This sounds like an unusual occurrence, as treatment shouldn't have resulted in leaf loss like this. Are you able to send us a few photos of the tree (overall) and its foliage? We'd like to confirm it is an Ash (as several other species can look quite similar, plus suffer from borers) and see what its general condition looks like. Some regrowth, depending on where it occurs on a stressed tree, can be signs of worsening damage, despite the fact that new growth is present.

Do you know what insecticide(s) the tree company applied? Some chemicals are systemic (absorbed by the tree's tissues and somewhat "curative") and some are contact, only effective against borers trying to lay eggs on the trunk as a preventative measure.

Was any herbicide used around the tree before the last defoliation event? Some weed-killer chemicals are risky to use around woody plants (trees, shrubs) due to the potential for root and bark absorption. They should state this precaution on the label, though. Sublethal effects could have included root loss (resulting in leaf loss) that the plant eventually, or at least partially, outgrew over time.

Miri

Hello Allan,

Photos will be very helpful, thank you; we'll keep an eye out for them.

We don't recognize that chemical name. Did you perhaps mean Milorganite, or was it something else? The tree company should have provided you with information (like the label or safety data sheet, or at least the product name and active ingredient) on what was applied.

The fungus they're referring to may have been anthracnose. They are common fungi infecting a range of tree species in spring (especially in cool, wet weather).

Miri