In 1998, we initiated a greenhouse experiment in which 8 different rootstocks were included. Preliminary data suggests that all rootstocks are susceptible to infection, i.e. none are completely resistant. However, some respond to infection faster than others and are able to form physical and chemical barriers that prevent further spread of the fungus. The most virulent species of Armillaria in California, A. mellea, is also the most widespread. This species is native to all grape-growing regions of the state. Armillaria mellea causes root disease of planted hosts and native hosts, but mortality due to root disease is higher in the former. Severe mortality of planted hosts is likely due to summer irrigation and build up of inoculum underground on roots of hosts that previously inhabited the site. Native tree species may be killed by A. mellea when watered during the summer (Raabe, 1966) . Mortality of native hosts may also be associated with logging or changes in host density due to fire-suppression. Other species of Armillaria in California include A. gallica, A. nabsnona, and NABS X. Based on our observations, all are weak pathogens of native hosts that exist mainly as saprobes. Armillaria mellea and A. gallica are common in the North coastal grape-growing counties. We sampled 436 trees on four oak woodland plots and found Armillaria on the root collars of approximately 50 %of the trees on each plot. Armillaria mellea was found on all four plots, but it was associated with noticeable mortality on only one. This plot also had a history of logging. It is likely that root infection existed, prior to logging, but was restricted to small lesions. Once the trees were cut, A. mellea fully-colonized their root systems and built up enough inoculum to attack healthy, neighboring trees. Armillaria gallica was found on three of the plots, commonly decaying dead trees. Based on our observations, A. mellea has the ability to kill healthy trees, while A. gallica is a very weak pathogen. The population structure of the two species is the same, in that a single individual can grow, vegetatively, to cover a large area (at least 50 m2). What limits or encourages the growth of these individuals, in addition to environmental factors, probably involves within-species and between-species competition. Based on repeated observations of a Sonoma county vineyard, Armillaria root disease can spread throughout a high density planting in several years. With isolates of A. mellea obtained from sampled vines and from a stump adjacent to a disease center, we determined that the entire block of vines is inhabited by a single individual. Mortality was first noticeable on the South edge of the block, suggesting that infection spread from the root system of this tree to the grapevines after it was cut. Based on the size of the trunk, it is likely that the roots of the tree extend throughout one-third of the vineyard. However, we also found small pieces of inoculum buried near many sampled vines, suggesting that inoculum was distributed throughout the vineyard before the vines were planted.
Armillaria root disease (also known as oak root fungus) is a long-term, chronic problem on grapevines and has been recorded in California since at least the 1880’s (Gardner and Raabe, 1963). The fungus attacks the roots of plants, killing the cambium and decaying the woody tissues. Symptoms of the disease include poor shoot growth, premature yellowing and dropping of leaves, dieback of vines, and eventually death of the vine. The disease occurs in distinct infection centers that expand over time. Spread of the pathogen is mostly by growth of fungal mycelium through susceptible host tissue and root to root contact, or through the formation of rhizomorphs (root-like aggregations of fungal tissue). The fungus can potentially survive for long periods in woody debris in the soil (up to 100 years) leading to long-term establishment on a site. Armillaria root disease is potentially a major constraint to hillside development of vineyards in north coastal areas and the Sierra foothills. Although most commonly associated with native oak stands, Armillaria species are natural components of most, if not all, forest ecosystems in California. In the coastal ranges, Armillaria is also associated with most native hardwoods and conifers (e.g., Douglas-fir, ponderosa pine). As an endemic species, Armillaria often does not cause major losses in native stands. The fungus is generally found in small lesions on the root system and root collar, and as epiphytic rhizomorphs. Lack of overstory mortality in these stands does not necessarily mean the absence of the fungus. As native trees are cut, the fungus colonizes the dying root systems, decays the wood, and slowly increases inoculum. Under the right conditions (susceptible host, favorable environment), Armillaria will then infect the planted crop. Little is known about the epidemiology of Armillaria in California vineyards. Large amounts of forest and orchard land are being cleared to establish vineyards. It has become clear that we need to identify the potential risk of Armillaria at sites that are being cleared of forest trees. If recommendations are to be made about site preparations and rootstocks, we must have a better picture of the identity and spatial distribution of the pathogen. We have identified two different species of Armillaria in forested sites of the coastal ranges and Sierra foothills: A. mellea, an aggressive pathogen, and A gallica, which acts primarily as a saprobe. By learning more of the ecology of these species in grape growing regions, we may be able to make risk assessment of new plantings. Control of the pathogen often relies on pre-plant fumigation, which is not feasible on hillsides. Due to the complete lack of control for already-infected vines, tolerance of infection relies on relative resistance of the rootstock and below-ground conditions affecting fungal growth. Given favorable growth conditions, changes in vineyard management to high-density plantings may compromise the relative resistance of the rootstock and exacerbate the problem. At the present time, there are no known Armillaria resistant grape rootstocks. Evaluation of rootstocks currently is done by planting vines in Armillaria infested sites. This method, however, often requires many years before adequate data is collected, and inconsistencies in the distribution of inoculum in the field make interpretation of these studies difficult. In addition, a screening program needs to take into account the variation in the pathogen. In 1997, we developed several techniques to inoculate grapevines with Armillaria. We will continue to refine these methods in 1998 and then expand the experiments in 1999 to encompass a wide range of rootstocks.