in low foliar disease severity from GS 31 to GS 55, thereby providing limited secondary inoculum to infect the flag leaf (<8% disease severity at GS 75) during excessively wet grain‐filling periods. Fungicides did not influence flag leaf CER and kernel weight, which was reflected in the limited grain yield responses to fungicide applications at GS 31 and GS 55 compared to no fungicide treatment (5.90, 6.07, and 5.81 Mg ha−1, respectively). The split‐N application increased grain yield compared to the single N treatment (5.58 to 6.27 Mg ha−1), especially in 1985 (5.47 to 6.40 Mg ha−1) when kernels per spike were increased (26.5 to 30.2). The data suggest that high rates of N, split‐applied, have potential to increase yield of winter wheat and foliar disease severity in the northeastern USA. Fungicides, however, should not be applied as a preventive measure for mildew and leaf spot control if severity is low from GS 31 to GS 60.

Specific resistance to powdery mildew of wheat (Triticum aestivumL.), incited by Erysiphe graminisf. sp. tritici, has been associated with several major genes that are relatively easy to select in segregating populations. Nonspecific resistance is more difficult to select because of genetic and nongenetic factors that lower heritability. The objectives of this study were to: (i) determine the effectiveness of recurrent selection for accumulating genes for nonspecific resistance to powdery mildew in two winter wheat populations, and (ii) examine the effect of changes in the level of resistance on other components of resistance. Two winter wheat populations (designated Early and Elite) carrying a dominant male sterile gene were subjected to two cycles of recurrent selection for powdery mildew resistance using phenotypic recurrent selection and modified half‐sib family selection. Ten‐day‐old seedlings were inoculated in the greenhouse with a spore suspension consisting of equal amounts of four isolates possessing combined virulence that overcame all known powdery mildew resistance genes except Pm3band Pm8. Beginning 7 d after inoculation, area under the disease progress carve (AUDPC) was determined four times at 4‐d intervals and used select individuals for the next cycle. The mean AUDPC on the primary leaves of S1families and the realized gains after two cycles of selection indicated that the Elite population averaged 46% gain over the two selection methods compared to 35% gain for the Early population. Over two cycles, half sib selection gains of 40 and 53% were realized in the Early and Elite population, respectively, while phenotypic recurrent selection reduced AUDPC 30% in the Early population and 40% in the Elite population. For both populations, selection for low AUDPC resulted in significantly decreased final disease severity ratings, colony number, and cumulative sporulation capacity. Duration of the incubation period was significantly increased only with two cycles of half‐sib selection for AUDPC. A high correlation between AUDPC and the final disease severity rating indicated that the breeder could select for nonspecific resistance using a final disease severity rating provided the test materials are all inoculated simultaneously.

To identify Stagonospora avenae genetically, 53 isolates from Europe and North America were assessed by RFLP analysis and internal transcribed spacer (ITS) sequence polymorphisms in comparison with isolates of S. nodorum, S. arenaria and Septoria tritici. Our results also provided insight on possible phylogenetic divergence within the species currently identified by characteristic conidial morphology as S. avenae. Isolates from Europe, Canada, Minnesota and North Dakota showed little genetic variation. In RFLP and ITS sequence analyses, S. avenae isolates from New York and ATCC 12277 (origin unknown) had a closer relationship with barley-biotype S. nodorum, and S. avenae f. sp. triticea isolates of ATCC 26370 and ATCC 26377 from Minnesota were closely associated with wheat-biotype S. nodorum. Enzyme restrictions of ITS PCR-amplified products could facilitate the identification of Stagonospora on cereals.