METHODS OF SCREENING ACER-PLATANOIDES L SEEDLINGS FOR RESISTANCE TO WILT (VERTICILLIUM-DAHLIAE KLEB)

Several methods of challenging young seedlings of Norway maple (Acer p latanoides) with Verticillium dahliae were investigated to develop a m ethod for screening seed provenances for wilt resistance. Seedlings we re inoculated by pouring conidial suspension (10(6) conidia ml(-1)) ov er roots wounded in situ, or by dipping roots in the same conidial sus pension at transplanting, or by transplanting germinated seed or seedl ings into compost infested with a beet-seed or straw-cultured inoculum . On the basis of foliar symptoms during the growing season, and on se edling height, and vascular staining and the presence of V. dahliae in the wood at the end of the season, root-dipping was the most severe t est; planting seeds or seedlings into straw-based inoculum was less se vere than root-dipping but gave similar levels of infection. Beet seed was a less effective form of inoculum than straw, and inoculation of roots damaged in situ resulted in disease escape. A comparison of the root-dip technique using three, five-fold inoculum dilutions and trans planting seedlings to two different composts showed that a reduction o f spore concentration to a fifth had no effect on disease, whilst the effects of the further dilutions suggested that spore concentration be came limiting at about 4 x 10(4) conidia ml(-1) significantly more see dlings were affected in one compost than in the other. This experiment confirmed that many plants which survive to the end of the season of inoculation die during the following winter, and that the incidence of deaths overwinter has to be considered in devising suitable screening methods for young seedlings and for making comparisons among seedling populations.