Nursery and Landscape - Needlecasts on Douglas Fir

Christmas tree growers and nurserymen will need to watch out for needlecasts on Douglas Fir. The following is an article on the subject.

Rhabdocline and Swiss Needlecasts of Douglas Fir - What’s the Difference?

In general, there are at least 40 needle casts in the United States; most affect pine, spruce, fir, larch, cedar, hemlock, and Douglas-fir. These diseases are caused by fungi that have only one infection period per year; in most cases (but not all), the new growth is infected with the disease agent and is eventually cast from the tree (hence the name) once the infection cycle is complete.

Two of these needle casts affect only Douglas-fir: Rhabdocline needlecast (caused by the fungus Rhabdocline pseudotsugae) and Swiss needlecast (caused by the fungus Phaeocryptopus gäumannii).

Rhabdocline needlecast is well established in Christmas tree plantations; incidence of Swiss needlecast, on the other hand, is very low. Still, growers want to know, “what’s the difference?” Disease development is similar, but symptoms and signs are different Symptoms of Rhabdocline needlecast include the development of redbrown spots that become evident during the winter on one-year needles. A week or two before budbreak, orange fruiting bodies develop on the lower needle surface. When the fruiting structures are mature, they rupture and release abundant spores during wet weather. These spores infect expanding buds. Once these new needles fully elongate and the weather is warm and dry, spores are no longer released, and the older, infected needles are cast from the tree. Symptoms on the newly infected needles do not appear until the following fall or winter, and spores are not produced again until the following spring.

Unlike Rhabdocline needlecast, the fruiting structures produced by the Swiss needle cast fungus consist of rows of very tiny, black fruiting structures emerging through the stomates of the lower needle surface. Needles affected by the disease may turn off color or brown, or may even remain green. Spore release from these fruiting bodies begins at budbreak but can continue until August, and it may be several years before fruiting structures are produced in these infected needles to complete the disease cycle. Once needles are shed, affected branches are thin and trees may be unfit for sale.

Cultural management for these two diseases is similar
• Use healthy stock and provide an optimal growing environment.
• Inspect trees during late winter/early spring and look for symptoms and signs. The pattern of infection will likely follow portions of the canopy or field where moisture in the canopy is most abundant.
• Remove sources of inoculum. For best results, remove old and severely infected trees now. Prune severely affected branches during dry weather.
• Manage the moisture. Choose sites with good air drainage. In existing plantations, reduce humidity and promote rapid drying of needles through spacing and row orientation, and remove weeds.
• Use resistant seed sources when available.

Chemical controls used for Rhabdocline needlecast are also sufficient for Swiss needlecast. For Rhabdocline needlecast, begin sprays of chlorothalonil when the first 10% of the trees in the planting first break bud (or the candles are about 1/2 inch long). Repeat the sprays at 7- to 14-day intervals until needles are fully elongated or conditions are no longer favorable for disease development. Add a spreader sticker to enhance coverage. Other compounds labeled for control include copper (hydroxide, salts, sulfate), mancozeb, Spectro (chlorothalonil + thiophanate-methyl), Stature (mancozeb + dimethomorp; nursery and greenhouse only), and thiophanate-methyl.

For Swiss needlecast management alone, spray chlorothalonil (one application) when candles are 1/2 to 2 inches long. Other labeled compounds include azoxystrobin, mancozeb (2-wk intervals through infection period), Manhandle, Spectro, or thiophanate-methyl. Refer to label for timing and rates. For all fungicides, thorough coverage is essential.

Reprinted from "Rhabdocline and Swiss Needlecasts of Douglas Fir - What’s the Difference?" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University. From the March 13 edition of the Plant and Pest Advisory, Landscape, Nursery, and Turf edition.