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.

Fig. 7 A petri dish showing a colony of V. dahliae. The black specks are Microsclerotia; the reproductive particle of the fungus. (Image used with permission from Steven Koike, UCCE).

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

Quiz

Question

Question: What advantage does runner inoculation have?

Looks Good! Correct: It has been shown that runner inoculation has the advantage of minimal genetic influence.
Question

Question: What main disadvantage does runner inoculation have?

 

Looks Good! Correct: Direct selection’s main disadvantage would be higher probability for disease escapes.
Question

Question: What is the main advantage of direct selection?

Looks Good! Correct: Direct selection has the advantage of being able to evaluate individual plants and having quick detection of resistance.
Question

Question: What main disadvantage does direct selection have?

Looks Good! Correct: Direct selection’s main disadvantage would be higher probability for disease escapes.
Question

Question: Let's say a researcher crosses a strawberry plant with a grade 2 with a plant with a grade 4, what would you expect in the progeny?

Looks Good! Correct: the offspring will have a range of grades from 1 – 5.. Each strawberry plant has multiple genes that control the ability to resist infection by Verticillium dahliae. The different genes control for numerous factors that result in resistance. When two parents are crossed together the resulting offspring may have a great deal of genes promoting resistance or causing susceptibility.
Question

Question: Why has it been important for researchers to look for strawberry lines that are resistant to verticillium wilt?

 

Looks Good! Correct: Increase yields and reduce fumigation costs. The value of Verticillium resistant plants is the reduction in crop losses that will lower a grower’s yield. The problem is managed today by soil fumigation which suppresses the Microsclerotia in the soil. However if the fungus develops a resistance to soil fumigants the process will become impractical. Also the process of soil fumigation is costly and time consuming which reduce the profit margin of the grower.

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.  

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