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Breeding for Resistance in California Strawberry to Verticillium Dahliae

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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

Review Questions

Question: What advantage does runner inoculation have?

Less resources needed
Minimal genetic influence
Requires fewer evaluations for resistance
Gives fast results

Question: What main disadvantage does runner inoculation have?

Low genetic diversity
High number of disease escapes
Requires constant monitoring
Multiple locations are needed

Question: What is the main advantage of direct selection?

Can only be done outdoors
Individual plants can be evaluated
Quick detection of resistance
B and C

Question: What main disadvantage does direct selection have?

High environmental influence
More resources needed
More note taking required
Higher probability for disease escapes

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?

All offspring will have high levels of resistance
All offspring will have a grade 3
All offspring will be very susceptible
The offspring will have a range of grades from 1 - 5

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

Increase yields
Reduce fumigation costs
Develop higher priced strawberry varieties
a and b

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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