Characterization and control of black foot disease of grapevine in California

Black foot is a recently identified but worsening soilborne disease of juvenile grapevines. Cylindrocarpon fungi have been found associated with this disease.; To identify which Cylindrocarpon species are causing the disease in California, thirty-one isolates were characterized based on DNA divergence, morphology and pathogenicity. Based on DNA divergence, I identified two species, C. destructans and C. macrodidymum. The morphology of these isolates was in agreement with published descriptions. I found that C. macrodidymum was reliably distinguishable from C. destructans in culture, by a unique orange-dark brown colony color on 2% malt extract agar and genetically, by a species-specific DNA marker. Each species caused typical black foot disease symptoms onto grapevine rootstock 5C. This was the first report of C. macrodidymum in California.; To determine the susceptibility of fifteen Vitis rootstocks to the disease, I inoculated C. macrodidymum to grapevine rootings. Ten months after inoculation with the pathogen, the percentage of infected plants, disease severity, and plant growth were evaluated. The pathogen was re-isolated from all rootstock varieties. Varieties were classified into four relative susceptibility groups: most susceptible: Salt Creek; highly susceptible: 110R, 1103P, 101-14, 44-53, Teleki 5C, Freedom, and Harmony; moderately susceptible: St. George, Gloire, 140 Ru, 420 A; and least susceptible: Carignane, AXR1 and Scharzmann. This study demonstrated the lack of disease resistance among currently available rootstocks. The pathogen being ubiquitous to many sites, and fumigation often lacking, I suggested protecting grapevine wounds with fungicide at planting time.; To examine the influence of an arbuscular-mycorrhizal fungus, Glomus intraradices (INVAM CA 501), on the disease, I inoculated mycorrhizal or non-mycorrhizal St. George (Vitis rupestris Scheele) rootings with C. macrodidymum. Eight months after inoculation, I evaluated vine growth, disease severity, and mycorrhizal colonization. Inoculation with C. macrodidymum significantly reduced dry leaf and root weight only when plants were non-mycorrhizal. Mycorrhizal plants had significantly lower leaf and root symptom severity than non-mycorrhizal plants. Mycorrhizal colonization was high and not significantly affected by inoculation with C. macrodidymum. Thus, G. intraradices pre-inoculated to V. rupestris, increased tolerance to the disease. I suggested pre-plant applications of endomycorrhizal fungi to reduce disease severity.