Extension Communications |
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4/28/03 Contacts: Yard and Garden Column for the Week Beginning May 2 What's Causing These Yellow and Brown Needles on Conifers? By Krystal Tentinger When looking out at your conifer windbreak or observing the stately Scots pine across the street, you may notice something alarming. Yellow and brown needles on conifers--also known as evergreens--are a common sight during the spring in Iowa. This discoloration can be due to several factors, including winter injury, Rhizosphaera needle cast, bark beetle damage and pine wilt. Winter injury is common when winters are dry and mild. Last summer and fall, our conifers were under drought stress. Then the dry winter caused even further stress. During the winter, water can be lost from conifer needles when air temperatures rise above freezing and the tree is unable to replace the it in sufficient amounts. The mild, sunny, winter days cause such an effect. Common symptoms of winter injury are brown to red needles and loss of needles in the newest growth. Winter injury often is most severe on the south and west sides of the tree. The new buds that remain on the tree in the spring usually develop without problems, but several successive years of winter injury can cause permanent injury to your conifers. Another common problem for conifers, especially Colorado blue spruce, is Rhizosphaera needle cast. This disease causes second-year needles to turn purplish brown and infected needles eventually fall from the branches. The symptoms of Rhizosphaera needle cast tend to first occur at the bottom of the tree and progress upward. The fungus that causes this disease produces very small, black spots on the needles. You can manage Rhizosphaera needle cast with timely fungicide applications in the spring and tree pruning to improve air circulation among the branches. A third common problem on pines is damage from bark beetle feeding. A variety of stress factors, such as drought, can weaken pine trees, lower their defenses to bark beetle infestation and cause tree death. You can check for networks of tunnels, called galleries, in the wood immediately beneath the bark of damaged trees. Careful observation also will reveal tiny holes caused by exiting beetles in the bark of trees damaged by these insects. "Pine wilt" is a phrase pine owners do not want to hear. Older, more established pines, such as Scots and Austrian pines, can quickly succumb to this deadly disease. When a tree is diagnosed with pine wilt, there is no hope for recovery. Death may come within a few weeks to a few months. The needles of infected trees quickly turn grayish green and then change to brown. The damage can be scattered in several spots throughout the tree to begin with, and then spread. This disease, which causes high mortality of Scots pine in Iowa, is caused by the pinewood nematode. These tiny worms travel from tree to tree only with the help of a large insect, the pine sawyer beetle. The nematodes are taken from dying and dead trees to healthy ones. This beetle can move the nematodes several miles. When introduced into a healthy pine, the nematodes move to the resin canals of the tree where they feed on the living cells lining the resin canals. they multiply very quickly, clogging the water transport system and ultimately disrupting the tree's circulation system. Little can be done to save a pine tree once it is diagnosed with pine wilt. Insecticides and nematicides are costly and ineffective. Sanitation, however, is an effective way to reduce the likelihood of nematodes being transferred from tree to tree. Nematode-harboring beetles emerge from dying trees and debris. Therefore, cutting the infected trees and burning, burying or chipping them can control the spread of the disease. Avoid Scots and Austrian pine when choosing trees for your windbreak or landscape. These exotic pines are very susceptible to pine wilt and other diseases. A windbreak planted entirely of Scots pine could be dead within months if stricken with this disease. Spruces, firs, white pine and arborvitae can be good alternatives for these pines. Conifers can be a beautiful addition to your landscape. However, many factors need to be considered before you choose what trees to grow. Because most diseases do not affect multiple types of trees, planting several different tree species will reduce the chances of all of the trees in a planting being wiped out by a disease. Also, it is important to select tree species adapted to the environmental conditions at the planting site. Finally, good management practices, such as watering and mulching, are necessary to maintain healthy trees and minimize the harmful effects of diseases and environmental stresses. Following these guidelines will help you foster happy, healthy trees for you to enjoy for decades to come. -30- ml: isugarden |
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