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Relative Resistance of Grapevine Rootstocks to Armillaria Root Disease

¹ Research plant pathologist, USDA–Agricultural Research Service, and ² Professor, Department of Plant Pathology, University of California, Davis, CA 95616.

^* Corresponding author [email: kbaumgartner{at}ucdavis.edu; tel: 530 754-7461; fax: 530 754-7195]

Grapevine rootstocks were screened for resistance to infection by Armillaria mellea, the pathogen that causes Armillaria root disease. The first objective was to determine which factors hasten colonization of grapevines by A. mellea in order to develop the most rapid inoculation technique. Results showed that wounding the root collar bark and vascular cambium did not significantly increase infection rate (p = 1.0), that young vines were infected significantly faster than older vines (p = 0.03), and that fine roots (5 mm diam) were not susceptible to infection. Based on these findings, the inoculation technique was modified and used to inoculate dormant rootings of eight rootstocks in the greenhouse. After two years, the root collars were examined for the presence of mycelial fans of A. mellea and infection was confirmed by culture. The rootstock Freedom had the lowest frequency of infection (7%) and was the most resistant rootstock (p = 0.0016). Rootstocks O39-16, 5C, Riparia Gloire, and 3309C had the highest frequencies of infection with 63%, 73%, 79%, and 85%, respectively. Rootstocks St. George, Ramsey, and 110R had intermediate frequencies of infection. Results suggest that Freedom and, to a lesser extent, St. George, Ramsey, and 110R will be useful components of an integrated management strategy for Armillaria root disease. Resistant rootstocks may decrease the rate of colonization of grapevines in A. mellea-infested vineyards, thereby reducing yield losses from Armillaria root disease.

Key words: Armillaria mellea, disease resistance, screening technique, Vitis vinifera