Studies in the Biology and Control of Armillaria Root Disease on Grapes
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.