Diseases that attack the needles, twigs and leaves of forest and shade trees with the exception of a few, do very little harm but nursery or Christmas trees can be damaged so that they lose commercial value.
Brown spot can be controlled in nurseries by spraying with an approved fungicide. Prescribed burning is recommended for control of the disease in plantations. Burns should be made only when the disease is causing damage and weather conditions are favorable for burning. Fire should not be introduced until the second winter then controlled burns can be done at three-year intervals until trees begin to elongate out of the grass stage. First year seedlings are very susceptible to fire. Kais et al. demonstrated the value of dipping seedlings in a benomyl/kaolin slurry to shorten the length of the grass stage (Figure 121).
Leaf blister of oaks is perhaps the most conspicuous leaf disease of the pointed leaf oaks such as scarlet and southern red. Certain of the entire leaf oaks are also attacked. The disease is more prevalent during wet springs. Infected leaves will have blister-like eruptions (Figure 122). Premature leaf fall is common while the leaves are only partially dead. The fungus over- winters on twigs and dead leaves. Raking up and burning the infected leaves as they fall will destroy over wintering spores thus reducing the source of inoculum for next year. Applications of an approved fungicide can be made in the spring just as the buds are swelling. Dogwood anthracnose caused by the fungus Discula destructiva, was discovered in Georgia in 1987. The disease was cultured from dogwoods growing on the Chattahoochee National Forest in northwest Georgia. The disease has been found in 38 Georgia counties (Table 4). Initial symptoms are small, purple-rimmed spots and large blotches on the leaves (Figure 123). Dieback of small twigs occurs initially with larger branches dying later. The dieback usually progresses from the bottom to the top of infected trees. Death of the
Anthracnose of sycamore and certain oak species is caused by Apiognomonia veneta. Infection occurs in the spring when spores erupt from infected twigs and fallen leaves. The spores germinate and grow into the newly expanding leaves and down into the petiole and twigs. Twigs are often girdled and killed. Secondary buds are formed below the girdled areas of the twig causing a noticeable angle from the parent twig. This ziz-zag appearance is characteristic of recurring heavy anthracnose infections. The disease is favored by cool, moist springs. Control is not recommended in forest stands but applying a fungicide as the buds are expanding can protect shade trees. A second application about two weeks later is suggested if cool wet weather occurs following the initial application. Raking leaves to remove inoculum is not effective since most infections initiate from previously infected tissues on the tree. Pruning infected twigs is recommended. Needle cast refers to the shedding of needles of pines due to various species of fungi (Lophodermium, Hypoderma). The fungi responsible for this casting produce black spots of different shapes on the older needles (Figure 124A). Affected needles turn brown and are shed. New tufts of green needles will be present on the branch ends (Figure 124B). Needle cast shows up in the spring and fall, and tends to show up more in years with lots of rainfall. Longleaf pines will naturally shed their older needles during dry years and this shedding should not be confused with needle cast. Needle cast very seldom causes permanent damage to trees, therefore, negating the use of fungicides. For fungicidal control of needle cast in Christmas tree plantations, refer to the Disease Control Section on page 138.
Sphaeropsis Tip Blight This tip blight is caused by the fungus, Sphaeropsis sapinea. The host range includes 33 species of pine but the disease is most frequently encountered on Austrian pine and is commonly seen on ponderosa, red, scots and mugo pines. It has also been reported to affect some species of Abies, Araucaria, Cedrus, Chamaecyparis, Cuppressus, Larix, Picea,
The fungus overwinters in pine shoots, bark, cones or litter and infects growing shoots in the spring. Spores are released during wet weather from spring through fall. Trees that are stressed because of poor sites, drought, or insect activity are very susceptible. In trees that are relatively free from stress, the fungus kills only current-season buds and shoots and second year cones. Fresh wounds, e.g., those created by insects, hail or pruning, on older branches and stems also are infection courts. Fully elongated needles and shoots and bark on the previous year’s shoots apparently are not susceptible to infection. However, the fungus has been found to persist on or in asymptomatic needles, cones and stems and it can proliferate to cause disease in stems subjected to water stress. Greenhouse inoculation tests conducted on slash and loblolly pines resulted in the killing of new shoots. The first test was conducted with stem-wounded inoculations using 20 seedlings of six-month-old slash and loblolly pines. Stem lesions developed in both species within two weeks of inoculations, but seedlings continued to grow and thrive. The second test was conducted using one-year-old loblolly seedlings. Inoculations were made in the new shoots using agar plugs. Shoots began to die after one week and after two weeks a necrotic lesion was visible around the shoots and extended through the lower area from the inoculation point. The most conspicuous symptom is stunting and browning of new shoots and needles, usually beginning in the lower crown. Successive attacks result in dieback of branches and tops, but progression to tree death is rare. Damage is often associated with climatic and site conditions. Tip blight sometimes displays a “shepherd’s crook” conformation making it indistinguishable from low-temperature injury due to late spring frosts. Epidemics have been attributed to excessive nitrogen from atmospheric sources in the Netherlands and from paper mill wastes spread in plantations in Wisconsin. It is feasible to associate tip blight in pine plantations located near chicken houses due to the atmospheric release of ammonia from the bird droppings. Powdery Mildews, Sooty Molds Powdery mildews are represented by seven genera. They occur on many species of plants and can cause economic losses on roses and certain other crops. Forest trees and shrubs are often attacked but no control is necessary. Crape myrtle and elm are two common species of trees often attacked by powdery mildews. Fungus growth covers the leaf surface giving the typical powdery appearance (Figure 126). Individual
Raking the fallen leaves and removing them from around the plant can achieve some level of control. Sooty molds like powdery mildews are superficial on leaf and other plant surfaces and cause little damage to trees. They grow in concert with the sweet honeydew caused by scales and aphids. Controlling the insects indirectly eliminates the sooty mold. |
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Forest Pests: Insects, Diseases & Other Damage Agents |
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