Black -Foot Disease of grape, caused by Cylindrocarpon obtusisporum (Cooke & Harness) Wollew. And/ or Cylindrocarpon destructans (Zinssm) Sholten, became a problem in vineyard establishment in California in the 1990s (Scheck et al. 1998a). Symptoms of the disease are sunken root lesions, black vascular streaking, leaf scorch symptoms resembling water stress, and vine stunting (Scheck et al. 1998a).
Taxonomy of the Causal Organisms
In California, both C. obtusisporum and C. destructans have been isolated from symptomatic grapevines (Scheck et al. 1998b). Based on morphological characters of isolates, whether C. destructans and C. obtusisporum are different species is unclear. In order to manage Black -Foot Disease properly, it is necessary to characterize the Cylindrocarpon species involved and to examine their respective roles in the disease. Our preliminary work on the ITS sequence show that morphological separation is not supported at the ITS level.
Epidemiology and control of the disease Control of Cylindrocarpon spp. By pre-plant fumigation with methyl bromide has been successful on peach and plum (Bulluck et al. 2001), on nursery grapevines (Stephens et al. 1999) and on strawberry (Yuen et al. 1991). Nevertheless, methyl bromide is a Class I ozone depleter which is scheduled for absolute use reduction in USA by 2005. Alternatives to methyl bromide are therefore needed.
In the 1990s, Black -Foot Disease was particularly obvious in areas extensively replanted with phylloxera-resistant rootstock (Scheck et al. 1998b). Because of the phylloxera epidemic, the AXR rootstock, used almost exclusively for 20 years has been replaced by other rootstock varieties. This suggests that this new presence of Cylindrocarpon on grapevine could be a problem of rootstock susceptibility. Our first year trial supports this hypothesis. AXR was among the least susceptible rootstocks. Our last trial showed that SO4, Salt Creek and Rupestris were highly susceptible. Scharzmann, 44-53, 110R and 5 C inoculated with the pathogen(s) had a higher but not significant root rot index. Riparia, 039-16 and Freedom showed no susceptibility to the disease. In California, factors of stress including poor water drainage, poor planting practices, and early fruiting were associated with disease damage in vineyards (Scheck et al. 1998 b). As vine age increases, susceptibility to the disease decreases. This could be a replant disorder that is amenable to biocontrol using mycorrhizal fungi in both nurseries and new or renovated vineyards. Very little research exists on the effects of mycorrhizae against fungal pathogens in grapevines. In our preliminary experiment on using separately 1 isolate of Glomus clarum (INVAM isolate WV227A) and 2 isolates of Glomus intraradices (INVAM isolates CA501 and UT126), G. intraradices (CA501) performed better in terms of disease control than the 2 others. Our last trial using only G. intraradices (CA 501) showed that the mycorrhizae significantly reduce the percent of root lesions (p=0.0175).