White Pine Disease: An Update

By Tom Doak, Executive Director

Once again this year, a lot of white pine needles are turning brown and dropping off in large numbers. The cause is several fungal diseases which attack one-year-old needles – those developed last year. For several years, we’ve seen significant needle drop from early June into early July. Wetter than normal springs and summers are believed to be the primary reason for the significant increase in fungal diseases.

The disease starts in spring, but symptoms don’t appear until the following year, when one-year-old needles turn yellow and brown and eventually drop. Current year needles do not seem to be affected, so the tree can survive. Trees with thin crowns will likely fill outsomewhat. Because the tree has only current year needles for photosynthesis, growth may be slower. Recent research indicates reductions of 25% or more.

Vigor may be affected, making the tree more susceptible to attack by other diseases or insects. Trees most likely to be severely affected are those already under stress – over-mature, growing in wet areas, or in poor health already.

While there are fungicides to treat small, ornamental trees, there are no practical or affordable treatments available for white pines in a forest setting. While landowners may want to cut trees before they die, don’t overreact. Most trees recover. For woodland owners, one of the best protections is proper forest management that helps keep white pines vigorous. Thinning in dense stands to increase air flow through crowns may be helpful. Because harvesting in severely affected stands can further stress trees, be cautious where there’s been heavy damage for several consecutive years.

One question we’ve been asked is whether harvesting and transporting wood spreads the disease. It’s not a concern. While infection levels vary across the state, fungal diseases have been found throughout Maine. Long-term impacts are unclear. It is not clear what the long-term impacts will be if we continue to have wet springs and summers. This spring was dry, so it’s possible next year may show a decline in severity.