Verticillium dahliae infects roots of a susceptible plant and becomes established in the root cortex. Most colonies remain limited to the cortex and cause no visible damage to the plant. However, some may eventually invade the water conducting tissue (xylem), which causes varying degrees of wilt, stunting and/or dieback.
Since 1994, the UC strawberry breeding program has selected genotypes for use as parents based on a multiple trait strategy that incorporates a rating for resistance to Verticillium wilt. This selection strategy has increased resistance scores for the parents used in the UC breeding population by 60%, and has increased the percentage of genotypes that are at least moderately resistant from 35.0% in the original germplasm to 78.5% in those genotypes used as parents for the most recent crosses (1).
Our work has shown that strawberry genotypes resistant to Verticillium wilt may differ in the extent to which they are colonized internally. Thus, in some genotypes the pathogen establishes an infection and colonizes the xylem but the plant shows few or no symptoms of disease. In resistant genotypes we found that 60% of the genotypic variation for visual symptoms was explained by the extent of colonization of individual plants by V. dahliae. However, the genotypic correlation between the percentage of pathogen-free petioles and resistance score for plants sampled in mid-season (May) was smaller than that for plants sampled in July. Together, these results suggest that the overall performance of strawberry genotypes in the presence of V. dahliae can be enhanced by both resistance and tolerance, but that tolerance may be less stable over the course of a season (2).
As an alternative to petiole isolations for assessing colonization intensity we have developed a quantitative PCR assay to provide an estimate of fungal biomass within plant tissue. A comparison of Q-PCR and petiole isolation assays shows the results of the two techniques to be highly correlated.
1. Shaw D.V., Gordon T.R., Larson K.D., Gubler W.D., Hansen J. and Kirkpatrick S.C. 2010. Strawberry breeding improves genetic resistance to Verticillium wilt. California Agriculture 64:37-41.
2. Shaw D.V., Gordon T.R., Hansen J. and Kirkpatrick S.C. 2010. Relationship between the extent of colonization by
Verticillium dahliae and symptom expression in strawberry (Fragaria x ananassa) genotypes resistant to Verticillium wilt. Plant Pathology 59:376-381. Verticillium wilt of strawberry