I planted Frans Fontaine Hornbeam in fall of 2020. I live is zone 6 and the native soil is reddish clay. I used triple mix and compost to amend the soil in the planting hole. These tree get almost full day of sunlight. The trees are watered through a smart sprinkler controller based on weather and watering needs. The first month I was very aggressive with the watering and noticed leaves turning yellow which I thought was due to weather changing. Last year I noticed leaves wilting and turning yellow during summer and shed their leaves early. I recently noticed extensive branches dieback and dead branches have blackish spots on them. Can you please advise if this is soil nutrient deficiency or fungal disease?
Hello: You have chosen a tree that is generally considered not to be bothered by diseases. However I suspect your trees have a fungal disease. It may be caused by overwatering in clay soil. Although I was not aware that Toronto has pockets of red clay, all clay soils can have drainage issues. You took the correct action by amending the clay soil with compost and manure prior to planting but perhaps this treatment was not sufficient to prevent the surrounding clay from preventing drainage. As directed, you made sure that the hornbeam was watered well during its first few years after planting. But it is difficult to determine how much is too much if the soil is improperly draining the water. Two other issues should be checked. If this is a hedge of Hornbeams, is the spacing correct between trees, and have the trees suffered from the cold as European Hornbeams are originally from warmer climates than ours. All these stresses could result in susceptibility to fungal disease.
I have included below links to two reference articles about European hornbeams that illustrate the concerns raised above.
I believe that you should consult a certified arborist from your area who can best advise you what to do at this stage. I think you should do this as soon as possible. He or she will be able to treat the tree with chemicals not available to the general public and also advise you on how to prevent further damage to your hornbeam.