Experts tackle readers’ garden questions.
Q: My Granny Smith apple tree has been in the ground for about 10 years. It has been pretty productive despite the best efforts of the codling moths. It is again covered with lots of developing apples. However, its trunk has a problem. Is there something we are doing wrong or that we could do to help it?
Crystal Elledge, Lincoln
Master gardener Cathryn Rakich: Apple trees, including the popular Granny Smith, are quite adaptable to many areas of California. With autumn now here, harvest time – from late September through early November – for Granny Smiths in the Sacramento Valley is upon us. Pick the apples early for green-skinned, tart fruit, or later for yellow-skinned, sweeter produce.
However, apple trees also are susceptible to disorders that can affect not only the fruit, but the bark and structure of the tree, and may even pose a threat to the plant’s life. In this case, it is difficult to diagnose the exact problem without more information, such as whether symptoms first appeared after large pruning cuts were made in the lower portions of the tree. Because the leaves and fruit of the apple tree appear to be unharmed, one possible reason for the injury to the trunk is a fungal disease called sappy bark, also referred to as papery bark.
Sappy bark occurs on older apple trees in most growing areas. The fungus enters limbs and larger branches at pruning cuts. As the infected bark and wood tissue decays, it becomes spongy and discolored. Affected bark frequently peels away, exposing decayed tissue beneath. During damp weather, affected bark appears spongy; when dry, it looks papery. Dark sap sometimes oozes from diseased areas. Bracket-like, spore-producing structures may form along the edges of affected areas. Sappy bark cankers can girdle branches. If infections occur on the trunk, they can kill the tree.
To reduce the incidence of sappy bark, all pruning cuts should be made flush with the limb. If stubs are left on the tree, they can be invaded by the fungus. Rain and wind can spread fungal spores to pruning cuts, so trees should only be pruned after at least 48 hours of dry weather.
If sappy bark is already present, the diseased bark and wood should be cut away and destroyed. In this case, cutting away the diseased wood is practically impossible, so it is probable that the tree will eventually die.
In addition, other wood-rotting fungi may have played a role in damaging the tree. Judging from a photo you supplied, one fairly large pruning wound is visible near the affected area in the upper center, as well as at least one on the right limb. Because wood-rotting fungi may infect wounds during the rainy season, large cuts made low in the tree should only be made from late April through August, and never when rain is predicted.
Another possible culprit is sunburn, which can result on the sun-exposed west side of a tree with little foliage for protection. Sunburn can split bark and expose trees to wood-rotting diseases that may progress up or down the branches. If the center of the tree was opened up by pruning, sunburn could have occurred from midday and late afternoon sun exposure.
For more information on sappy bark disease, check out the University of California, Integrated Pest Management, website at
http://ipm.ucanr.edu/PMG/GARDEN/FRUIT/DISEASE/sappybark.html. For more information on sunburn, visit the Master Gardener of Sacramento County website at http://ipm.ucanr.edu/PMG/GARDEN/ENVIRON/sunburn.html.
Cathryn Rakich is a UC Cooperative Extension Master Gardener for Sacramento County.
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