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

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.