Horticulturalists are reporting sudden leaf and twig dieback on several different species of deciduous trees across southern Ontairo.
The twig’s sapwood (xylem) is brown and looks like it has been ( at least partially) killed from extreme winter temperatures. Either buds will fail to emerge in the spring or leaves will emerge to almost complete size and then fail when the water demands of the foliage exceed capability of the winter-killed sapwood.
The dieback resembles a systemic wilt disease but all samples we have inspected so far show no signs of xylem staining that would resemble common wilt diseases (such as Verticillium). With such a widespread area showing symptoms on so many different species, a systemic wilt disease doesn’t quite fit for an explanation.
Just this week an arborist called me about green fruit dropping from a homeowner’s apricot tree. This is a common occurrence on apricot in June. Many fruit trees produce too many flowers that they can’t possibly sustain into mature fruit. Another factor is the extreme winter temperatures they tender fruit trees withstood. And yet another factor is the hot, dry conditions experienced during bloom and fruit set. Hot, dry conditions during fruit set can sometimes results in incomplete fertilization and seed development. Without a viable seed, there is no hormonal draw for carbohydrates to the fruit and it will instead senesce and drop from the tree.