GREENSCAPE GARDENS TREE TIPS
White Pine Decline
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The past couple of years, we have seen an excessive amount of mortality among the urban white pines. Reported causes range from ozone and winter damage to root rot. The actual causes are not known because too few cases have been verified and many are inconclusive. Among the possible contributing factors are growing conditions such as poorly drained soils, compacted soils, high pH or heavy clay soils, salt damaged, coupled with severe weather.
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White pines are intolerant of poor growing conditions. Trees growing on marginal sites, develop poor root systems which reduces their ability to tolerate stress. Minor stresses such as abnormal fluctuations in temperatures and precipitation can deplete food reserves and lead to decline. Although declines are thought of as slowly developing syndromes, they can also be characterized by sudden deterioration. The "straw that broke the camel's back" may be so inconsequential as to be unrecognizable. Some pines looked perfectly healthy last year declined and died in one year. There appeared no signs of disease or insect activity, simply decline and death in one season. However, the growing conditions and care were questionable as well as a heavy clay soil in the area did not help the situation.
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Similar disorders, also called declines, are the result of both biotic and abiotic factors. The abiotic factors are the same as described above, poor growing conditions. But to these are added, weevils, bark beetles and root rot pathogens. Two weevils, pales weevil ((Hylovus pales) and the pine root collar weevil (Hylobius radicis) have been found associated with declining white pine. Both weevils are also known to transmit the root pathogen Verticicladiella procera. If present, the adult weevils emerge during warm periods of the winter and by early April are actively feeding. They feed on bark, buds, and needles of healthy trees. In the process they can introduce the fungus into feeding wounds. The adults mate and lay eggs in early summer in fresh cut stumps or in the base of stressed trees. They require these dead or dying trees to reproduce successfully and these provide the exposure of fungus. The larvae are small, maggot-like grubs that feed in the crown area, producing extensive damage. The fungus produces spores in wounds, on infected roots in the soil and inside insect tunnels. The spores which are sticky, readily adhere to the pupae and emerge as adult weevils. The spores are carried from infected trees to healthy trees. There the fungus colonizes phloem tissue and kills the cambial layer. The infected portion becomes resin soaked with black streaks. These streaks can extend up to 18" above the soil line.
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Signs of streaking and excess resin production at the base of symptomatic trees indicate infection. Early symptoms of infection may be limited to reduced growth and last for several years. When root damage becomes extensive or the crown becomes girdles, the top declines. Browning may take a season or two or as little as a few months. In later cases an apparently healthy tree may be dead by mid summer.
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Other pathogens can cause resin soaked lesions, for instance Valsa (perfect stage of Cytospora). But Valsa cankers are usually associated with wounds higher on the trunk or lateral branches. Phytophihora root rot can also cause similar symptoms, but instead of resin soaked wood with black streaks, roots will be black and mushy. Rarely does Phytophihora extend above the soil line, unlike Verticicladiella. Surveying the situation is necessary in determining the role of these various factors. However, all are associated with stressed trees, usually resulting from poor growing conditions. Avoiding or correcting poor soils with inadequate drainage will help reduce future losses.