Breeding and genetics key to stemming Pierce's disease

“Breeding grapes can be frustrating,” Walker says. “Wild species are often hard to classify; they are hybrid forms that can vary between parental extremes. For instance, the same species could have members that are very resistant or very susceptible to Pierce’s disease. We can not select parents for breeding without extensive pretesting to ensure they have the high levels of resistance we need.” The process of identifying genes for resistance to Pierce’s disease could be shortened by using genemapping techniques similar to those used to map the human genome. It may be many years before the grape genome is completely mapped, but Walker and colleagues have begun to build basic maps that will help them find genes that confer resistance, as well as identify better hybrids for breeding purposes. However, Walker warned: “You can isolate a desirable gene from one grape species and splice it into the chromosome of another, but you currently have little control over where the new gene is placed on a chromosome, or how it is expressed. In many ways, gene-splicing techniques involve as much trial-anderror as traditional breeding techniques.” The authors respond: Mr. Olson asks how we can make a statement about the chemical soil quality when we do not investigate all the possible elements that make up soil. Nitrogen, organic matter content, some measure of microbial activity and soil density are measures often used to assess the suitability of soil for agricultural purposes. These properties are indicators of change in the chemical, microbiological and physical properties of soil. Would our conclusions have been different had we measured many other parameters? Perhaps, but those that we measured provide a good look at some of the most important soil constituents that are likely to change the most on agricultural lands over the time period in question. Mr. Borst suggests that the changes we measured are incompatible and that the clay data are a function of the method used to measure the clay content. All samples were treated the same in the laboratory, and we know of no reason to suggest that the archived samples would behave differently than the new samples in the particle-size analysis. Soil quality is, as Mr. Hoekstra observes, a qualitative not quantitative parameter. We agree that the concept is qualitative, but as scientists, we try to inform the qualitative with quantitative information. The clay percentage changes may be a function of erosion, deep plowing, land-leveling or natural variability. Among the variables measured, we have the least confidence in the differences reported for the clay values. The samples were taken where landowners would give us permission based on the location of samples collected long ago. Analyses were carefully done, appropriate statistics were applied and conclusions drawn. A more complete statistical analysis of our findings can be found in Geoderma 113(3-4):215–30.