For years we had beautiful veronicastrum (alba) that was a main feature in our garden. In subsequent years the plant would grow vigorously in the spring and early summer and then one by one the leaves would start to wither and the stems would turn brown. The plant would die back before flowering. The following year the same thing would happen. Strong growth then they would wither. It was not from lack of water or overwatering. We dug out the plant and replaced it with new ones. We incurred the same problems with the new plants. Could it be a virus or root borer? Is there anything we can do? We don’t use pesticides or sprays.
The cause of the withering leaves could be the Papaipema sicata, or culver’s root borer moth, found in North America, where it has been recorded in the United States and Canada. It is listed as a species of special concern. The larvae feed on Veronicastrum Virginicum and Alba, the moth bores the roots of their host plant: please see Culver’s moth
Your suspicion of a root borer can be attributed to this moth, its life cycles is similar to the decline of your Veronicastrum, observations of its egg laying cycle matches your description. As a Michigan State University paper states: “Eggs are laid on or near the food plant in the fall and hatch in late spring or early summer. Larvae can be found in the root and lower stem of the host plant in most years from 21 July through 14 August. Feeding and tunnelling in the root causes the plants to wilt, dry and become black. In extreme cases the stem becomes broken and dies. The final developmental stage leaves the root, and pupates in the soil near the plant.”
This could likely also explains why the problem did not change when you bought new plants, as the larvae could still be in the area.
The paper goes on to list predators of the moth: “Major natural enemies of Papaipema: include mammals such as rodents and skunks (Hessel 1954, Decker 1931, Schweitzer 1999), woodpeckers (Decker 1930) as well as numerous parasitoids and predatory insects. Small mammals, in some cases, can completely eradicate small populations of Papaipema, (Hessel 1954). A tachinid fly, Masicera senilis, and a braconid wasp, Apanteles papaipemae, are probably the most important parasitoids of Papaipema larvae (Decker 1930).”
The lifecycle and habits of other insects listed as pests do not resemble the damage you have described such as a the genus Lygus.
Veronicastrums are free from most pests and diseases although black leaf spot (Diplocarpon rosacea) can occur on the lower leaves of some cultivars in dry seasons. Based on your description it is unlikely this is what is affecting veronicastrum (alba), but should not be ruled out. Here is a video on how to identify black leaf spot Black Spot Disease
This summer, if possible, submit a photo of the leaves and stems, for greater clarity. At this time the information provided closely resembles the above two causes.
Thank you for contacting Toronto Master Gardeners.