| Abstrakt: |
An apple scab, resistant selection, 1569-100 with field resistance to powdery mildew and fireblight, growing on EMXXVI rootstock of Malus domestica (Borkh) in the greenhouse, was artifically infested with European red mite (ERM), Panonychus ulmi (Koch) and evaluated for damage. In the first study (1984-1985) one side of the tree was treated with acaricides and the other side untreated, and 17 randomly selected branches were sampled from each side. In the second study (1986-87), the tree was divided into quadrants and 10 randomly selected branches within each quadrant were used for application of the treatments. We induced ERM infestations in the first year of each study using dormant eggs in 1983 and live ERM in 1986. Significant differences (P < 0.05) in total numbers of ERM that developed from dormant eggs were found in 1984, between the untreated and acaricide treated sides of the tree. The branches on the untreated side were significantly shorter (P < 0.05) than those of the treated on two of six counts. There were significantly (P < 0.05) fewer leaves on the unsprayed side of the tree on five occasions. No differences in fruit numbers were found in 1984. Significant (P < 0.05) positive correlations were found between total ERM numbers and both length of branch and number of leaves in 1984, which meant that total ERM numbers increased as length of branch and number of leaves increased. A highly significant (P < 0.01) carryover effect was evident, in the mean number of flowers and fruits found in 1985, from an infestation of a mean of 11.43 ERM per leaf in 1984 on the unsprayed side. Fewer flowers and fruits were present on the unsprayed side. No such effect was present in branch length as differences were nonsignificant (P > 0.05). The second study (1986-87) confirmed that ERM infested (water treatment) branches, had significantly (P < 0.05) reduced leafe numbers in 1987, which meant a significant carry over effect. But the differences in mean branch length and flower and fruit numbers were nonsignificant (P > 0.05) between the acaricide treated and control water branches. It was also apparent, in 1987, that branch length was greater in the water treatment but that this was not significantly (P > 0.05) different from the branch length for all acaricide treatments. It did suggest, however, a possible compensating mechanism operative in the tree. Our data suggest that interruption of the physiology of apple by ERM if minimal, will allow the tree to overcome the stress caused by ERM if grown under suitable conditions. |
