QuestionWe have a 3 year old silver birch in our garden which is clay soil. The tree has done really well -until last week! We woke up one morning to find a black discolouration on the gravel under the tree, extending about 5 foot diameter, as if the tree had "rained" a liquid on the ground.The stain is not sooty, however, or wet. The tree looks less healthy and there are pinhead sized spots on the backs of some leaves.The leaves have yellowed but I think that this is the normal Autumn drop.Any ideas, please?Thanks.
AnswerSounds like an insect called aphids. These soft bodied insects suck the plant juices from the tree and secrete a substance called honeydew. Honeydew is high in sugar content and a sooty mold will grow on the honeydew. This is the black you are seeing. The liquid should be sticky. To stop the honeydew you will have to control the aphids.
This late in the fall I would not do anything this year since the leaves will fall soon. Next spring check the tree and see if the problem still exists. Insect populations will vary greatly form year to year and there is a good chance that they will not be there next year. If you see the same problem you will need to treat the tree with an insecticide for aphids. You can either spray the foliage or put a systemic insecticide on the soil under the tree. Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil.
Check with your local nursery/garden type store for an insecticide for aphids.
