Direct genotype evaluation
A second test performed was the direct screening of genotypes by inoculating peat pots containing the experimental seedlings. The seedlings were derived either from the original pure strains to use as a control or from previous breeding efforts. Peat pots were placed in a 1 cm deep pan holding 1 liter of inoculant solution until they had absorbed about 15 ml. The peat pots were randomly distributed for planting in an assigned test plot.
This is beneficial because individual specimens can be measured separately. This allows for the quick detection of resistant types. However, the plants that show no signs of disease may have escaped treatment due to the heterozygous distribution of spores in the inoculant.
All test plots from either evaluation method were graded on the 1 to 5 scale for disease symptoms; 4 to 7 times during the trial year at approximately 3 week intervals. The final score for each individual seedling across the year was calculated by taking the average of all ratings during the time of the research period .3
All plants involved within the study were tested for V. dahlia by petiole assay in order to validate the presence of infection.Petiole assay is the culturing of a cross section of a plant’s vascular system on agar media plates to determine the rate of infection within the test plots. All plates showed a positive result of V. dahliae during this study.2 The process used by UC Davis in this e-library article proves that a greater level of resistance to fungal disease can be obtained by conventional plant breeding practices. Development of this technique of verticillium resistance can be used in other horticultural crops such as tomatoes, lettuce, potatoes and Chrysanthemum.1 An additional benefit found by Wilhelm is V. dahliae resistant plants also exhibit a cross-resistance to another dangerous plant disease, powdery mildew.