Frukttrekreft på eplegrunnstammer

Abstract

The apple canker disease (aka European canker), which is caused by the fungus Neonectria distissima, infects apple trees and can cause severe damage on both the fruit and the tree itself. For some time it has been assumed that apple trees may be infected as early as the planting stage. There is, however, no previous documentation on rootstock infection. If rootstocks can get infected, how does this happens and what is the entry point for the infection? This project has investigated the issue by means of testing different kinds of rootstocks and different methods of inoculation. The different methods used were inoculation by pricking the rootstock with needles containing inoculum, spraying the rootstocks with a spore suspension and dripping spore suspension in the wound made after cutting off the top shoot of the rootstock. Inoculation occurred at different dates, and the rootstocks received different treatments of pinching and topping. The experiments showed that all rootstocks may become infected by apple canker, although there was a difference in how effective the different methods of inoculation were. The rootstocks which were inoculated by needles containing inoculum had a high probability of developing infections and showed symptoms of infection after four weeks. Needles with inoculum inserted into the shoots provoked earlier symptoms than those inserted into the stem. Inoculation by spraying the rootstocks with a spore suspension developed very little infection, and did not show symptoms until eleven weeks after inoculation. Results also showed that, the rootstocks were most prone to develop infections if they were pinched the same day as they were sprayed with spore suspension. Rootstocks pinched one or three days before being sprayed with spore suspension developed fewer infections. All rootstocks tested, M9, B9, M26 and MM106, developed infections of apple canker, and there was no difference in susceptibility. When wounds were made in the rootstock, either by pricking with a needle or by cutting the top off, the infection levels after inoculation were high. If the wounds were made by pinching off the shoots, however, the probability of infection was low. Wounds in younger tissue were more susceptible for infection than wounds in older tissue. The results show that to reduce the risk of infection of apple canker, it is important to be considerate while handling rootstocks to avoid damages. When pinching, topping and grafting one should take precautions to avoid apple canker and be aware of potential sources of inoculum in the near vicinity.