Spring weather is here and many people are out and about noticing that some evergreens aren't looking as green as they did before winter arrived. The Iowa State University Plant Disease Clinic has received several samples of browning conifers, probably because the winter was quite dry. It seems there are several causes for the trouble.
The first is winter desiccation. Many trees have brown needles because of our dry winter weather. Many areas of the state also had a drier than normal summer and fall, stressing trees before winter set in. The browning on these plants tends to be on the tips of branches and commonly on one side of the tree. Many of these trees should survive as long as buds are still alive. However, several consecutive years of drought stress can permanently injure trees.
The second problem we're seeing is Rhizosphaera needle cast on spruce, particularly Colorado blue, which causes second-year needles to turn purplish brown. Infected needles tend to fall off the branches. Rhizosphaera needle cast usually starts at the bottom of the tree and works its way up. Spruce trees with Rhizosphaera needle cast can be easily identified by tiny, black fruiting structures of the fungus that form in rows on infected needles. Rhizosphaera needle cast is managed by improving air circulation around trees through pruning and applying protective fungicide applications starting during the last 2 weeks of May.
The third problem that's been through the clinic is pine wilt. A microscopic worm called pinewood nematode causes this lethal disease. Beetles carry nematodes from tree to tree. The pinewood nematode is native to the United States; therefore, we don't typically see it cause problems on native plant species, such as white pine, because they evolved together. However, exotic pines, especially Scots (Scotch) pine, don't have natural defenses against the nematode. The primary management recommendation for pine wilt is tree removal to prevent the beetle from spreading nematodes to healthy trees. It's important to remove and destroy infected trees because beetles are likely to emerge from wood even after the tree is chopped down.
The best defense against conifer problems in the long run is to plant several different tree species, allowing adequate space between each one. Proper planting, mulching, and watering through dry periods are also important for maintaining plant health.
This article originally appeared in the April 11, 2003 issue, p. 39.