Susceptibility to Armillaria mellea root rot in grapevine rootstocks commonly grafted onto Teroldego Rotaliano

Armillaria root rot is an increasing problem in some grapevine-growing areas in north-eastern Italy (Trentino Province). The susceptibility of seven grapevine rootstocks (Schwarzmann, 3309 C, 101-14, Teleki 5C, SO4, Kober 5BB and 41 B; all grafted with Teroldego Rotaliano) to Armillaria mellea was evaluated in a five-year investigation. Two inoculation methods were also compared: young grapevine plants were either transplanted to a substrate that had been inoculated withA. mellea (Method A), or A. mellea rhizomorphs were inserted under the root bark after the root bark had been lifted up with a scalpel (Method B). Plants inoculated with Method A had higher infection and mortality rates than plants that were inoculated with Method B, demonstrating that root wounding does not lead to higher A. mellea infection. The significantly higher mortality and infection rates of 3309 C as compared with Teleki 5C in the final year of the study suggest that a Teroldego Rotaliano vineyard established on 3309 C will suffer greater losses than would a similar vineyard established on Teleki 5C. Rootstocks that were intermediate in their response to infection (Schwarzmann, Kober 5BB, and 41 B) may offer moderate levels of resistance since with these rootstocks the mortality and infection rates were not significantly different from those of Teleki 5C. Since all rootstocks became infected, however, no rootstock is completely immune.