Over the past few weeks I’ve received numerous calls and emails from clients concerned over the welfare of their beloved hemlock trees. The trees of concern have all been described as having browning needles and dieback. Despite having been treated for the Hemlock Woolly Adelgid, these trees do not seem to be doing well. So, what’s causing their needles to turn brown? Upon inspection of these trees, I’ve been able to confirm that the dieback is due to Rosellinia needle blight (caused by the fungus, Rosellinia herpotrichoides).
Over recent years, Rosellinia needle blight has become more common across North Georgia, especially on young plants in dense groupings along streams or areas where it is wet. Disease incidence increases under cool, wet conditions. Rosellinia needle blight affects conifers, including hemlocks, of all sizes and ages. The disease has been observed killing young trees, but it generally does not kill large, healthy trees. In that vein, it is possible that weakened trees compromised by the hemlock woolly adelgid would be more susceptible if they were to contract Rosellinia needle blight, so be sure to inspect your trees and treat them for the hemlock woolly adelgid if you have not already.
Additionally, trees planted in clumps or in a hedgerow may be more severely infected due to close proximity and ease of spread between them. On larger trees, this fungal disease tends to be first noticed spreading on foliage on a tree’s lower and inner branches. Needles first turn a light brown, and then mat together. Sometimes white fungal hyphae and black ball-shaped structures will develop on the underside of the needles. A 10x hand lens makes viewing these fungal structures easier. The fungus becomes active in the spring worsens during periods of cool, wet weather, though damage is not usually observed until summer.
As with other fungal diseases, Rosellinia needle blight is thought to be spread through the air, early-season infections continue to spread through the summer months and worsen along with rainfall and humidity. Unfortunately, there is not much that can be done for control of the disease, as once present, it will continue to express itself when environmental conditions are favorable. Since no fungicide is labeled to control this disease on hemlock, and many fungicides are restricted from use near open waters and streams due to fish toxicity, control via fungicide is generally not an option. In a home landscape, pruning out affected branches and destroying them can reduce spread upward in the tree. Thinning dense stands to remove severely affected plants and avoiding overhead watering can also slow its spread.
The Gilmer and Fannin Extension offices are seeking fresh samples of this disease. If you believe you have Rosellinia needle blight and you’d like to submit a sample, please bring a fresh sample with both living and infected tissue. Please include the tree’s GPS coordinates with your sample.