Black-foot disease caused by Cylindrocarpon obtusisporum and C. destructans, and Phaeoacremonium grapevine decline are affecting vineyard establishment in California. Although it is unclear just how widespread the problem is, vineyards throughout California are reporting economic losses resulting from replanting costs.

Symptoms

These diseases produce overlapping symptoms and generally follow a similar pattern of development. Affected grapevines grow slower, evidenced by reduced trunk diameter, shortened internodes, reduced foliage, and reduced leaf size.

When trunks of declining grapevines are viewed in cross-section, dark-brown to black streaking in the vascular elements is evident. This discoloration may occur in a few to most of the vascular elements. Foliar symptoms may appear as interveinal chlorosis, followed by necrosis and early defoliation. Uneven wood maturity, which is usually associated with a rapid desiccation event, is another possible symptom. Below-ground symptoms include a reduction in total root biomass, low numbers of feeder roots, and sunken, necrotic root lesions.

Sample sources / isolation results

Symptomatic young grapevines were submitted to our lab by University of California extension farm advisors, pest control advisors, and vineyard managers. Additionally, several collection trips were made to observe affected vineyards. We have consistently isolated four species of plant pathogenic fungi from symptomatic grapevines from northern, central, and southern California counties.

Cylindrocarpon obtusisporum was isolated from the roots and crowns of two- to five-year-old declining grapevines in Contra Costa, Lake, Madera, San Joaquin, Sonoma, and Tulare counties (Table I). Root isolations were made from the margins of necrotic brown-colored and healthy tan-colored root tissues. Isolation attempts from rootstock trunks (above-ground and below-ground) were from discolored areas of the pith and vascular tissue. Cylindrocarpon obtusisporum grew rapidly from infected tissue pieces and sporulated readily.

Cylindrocarpon species are soil-borne and infect grapevines through natural openings or wounds on the roots or other below-ground portions of the rootstock such as the pith. Over time, the fungus invades lignified tissues of the plant, resulting in a brown or black necrosis of the roots and vascular elements.

Cylindrocarpon destructans, a pathogenic species closely related to C. obtusisporum, was first reported in France in 1961 as the cause of “black-foot disease” on young vines. A recent study from the Bordeaux region of France reports that up to 50% of the grapevine mortality in new plantings there was due to this pathogen. Cylindrocarpon destructanshas been found in California, but it is uncommon.

Three species of Phaeoacremonium have been isolated from declining young grapevines in northern, central, and southern California between 1995 and 1997. (Previous names for the fungus now called Phaeoacremonium include Cephalosporium, Acremonium, and Phialophora.) Rootstock isolations were made from the pith and woody tissue of below-ground portions of the rootstock. Diseased pith tissue was dark in color, and wood outside the pith was dry with a silvery sheen. Phaeoacremonium spp. were found in the crown and lower portions of the rootstock of young grapevines with decline symptoms.

This fungus grew slowly out of infected tissue but sporulated abundantly in culture. The species Phaeoacremonium chlamydosporum was isolated from wine grapevines in Lake and Sonoma counties; P. inflatipes from wine grapevines in Contra Costa, Lake, San Joaquin, and Riverside counties; and P. aleophilum from table grapevines in Riverside County (Table II).