Evaluation of pruning wound protectants on nursery apple trees

Wounds made when rootstock shoots are pruned back on nursery trees can get infected by canker and wood rot pathogens. These infections can develop into cankers causing dieback of young apple trees. Alternatives to current wound protectants need to be investigated to improve the protection provided on these wounds.

Four fungicides were evaluated for their efficacy towards mycelial growth inhibition of three canker pathogens and one wood rot pathogen. Nursery field trials assessed three pruning wound products, six fungicides and a Trichoderma product over two seasons at two nurseries. The rootstock varieties M793, MM109 and CG202 were included. Natural infection of canker and wood rot pathogens were assessed with re-isolations. Pruning wound treatments used in the nursery trials were also assessed on dormant rooted rootstock M793 plants with artificial inoculation of Diplodia seriata. Trials were conducted over two seasons and included the assessment of efficacy 1 and 7 days after pruning wound protection were applied (season 1) and protective and curative action of treatments (season 2). The efficacy of the treatments were assessed four months after treatment through re-isolations from the pruning wounds.

The fungicide sensitivity assays showed that isolates of Diaporthe eres (B), Didymosphaeria rubi-ulmifollii, Diplodia seriata and Trametes versicolor are sensitive towards guazatine, imazalil and tebuconazole. Isolates of Didymosphaeria rubi-ulmifollii and Diplodia seriata were also sensitive towards carbendazim. Trametes versicolor was found to be resistant towards carbendazim.

The testing of the pruning wound protectants and fungicides when challenged with Diplodia seriata conidia gave valuable results. The best pruning wound protectants over the two time periods (challenged 1 and 7 days after application) were carbendazim and MT1+carbendazim. The industry standard (ABE Tree Seal) inhibited infections by 68% at 1 day and 86% at 7 days, though higher levels of inhibition were obtained with Neocil-plus, prochloraz, carbendazim and MT1+carbendazim. The protectant trials clearly showed that carbendazim and MT1+carbendazim are effective to protect the pruning wounds (no pathogen could be re-isolated from these wounds). ABE Tree Seal and prochloraz were also very effective in preventing infection. The curative trials, where the pathogen were applied first and two days later the pruning wound treatments, showed that only carbendazim and MT1+carbendazim was effective to inhibit the colonisation of the pathogen in the pruned wound. This trial clearly demonstrates the effect of applying a pruning wound sealant such as ABE Tree Seal a few days after pruning. The sealant provides an ideal environment for pathogen spores that have landed on the pruned surface to infect and grow. It is clear that carbendazim is the most effective fungicide tested in relation to Diplodia seriata. The only drawback of this fungicide is that it is not effective against wood rot fungi. Next to this fungicide did prochloraz show good inhibition over the different trials. ABE Tree Seal gave the highest levels of protection of the pruning wound sealants for both seasons tested, however, applied protectively.

The nursery tree trials’ results could unfortunately not be used to compare treatments. Recommendations in terms of products need to rely on the outcomes of the artificial inoculation trials.

Canker and wood rot pathogens were isolated from the nursery tree pruning wounds and together with the brown streaking running from pruning wounds, confirm the importance to protect these wounds. The fungicides that can be recommended include carbendazim and prochloraz, however, carbendazim is not effective against the wood rot fungi. Of the pruning wound sealants, ABE Tree Seal and Neocil-Plus would be recommended, however, the rain fastness of Neocil-Plus would need to be determined. Infections from the bud union could be investigated further to determine if the application of a fungicide could hold potential to prevent any infections via this wound site.

Wounds made on nursery apple trees when cutting back the rootstock need to be protected timely and with an effective pruning wound protectant.