Although stripe rust, caused by Puccinia striiformis f. sp. tritici, has been an occasional problem on wheat in the south-central United States from 1941 until 1999, the disease has been consistently severe in the region since 2000. Furthermore, since 2000, the geographic range of stripe rust in the eastern United States has expanded, and the old population of races has been replaced by a new population. The objective of this study was to determine whether new isolates of the pathogen were more aggressive and better adapted to warmer temperatures than old isolates. In all, 6 old isolates (collected before 2000) and 14 new isolates (collected since 2000) were evaluated at 12 and 18°C for latent period on wheat seedlings and urediniospore germination on Noble agar. At 12°C, old and new isolates had similar latent periods and spore germination percentages. However, at 18°C, new isolates averaged 2 days less for latent period and double the spore germination compared with old isolates. Therefore, the new isolates are better adapted and, thus, more aggressive at warmer temperatures than the old isolates. These differences may have contributed to the severity of recent epidemics in the region and to the expanded geographic range for stripe rust.

Yield component analysis provides a framework for identifying potentially useful traits for yield improvement. Consideration of how population density affects other yield components has not been addressed specifically for short‐season soybean [Glycine max (L.) Merr.] production. We assessed the direct and indirect contributions of population density for short‐season soybean yield and its components over a wide range of population densities (6–134 plants m−2) using path‐coefficient analysis. Data were from field tests conducted in 1997, 1998, and 1999 at Keiser, AR. Although population density had a large inverse association with pods plant−1, the large direct effect of population density on yield was greater than its negative indirect effect via pods plant−1. The direct effects of pod number plant−1 and seeds pod−1 on yield were positive, whereas mass seed−1 had a negligible effect. Pods fertile‐node−1 differed between cultivars, and it was reduced by increasing population density. For early sowing, the contribution of population density to yield was less because pods m−2 could be achieved at low population densities by a large number of fertile‐nodes plant−1 and pods fertile‐node−1. In contrast, at late sowing, the decreased potential for fertile‐nodes plant−1 was compensated by increasing plant population density. In short seasons, maximizing nodes m−2 and pods m−2 can be achieved by high population densities and early canopy closure, rather than the conventional system of larger plants with greater numbers of pods plant−1 and pods fertile‐node−1

The parents, F1, F2, and backcrosses derived from two hard red winter wheat (Triticum aestivum L. em Thell.) cultivars were studied in a field experiment grown on the Agronomy Research Station at Stillwater, Okla., during the 1971‐1972 crop season. Data were taken on individual plants to estimate gene action, heritability, and interrelationships of grain yield and selected yield‐related traits. Characters studied were 1) grain yield, 2) tiller number, 3) kernel weight, 4) kernels/spike, 5) spikelets/spike, and 6) kernels/spikelet. The direct and indirect effects of characters 2, 3, and 4 on grain yield were also studied.Additive and dominance variances were much smaller than the environmental variance for each character except kernel weight. Kernel weight, tiller number, and spikelets/spike displayed rather high broadsense herRabilities while kernel weight was the only character to display a high narrow‐sense estimate. Other heritability estimates ranged from intermediate to low. On the basis of gene action and heritability estimates, direct selection to improve a trait, should be more effective for kernel weight than for any of the other traits.Tiller number had a high positive phenotypic correlation and an intermediate genetic correlation with grain yield. Phenotypic correlations of kernel weight and kernels/spike with grain yield were intermediate and low, while their genetic correlations were low and intermediate, respectively. Negative associations observed between kernel weight and tiller number and between kernel weight and kernels/spike suggests that simultaneous improvement of these characters will be difficult. Large positive phenotypic and genetic correlations were observed between kernels/spike and spikelets/spike and between seeds/spike and kernels/spikelet.Path coefficient analyses at the phenotypic level indicated that the direct effect of tiller number on grain yield was large while the direct effects of kernel weight and kernels/spike were intermediate and low, respectively. At the genetic level, the direct effect of each component was intermediate and about equal in magnitude. This implies that the large direct effect of tiller number at the phenotypic level was due largely to nonadditive genetic or environmental effects or both.The results of this study indicated that selection for kernel weight in early generations is the most important factor in a breeding program for increasing grain yield. Tiller number makes a greater contribution toward grain yield but is much more difficult to improve by selection in early generations.

during the cropping season when water supply is likely to be more abundant. This approach has been an effec-Seasonal rainfall is a key factor determining yield of nonirrigated, tive tool for crops grown in monsoonal climates where summer crops. In temperate regions, however, systematic analyses of they are sown near the beginning of the wet season and long-term weather data have not been used for directing breeding programs or for crop management options. We evaluated long-term mature before the dry season (Monteith and Virmani, weather data (36-98 yr) for 16 sites in four geographical regions in 1991). Similarly, soybean [Glycine max (L.) Merr.] prothe USA to assess the potential for drought avoidance. For each day duction in the southern portion of the Mississippi Delta of year when the probability (P ) of having a minimum temperaturehas shifted to early sowing dates combined with early Ͻ0؇C was less than 0.05, water deficit was estimated as the difference maturing cultivars to avoid droughts, which frequently between the 7-d running sums of rainfall and potential evaporation.occur in August (Bowers, 1995;Heatherly, 1999). For comparative purposes across locations, a 7-d water deficit Ͼ50 mm Despite the recognition that matching crop phenology was defined as a drought. For the Midsouth, there were approximately with seasonal water supply is an effective means of 62 d at both the beginning and end of the growing season with P Յ avoiding drought, tools for evaluating weather patterns 0.20 of drought. In the Southeast, there were approximately 48 d and for the potential of avoiding drought are not widely 121 d at the beginning and end of the growing season, respectively, with P Յ 0.20 of drought. For the Midwest, P of drought was Յ0.20 used. Several of the most common meteorological meththroughout the growing season for three of the four sites, and it was ods for assessing drought have limited utility for evaluatconcluded that a 50-mm water deficit was not likely to be a production ing the potential for a given crop production system to constraint on the deep soils of the Midwest. For the Northern Great avoid drought in a particular region. For example, the Plains, P of drought was Ͼ0.20 for more than half of the region's Palmer Drought Index (Palmer, 1965) uses a soil water growing season. This meteorological approach for assessing drought balance approach to evaluate drought severity, which may provide insights for drought avoidance in breeding and crop manis normalized for the departure from climatological agement.norms. Thus, a Palmer Drought Index of Ϫ4 for an arid region and for a very humid region would both indicate extreme droughts, but because this index is normalized 1566

Genetic correlations provide useful information to plant breeders for developing selection schemes. Genetic correlations among yield and yield components (panicle number, panicle weight, panicle length, primary branches, and plant height) for U.S. southern long‐grain rice (Oryza sativa L.) have not been reported. The objectives of this work were to estimate and use genetic correlations in developing selection methodologies in rice breeding programs. In 1989, two 4 × 4 crossing factorials (Design II) were completed, and the 32 F1 hybrids and the 16 parents were evaluated in 1990 at two Arkansas locations (Stuttgart and Marianna). Additive genetic and broad‐sense genetic correlations were estimated. At both the additive and broad‐sense genetic levels, yield was positively correlated with panicle weight. Yield was negatively correlated with panicle number, but the effect was diminished at the broad‐sense genetic level. Panicle weight was negatively correlated with panicle number. Path analysis, however, revealed positive direct effects for both panicle number and panicle weight on rice yield at both the additive genetic and broad‐sense genetic levels, with panicle weight exhibiting larger direct effects on yield than panicle number. Selection indices were developed from the additive genetic and phenotypic variances and covariances. The selection indices indicated that selecting for increased yield via selection for either panicle weight or panicle number alone would be ineffective. A selection index that included selection for both increased panicle weight and panicle number to increase yield was estimated to be 91% as effective as selecting for yield directly.