What disease are my newly planted beech trees having?


I had a landscaper plant 5 Dawyck (columnar) European beech trees for me along a tall concrete retaining wall to form a hedge. There’s plenty of space for the trees to grow. The planting was done about 6 weeks ago in early spring. The trunks were about 40-45mm thick. Four of the five have leafed out growing beautifully, the biggest one is taking much longer but is about to open all its buds. But just yesterday I discovered white powdery things on all the leaves of all of the trees, look like white flour, and feel a bit sticky. Mostly on the upper side of leaves but some are on the bottom and looking pretty bad. Can also see some curled leaves with lots of powders and maybe some kind of worm in there? I didn’t open up the leaves because I was afraid to get anything else there infected. I’ve never seen this before. Did a little research, suspecting it’s powdery mildew. I don’t understand how they got infected. I’ve been so diligent looking after them, watering about 3-4 days a week. The only concern is I’m not sure if they are getting enough direct sun. I think some are getting about 4 hours and some are getting about 3 hours. Sometimes they only get sunlight on the upper part of the trees due to the retaining wall that is taller than the trees. Before landscaping, we thought hard choosing between hornbeam and beech. We chose beech eventually for its narrower and taller form, and it’s leaves retain better in winter. We spent so much money on getting these trees and planting them, as well as installing the rest of the garden, it’s really hard to even think about I might be loosing them. What did I do wrong? Is it something treatable? Also even if it’s treatable, am I fighting a loosing battle because they are so easy to get this disease for some reason? Is it a sign of insufficient sunlight with the 5th tree not leafing out yet by now? (I thought it’s just taking its time to establish) I’m hoping the trees will grow over 12 metres to form a ore act hedge over years. If this disease keeps coming back then I need to seriously decide whether to keep them or start over. I want to protect the rest of my garden too!


Thank you for contacting the Toronto Master Gardeners.

from your description of the leaves appearing sticky I suspect that your beech tree is suffering from an attack of Woolly aphids.

Woolly aphids get their name form the fluffy, wax-like substance that covers their bodies. At first glance you may mistake this for fuzzy mold. These aphids are pale yellow, sap-feeding insects, which are covered by fluffy white fibers, forming dense colonies on young shoots and the underside of leaves where they tend to congregate. Often large numbers of the molting or cast “skins” will be attached to leaf hairs, which gives the leaf a whitish appearance.

These sap-sucking pests generally use two hosts: one for overwintering and laying eggs in spring, and one for feeding in summer. Woolly aphid insects generally feed in groups. They can be seen feeding on foliage, buds, twigs and branches, bark, and even the roots. Damage may be recognized by twisted and curled leaves, yellowing foliage, poor plant growth and branch dieback.  Aphids exude a sticky substance called honeydew. If left untreated, plants may become covered with sooty mold, an unsightly black fungus that resembles soot. Though this does not normally affect or damage the plant itself, getting rid of the aphids and their honeydew will help control the sooty mold.

Natural enemies such as lacewings, and parasitic wasps may help to control aphids, but may not appear in sufficient numbers until aphids are abundant.

The best solution to ridding your tree of these pests is your garden hose set on full blast. Spraying them will knock them to the ground and they will be unable to return to the host. Do this every few days until you no longer see any signs of them. Where aphid infestation is abundant you can use insecticidal soap, which is available at your local garden center. Make sure to follow directions on the label. Repeated treatments of both the upper and lower surface of the leaf are necessary in order to control your infestation. You may wish to contact a certified arborist to apply the soap.

Beech trees are also susceptible to powdery mildew. As the name implies, powdery mildew looks like white or gray blotches on the leaves and stems of the plant or tree. There are actually several types of powdery mildew fungi, but they all look basically the same. You may not notice a problem until the top surfaces of the leaves turn powdery, but powdery mildew can also affect the lower leaf surface.

The weather has been ideal for this fungus, with hot, humid days and cool nights. Although powdery mildew is unattractive, it is rarely fatal. However it does stress the plant and severe or repetitive infections will weaken the plant, thus opening the plant up for infestations by other insects such as aphids. If enough of the leaf surface becomes covered with powdery mildew, photosynthesis is impaired and infected leaves often fall prematurely.

There are some fungicides approved for use in Ontario; see your local garden centre for specific products. However, they must be applied at the very first sign of the disease.To prevent this disease in the future, make sure that your tree is planted in sun, and not crowded, with plenty of air circulation around it. The fungus overwinters in plant debris, not the soil, so a very thorough fall clean up will be important.

You mentioned that you beech tree receives at best 4 hours of sunlight. which I suspect is the reason for powdery mildew.  Beech do best in in full sun (6 hours direct sun or 4-6 hours of partial sun/shade. It prefers rich, moist well-drained soil.

Similar to beech Hornbeams serrated leaves will hang on the branches for most of the winter, but not as long as the beech. European Hornbeams grows to a mature height of 35‘ and mature spread of 20’.  In general European hornbeams will grow in full sun (6 hours of direct sun), as well as partial sun/shade (4-6 hrs of sun) or even in full shade. They are also tolerant of all soil types.

You may wish to consult a certified arborist for his opinion. To find a certified professional arborist in your area to help you visit the Ontario branch of the International Society of Arboriculture here.

Good Luck