Summerfruit NZ's research and development manager, Jack Hughes, looks at the problem of cankers on young cherry trees and the research being carried out to find solutions.
What’s the problem?
Many young cherry trees are suffering from trunk and limb damage in Central Otago – as can be seen from Photo 1 on the right.
Photo 1. A trunk canker has developed above the painted area.
Reports suggest that severe Pseudomonas syringae infections have killed up to 50% of trees in some new plantings, and up to 68% of unprotected second year trees had trunk cankers in a trial block at Plant & Food Research’s orchard in Clyde. While we know that vulnerability to infection reduces as trees get bigger and bark thickens, we also know that cankers are incurable once established.
Cause of the damage
There is strong evidence to show that trunk damage is initially caused by late winter/early spring cycles of day time warming of cambial tissues and sap movement, followed by night time freezing. Damage is observed on both sides of the trunk, so both morning and afternoon sun can cause damage. Bacterial blast then invades the damaged tissue.
Temperature sensors at the Clyde orchard showed daily temperature fluctuations of unprotected cambial tissue from -5°C to 22°C. The trunk warming breaks dormancy which results in sap movement and vulnerability to freezing injury. So called ‘sun scald’ is well documented with deciduous trees grown in high latitudes around the world.
A remedy to prevent damage
The aim is to insulate the trunks to reduce the amount of warming on sunny days. This reduces consequent sap flow and resulting higher vulnerability to freezing damage. It’s protection rather than cure.
Photo 2. Spray guards and white acrylic paint applied to trunks and scaffolds of young UFO trees.
Painting the trunks and main branches with interior white latex or acrylic paint, is currently thought to be one of the most efficient methods of protection and can be seen in Photo 2.
Photo 3. The DIY approach.
A paint brush was used in the trial but a mini-roller, as shown in Photo 3, can be used to speed up application and make it easier to protect larger areas. Spray gun application may be worth considering for large areas. Exterior paints that may have plant toxic additives should be avoided.
Photo 4. Spray guard protection is evident
Spray guards also give very good protection and can be doubled up (two consecutive spray guards along the trunk) to increase the area protected.
Photo 4 shows the protection given by the spray guard and the trunk damage suffered on the unprotected section of the trunk.
Wrapping trunks with frost cloth is also effective but creates a high humidity environment that favours disease development. Frost cloths should be removed before the growing season begins and so they require a lot more work.
These two methods gave the best protection and reduced trunk canker incidence on second leaf Lapins by 79% for the double spray guard, and by 54% for the white paint.
Ongoing work and further information
These provisional suggestions are based on work to date and may be updated as new data comes to hand.
Virgina Marroni's articles below provide more reading on the subject and members with access to the portal can see her presentation at the 2019 Summerfruit NZ conference in under Main Menu/Conference/2019 Speaker presentations/Virgina Marroni.
See Warming up to new ideas and Cherry tree trunk protection
Our thanks to Virginia Marroni et al at Plant & Food Research.