Apple Tree Disease

Asked October 28, 2019, 8:01 PM EDT

I have a Braeburn semi-dwarf. I looked up into the inner part of the tree and most of the branches were covered in a white gauzy-like growth. What can this be and how do I fight it?

Washington County Oregon trees and shrubs

5 Responses

From your description of white guazy growth on the branches your apple tree may be infected with powdery mildew. To get a more definitive identification bring infected leaves and shoots to Washington County Master Gardener Office (1815 NW 169th, Suite 1000; Beaverton OR 97006; mastergardener.wc@oregonstate.edu; 503-821-1150 ex 2; open M-F 9am-noon, 1-4pm). They can look at the leaves and shoots under magnification and give you a more conclusive answer.

Braeburn apple trees, unfortunately, are susceptible to powdery mildew. Other cultivars, like Fuji, Liberty, etc. are resistant. Powdery mildew looks like whitish fuzzy covering of leaves and shoots. It can affect the fruit, and if severe, the health of the tree. Temperatures of 66-71 and high humidity are optimal for powdery mildew infections, so it's commonly seen in the spring. Our weather conditions this fall, with rain and moderate temperatures have also been good for the fungus.

To control powdery mildew remove affected branches and any buds with the white fungal material on them. The fungus overwinters in the fruit and leaf buds, then grows and forms spores to multiply in the spring. Clean up fallen leaves and fruit and compost them in another part of the garden - away from apple trees.

To prevent this disease prune the tree in late winter or early spring to promote good air movement that will dry the foliage and buds. Next spring as the buds are developing use fungicidal spray to control the fungus until . With a small tree you can cover it well, and see any fungus in the buds. Repeated sprays are needed until lateral growth is complete. For home use horticultural oils, like neem, and horticultural mineral oil sprays work well. Lime-sulfur sprays work, but must be done under the right conditions. Spectracide immunox @ 0.67oz (1Tbsp+1tsp)/gallon water works. Fugicicides using the bacteria, Bacillus thuringensis (Prevont, kBayer Advanced Natria Disease Control and other fungicides) work well. It's best to rotate sprays, using one for a while then switching to another. Also, prune or rub out any buds with white on them. Also, be aware on bees on blossoms and try to avoid spraying when bees are plentiful. In this article, any sprays approved for home use are labeled with an "H" in a square label, Apple (Malus) - Powdery Mildew https://pnwhandbooks.org/plantdisease/host-disease/apple-malus-spp-powdery-mildew.

Basically, for now prune out any infected shoots. Remove fallen leaves and shoots to another part of the garden for composting to contain spores. A spray of a horticultural oil like neem now would lessen the infection. In late winter - early spring prune out some center branches to open up the middle of the tree to sunshine and air movement to improve the health of the tree and help with air circulation. This article has additional information, Powdery Mildew of Apples http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/plant-diseases/fruit-and-nuts/pome-fruit-diseases/powdery-mildew-of-apples

That is not powdery mildew. Scale or wooly apple aphid insects are much more likely and so we will stop guessing and send this long to an entomologist.

Thjanks, I almost did not send the pix in so glad I did

Hi,

Yep, Jay was right. Those look like colonies of woolly apple aphids, Eriosoma lanigerum. If you scrape some of that material off and pop it into 70% rubbing alcohol, some of the white material should dissolve and you should be able to find the little aphid bodies. That would be a good way to double-check that the ID is correct.

Here is some info about these insects and some control recommendations from the PNW Insect Management Handbook....

Pest description and crop damage Adult woolly apple aphids are reddish to purple and are completely covered with a thick, woolly white wax. The insects feed on roots, trunks, limbs and shoots, producing galls at the site of the infestation. Heavy infestations on roots or above-ground portions of the tree can stunt growth and even kill young trees.

Biology and life history These aphids overwinter as adults on roots and aerial parts of apple trees. In severe winters the aboveground colonies may be killed. In spring and early summer, nymphs move up and down the tree to find a new spot to settle and produce new colonies. Preferred feeding sites are leaf axils on new shoots.

Scouting and thresholds Inspect trees during the growing season for the characteristic white, waxy colonies. Aerial colonies are mainly found in rough areas of bark and in wounds caused by pruning.

Management-biological control

Lacewings, ladybeetles and syrphid fly larvae are predators, as well as the parasite Aphelinus mali. These predators will consume all but the waxy secretion left by the aphids.

Management-cultural control

This aphid is considered a minor pest in the PNW. If colonies are found on trees, they can be physically removed. Many Geneva and Malling-Merton rootstocks are resistant.

Management-chemical control: HOME USE

Spring and summer

  • carbaryl-Highly toxic to bees.
  • imidacloprid-Soil drenches may have residual activity in woody plants lasting for 12 or more months. If short-term management is the goal, consider other approaches. Highly toxic to bees.
  • insecticidal soap-Some formulations are OMRI-listed.
  • malathion-Highly toxic to bees.
  • pyrethrins-Highly toxic to bees. Some formulations are OMRI-listed for organic use.
Hope that helps!
Yours,
Bill Gerth
Faculty Research Assistant
OSU Plant Clinic