Breeding for Resistance

Now that we have described the impact of V. dahliae on strawberries and the importance of researchers developing lines that are resistant for farmers to use, here are the details of how resistance was increased. Plot-mean evaluations of commercial varieties have shown that the genetic basis for the V.  dahliae resistance trait in strawberry cultivars is highly heritable.2 This means that it is possible to breed gene traits responsible for resistance and the increase can be shown in Figure 4 by statistical analysis. To determine the value of any plant in the study, a regression line graph was used that charted traits of interest such as disease progression and yield.  This was done to measure the level of resistance in the test plot being studied and determine which plants should be crossed together to produce offspring with increased resistance levels illustrated in Figure 5. 6

Fig. 5 The bidirectional selection for resistance as well as susceptible strawberry plants. (Image used with permission from Douglas V. Shaw, UC Davis)

Each trial year, a percentage of strawberry lines that had been graded with at least a 3 for moderate resistance were determined.  Strawberry plants that had been graded at least a 3 or greater rating were included in the next year’s breeding program. 6 The majority of these resistant plants had dark green glossy leaves; a visual characteristic that was found to be correlated with resistance. 8Breeding for resistance of  V.dahliae has had the largest significant result with strawberry lines that showed a moderate resistance of 3 or greater. 6

In contrast, strawberries with a score of 2 or less were eliminated unless they contained excellent horticultural characteristics. A program of continuous improvement was enacted where progeny who had a parent with a rating score of 2 or less had fewer seeds or runners saved, The remaining germplasm had a moderate to high level of resistance which was used to enhance the overall breeding program. 6

Fig. 6 Shows results from breeding experiments from 1987 (original germplasm) to 2005. This regression line demonstrates the change in percentage of genotypes with at least moderate resistance (Image used with permission from Douglas V. Shaw, UC Davis 2010)