The Douglas-Fir Needlecast Blues

A landscape contractor asked me about a customer's weeping Douglas-fir (Pseudotsuga menziesii 'Graceful Grace') late last winter. She had called him to say that she thought the tree was dead because it had dropped all of its needles. I suggested that she wait and see if the tree put on new growth this spring before doing anything drastic. Several weeks later, she called to tell him that the tree was putting on new growth!

Douglas-firs are susceptible to two needlecast diseases that require management to keep them looking attractive and keep them from slowly dying: Rhabdocline needlecast and Swiss needlecast. Sometimes, the only foliage present on an affected tree is the current season's growth. And it is common for trees to be infected by both diseases at the same time.

Rhabdocline needlecast causes needle browning and premature defoliation. When this happens for successive years, trees are weakened and become more susceptible to other diseases and insect damage. Symptoms of Rhabdocline show up in late summer or fall as yellow or reddish-brown spots on the upper and lower surfaces of the current season's growth. Needles can have one to several spots that continue to enlarge and may cover the entire needle. The lesions swell and split lengthwise along the underside of needles in late spring, releasing fungal spores that are spread by wind and rain. This coincides with new growth coming on, and the tender new growth is most susceptible to infection. Rhabdocline is generally most severe on the lower portion of the tree canopy due to increased shading and reduced air circulation compared to the top of the tree.

Managing Rhabdocline needlecast starts with the planting site. Douglas-firs are shade-intolerant, so full sun and well-drained soils are a must. Maintain good weed control around them and avoid underplanting them with anything that impedes good air circulation. Prune dead and dying branches during dry weather.

Start making fungicide applications at bud break and follow with a second application one week later. Make a third application two weeks after the second. Make the fourth application three weeks after the third if the weather remains cool and wet. Fungicides containing chlorothalonil, copper hydroxide, copper sulfate, mancozeb, and thiophanate-methyl are labeled to control Rhabdocline.

Douglas-fir trees infected with Swiss needlecast may not show symptoms until they have been infected for more than one year. While new growth is easily infected, Swiss needlecast generally does not kill needles for two or three years. Check older needles for the characteristic black fruiting bodies of the fungus erupting through the stomatal lines on the underside. They are easily seen with a hand lens; they may appear as lines of soot to the unaided eye. Spore release continues for a longer time than with Rhabdocline. Needles turn yellow or brown prior to being cast.

Like Rhabdocline, Swiss needlecast is often the most severe lower in the canopy due to shading and reduced air circulation. The same cultural controls are important – full sun, well-drained soil, and good weed control around the base of trees.

Fungicides should be applied when new growth is one-half to two inches long. Make a second application two or three weeks later. If cool, wet weather continues, make a third application two weeks after the second. With Swiss needle cast, fungicide applications should continue for three consecutive years because infected needles remain attached to the tree and continue producing spores for that time. Fungicides containing clorothalonil or mancozeb are labeled for Swiss needle cast.

With both diseases, seed source is very important, and you should avoid those from Rocky Mountain seed sources. Seed sourced from Pacific Coast trees appears to have more resistance.

Given the frequent occurrence and severity of these diseases, they are no longer recommended as landscape trees. Some Christmas tree growers are even moving away from them because of these needlecast diseases.