Sudden death of pines or other evergreens may often be caused by the pinewood nematode. It is usually transmitted by long-horn beetles and has been found in more than 25 species of evergreens. Scotch (Pinus sylvestris), Austrian (Pinus nigra), and Japanese species are very susceptible to this disease. Since the pinewood nematode is native to the U. S., most native pines are resistant to it. The sawyer beetle appears to be the primary carrier (vector) for this disease, although other wood-boring beetles may also transmit the nematode.

The symptoms are minimal but dramatic. If a tree is infected in the fall, brown limb die-outs (flagging) in or near the crown, or an absence of candle growth will usually occur. The most noticeable symptom appears in mid to late summer when the greatest need for moisture is required, and blocked by the nematodes in the resin canals. In a matter of six to eight weeks the tree becomes a dull green, to yellow, and finally brown. Depending on the age and condition of the tree, it may take up to one year before it is fully destroyed by wilt. In some cases, death occurs from a combination of pests.

The disease cycle is simple and direct. In early to mid-summer the sawyer beetles emerge from dead pine trees with pinewood nematodes in their air passages. As they feed on several trees, nematodes leave the beetles and enter through the feeding wound. From this point the nematodes invade the resin canals, cambial tissue, and cortex. With a short life cycle, and a complete generation of nematodes every four days in optimal conditions, the population multiplies exponentially. As a result, the water transport system is blocked and the tree dies.

Management of the nematode population is to reduce the spread of the disease. Dead trees must be cut down and burned before the sawyer beetles emerge in early to mid summer. Pruning out flagged branches in the crown may only postpone tree mortality. If you want positive ID, these branches can be sent to the department of entomology.

Wood from infected trees must be handle properly to prevent spreading the disease. The key here is to stop the reproduction of nematodes and beetles by heating the wood. This can be done by immediate burning, fumigation of chopped wood for later burning, or placing chopped wood under black plastic (completely surrounding wood) for several months allowing the sun to heat and destroy the pests.

When replanting pines, use native disease resistant species. Avoid planting susceptible pines such as Austrian, Japanese, or Scotch, keep properly watered and fertilized, and control other pests that will stress trees.

At this time, there is no cure for pinewood nematode infestation. It is found in over 35 states, being native to the Americas, and is devastating to ornamental and Christmas tree plantings. 

 

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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