Most pines infected in the spring are often dead by late summer to early fall. Large trees may take two years to die. Vigor of plant does not seem to have a bearing on which plants become infected. Infected plants quickly become stunted. Foliage begins to fade to an off green or slightly yellowish color before turning brown. Dead needles hang on to the branches for a long time. On large trees that take two years to die, the older needles turn yellow first and fall off before the younger needles turn brown. These symptoms can occur any time midsummer to late fall or late winter to spring. Trees infected in the fall do not break bud the following spring.
Prior to foliar symptoms, the resin content of the wood decreases significantly, but this symptom may go unnoticed unless the tree is pruned. Pines declining from environmental stress or transplant shock will develop symptoms similar to pine wilt. In addition saprophytic nematodes and predator nematodes can be found on the dead pines. Proper identification of the nematodes is important. A laboratory test is necessary to confirm this disease.
Pine sawyer beetles (Monochamus carolinensis) which are native longhorns, are known to transmit this nematode. The beetles that nest in dead trees and feed on "healthy" trees are the most likely to transmit the nematode. One longhorn can carry several thousand nematodes on and in their bodies. The nematode presence does not necessarily prove that they killed the tree. Research has shown that under certain growing conditions the pines do not die. Pines under stress are more likely to be killed by the pinewood nematode.
The nematode can alter its development based on whether it is in a dead or drying out tree or in a tree still somewhat viable. The nematode can take between four and twelve days to go from egg to adult depending on temperature. Females lay dozen of eggs over several weeks before dying themselves. The nematode breeds fairly rapidly and sometimes, in association with bacteria, quickly causes the vascular tissue to plug-killing the tree. It has been proven that the nematode alone can cause the tree to die. The nematode feeds on plant tissue or fungal mycelium such as blue stain canker mycelium.
Have suspected plants tested. If results are positive, remove diseased and dead evergreens as soon as possible to reduce breeding sites for the insects that transmit the pinewood nematode. Infected trees must be destroyed (burned or buried) to eliminate the breeding sites for the beetle vector. Maintaining the health of landscape pines through good cultural practices will reduce the number of stressed and dying trees available to attract the insect vector, thereby reducing spread of the nematode through insect oviposition.
