Insect Vectored Pathogens On Woody Ornamentals
Gary W. Moorman, Professor of Plant Pathology
Some devastating pathogens in woody ornamentals are moved from plant to plant or 'vectored' by insects. These include the fungi Ceratocy s tis fagacearum (oak wilt) and Ophiostoma ulmi (Dutch elm disease), the bacteria Xylella fastidiosa (bacterial leaf scorch), Erwinia amylovora (fire blight; see additional fact sheet), and the phytoplasmas that cause elm yellows (elm phloem necrosis) and ash yellows. In general, these pathogens are not spread very long distances by the insects but the insect association makes controlling them very difficult. In some cases, controlling the insect helps control the disease but in other cases, insect control does not effectively control the disease spread.
Crucial to developing a disease management strategy is knowledge of what insects vector the particular pathogen and whether it is the same insect all the time or different ones at different times. Something must be known of the insect's life history, when it is active and whether the pathogen is carried on the outside of the insect in inside the insect. It is important to also know whether mere insect probing by the insect results in transmission of the pathogen or whether a long feeding time is required for transmission. You must know what plants harbor the pathogen and which ones support populations of the insect. These facts help determine whether an insecticide can be useful in controlling the disease.
Oak wilt, caused by fungus Ceratocystis fagacearum, infects most oaks, especially red oaks, and causes obvious symptoms. Initially, upper leaves die at tips and margins and this is soon followed by massive and rapid defoliation of dull green leaves.
Cutting into the sapwood reveals brown streaking primarily in the outer annual ring. Symptoms appear from 6 weeks to 1 year after infection. A black fungal mat develops under the bark and erupts through the bark in the spring to produce sticky spores. Sap-feeding beetles feed on the fungal mat, acquire spores on the outside of their bodies, and are attracted to fresh wounds on oaks where they feed and deposit the spores. Once the tree is infected, the fungus produces spores in the xylem and these spores can move downward into the roots. Neighboring oaks with naturally grafted roots then become infected through grafts. Beetle vectoring and root graft movement spread the pathogen relatively short distances, often killing a few oaks locally but not spreading rapidly or far from that locale. The pathogen is moved long distances when the tree is cut down, the bark left on, and the logs transported for use as firewood. Several sap feeding beetles are involved in vectoring the pathogen. When taken as a group, it is likely that vectoring can occur at any time during the growing season. As a result, insecticides are not practical for controlling oak wilt. Cutting root grafts among infected trees and nearby healthy oaks, destroying infected trees, and injecting healthy trees with an appropriate fungicide (usually once every 2 years) can be combined to manage oak wilt.
Dutch elm disease, caused by the fungus Ophiostoma ulmi, was once called Ceratocystis ulmi . The pathogen and its insect vector relationship were first thoroughly described by several, now famous, women plant pathologists in Holland, thus the 'Dutch' name. The disease results in 'flagging' or wilting of leaves on one or two limbs. Those leaves die and fall. These symptoms spread to the rest of the tree over 1-3 years. Brown streaking can be seen in the xylem vascular tissue. Elms within 50 feet of one another often have root grafts and the fungal spores can move from tree to tree through these grafts. Bark beetles are very important vectors of the fungus. They feed on twigs and introduce the fungus. The fungus kills the twigs and later in the season the beetles are attracted to these freshly killed twigs where they lay eggs. The larvae that hatch from the eggs create galleries or tunnels beneath the bark and the fungus forms spores in these dark, damp tunnels. As the beetles mature, they emerge from the galleries, covered with Ophiostoma spores and seek a feeding site, thus carrying on the cycle of the disease. If it is known when the beetles are active in a particular area, elms can be protected with insecticides in order to reduce the bark beetle population and thereby reduce the chances of spreading the fungus. In addition, root grafts must be cut among neighboring elms. Wood from the infected elm saved for fire wood and the stump must be debarked. Otherwise, the bark beetles and the fungus will remain active under the bark. The pathogen and the beetles are spread long distances through the transport of infected wood that has the bark remaining on it. There are several Dutch elm disease resistant cultivars of elms now commercially available.
Bacterial leaf scorch in oak is caused by the bacterium Xylella fastidiosa . Although many different oaks are susceptible, red oaks in the northeastern U.S. are particularly susceptible. In addition to oaks, sycamore, mulberry, red maple, sweetgum, Boston ivy, and many herbaceous weeds can also be affected by Xylella . It is thought that strains from different plants are actually different species of Xylella because, for example, bacteria taken from elm do not infect sycamore or vice versa. Symptoms include the death of the tips and margins of leaves, beginning on the lower and inner part of the canopy and spreading upward and outward. The death of the leaf tissue proceeds from the margin of the leaf toward the midvein and there is a distinct yellow border between the dead and live tissue in an undulating line. Most herbaceous weeds that harbor Xylella exhibit no visible symptoms. Spread of the bacterium is by nymph and adult spittlebugs and leafhoppers. The bacterium is immediately transmittable by some insects but must be in some other insects for 2 hr. before transmission can occur. In that case, the insect will continue to carry the bacteria until it molts and the bacteria actually multiply within the insect. Initially, just a few branches will exhibit symptoms but more branches become involved year after year. It is not unusual to have neighboring trees of same species with one tree severely affected while the other is healthy. This indicates that root graft transmission is not important. Because many insects are involved, little is known of their identity or life histories, and the bacteria can be spread very rapidly, insecticides are not helpful in managing this disease. It has been demonstrated that tetracycline injections every year put symptoms into remission but does not cure an infected tree. It has also been clearly demonstrated that injections and soil drenches with other materials have no effect on bacterial leaf scorch.
Elm yellows (formerly known as elm phloem necrosis), is caused by leafhopper vectored phytoplasmas. Phytoplasmas are like bacteria in that they do not have a true nucleus and their internal structures are similar but lack the rigid wall characteristic of bacteria. Adult white-banded leafhoppers (Scaphoideus luteolus) feeding on the phloem of elms acquire the phytoplasmas. (The possibility that several other leafhoppers are also vectors is under study.) The phytoplasmas must incubate inside the leafhopper for 3 weeks and move from the gut to the salivary glands, after which, it can be transmitted to trees for the remainder of the leafhopper's life. The phytoplamas do not pass into the eggs of the insect from the adult. The insect overwinters as eggs on the bark of live elms. Egg hatch begins at budbreak and continues for about 3 weeks. The instars take about 5-6 weeks to develop into adults. Peak activity of the leafhopper in some areas is late summer but they may be present throughout the growing season until frost. Leaves yellow and wilt over the entire crown of the tree and fall prematurely in summer. Trees usually die 1 to 2 year s following infection. Roots are killed early in disease development. The inner bark (phloem) and the outer most layer of xylem (water conducting tissue) is yellow or butterscotch in color and has a definite oil of wintergreen odor in American elm (Ulmus americana) and maple syrup odor in red or slippery elm (Ulmus rubra) if held to the nose immediately after cutting. If a piece of the discolored bark is placed in a tightly sealed jar, the smell will accumulate. Three to 6 months of incubation after infection are required before symptoms appear and obvious symptoms of systemic yellowing and browning may not appear until the following season. It is common for a disease outbreak to result in the death of an entire stand of elms locally but very little spread to occur from that locale. No effective control for the disease is available and removal of the infected tree is recommended.
Another important phytoplasma disease is ash yellows. In this disease, tree h eight and diameter growth are suppressed, leaves may be in tufts or rosettes on branches, the leaves are smaller and lighter green than normal, branches die and there is a gradual decline and death of trees. When the smooth areas of bark are exposed to wind or sun they are pink rather than gray. Numerous water sprouts may form on the lower trunk. In general, symptom development is slow and the tree declines over several years. White ash (Fraxinus a mericana) is most susceptible while green ash (Fraxinus pennsylvanica) is considered tolerant (it gets the disease but less severely). The phytoplamas is vectored primarily by leafhoppers in the genus Scaphoideus during summer and fall feeding and the next spring, initial symptoms appear. Transmission of the phytoplasma is too fast to control the disease spread with insecticides.
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