Swedish Columnar Aspen (Populus tremula ‘Erecta’.)

Question from Lyne:
I have a 8 foot Swedish Aspen that I planted last year, June. I am up in Alberta Canada. I have noticed the last couple of days it did not look as well as it did. I have noticed that there are leaves that have black spots on them. I have other Aspens which do not seem to be effected. We did have a hail storm about a week ago. Should I be worried about this tree? The research I have been able to do so far has been no help at all.

Answer from Pat:
Sounds as if you have a type of European aspen sometimes called “Swedish columnar aspen,” (Populus tremula ‘Erecta’). This one is tall and narrow and turns red in fall. Also the bark of European aspen is darker in color than the American quaking aspen (P. tremuloides). Aspens are members of the poplar genus and unfortunately all poplars, including aspens, are subject to a flock of pests and diseases. That said, we can’t count out the possibility that the leaves were damaged by hail and I hope that is what the problem was. Hail damage is most often seen on vegetables. On soft leaves it will make round holes right through, but on tougher surfaces it will sometimes bruise or dimple fruit, leaves, or stems, but these bruised marks will usually turn brown, not black and though they remain and don’t go away, they don’t get any bigger.

Among the diseases of poplar trees that can produce black spots on leaves, however, are five different fungus diseases. If already established on the tree, these diseases are likely to strike young foliage first thing in spring. If this is a new infection then you may first see it in early or mid-summer. Most of these diseases come on worse in wet, cool weather.

The first of these diseases is Marssonina leaf spot, which causes small black spots on leaves of poplars and brown spots on aspen. These spots gradually grow larger. Another fungus disease is Septoria leaf spot. (As a diagnostic tool for Septoria, look for a small black pimple in the center of the spots.) Ink spot of aspen is yet another disease. This one usually begins earlier in summer as brown areas on the tops of leaves. Then there is leaf and shoot blight (Ventoria) but it usually scorches whole leaves and twigs rather than spotting them, and finally leaf rusts (Melampsora) create a spotted look which then spreads to be larger.

It is not easy for me to figure out which of these problems is afflicting your aspen but now that you know the names of the diseases, you can research them and decide which one is afflicting your tree. You may even be able to find photos of each. Leaf scorch is another problem but it is not related to spotting and not a disease. This can be caused by environmental factors and it affects leaf edges and tips.

All the above diseases survive winter on leaves and twigs left lying on the ground under the tree and sometimes also on the bark. Then in spring the fungi that have been dormant on the ground awaken and send their spores up into the air back onto the tree. Be sure to rake up all that debris and destroy it, don’t compost it. Severe attacks of these diseases can cause defoliation. Keep all fallen leaves raked up. Check branches for cankers. Most arborists suggest if you see signs of disease or cankers on some branches or twigs, prune them out. (Dip pruning shears or saw into alcohol or bleach between cuts. Bleach is very effecting but is too easy to splash onto clothes and ruin them.) I am worried however, that pruning the tree now might stimulate growth and that would not be good at this time of year, since fall is around the corner and you want the tree to harden off for winter. Thus I think it’s best to wait until after leaves fall to do any pruning.

Other things you can do include watering in early morning, never at night. Don’t let the leaves get wet and stay wet. Space trees so there is plenty of air circulation.
Prune out blighted wood and twigs in fall or winter. Apply fungicides early in the season so these diseases don’t get started up in spring. Serenade is an organic fungicide that might be helpful, but before using any product, I suggest you contact your closest Canadian Association of Farm Advisors (CAFA) for advice. You might also call in a certified Arborist, or talk to your local nursery regarding common local solutions for poplar trees suffering from fungus disease.