Christmas Trees: Traditions, Production, and Diseases

The Mycosphaerella-like fungus associated with symptomatic needles produces partly erumpent, black pseudothecia that are ca. 125 µ in diameter. Ascospores are hyaline, fusiform, 1-septate, slightly constricted at the septum and measure 13.4 (9.8-17.9) x 2.9 (2.2-4.0) µ. Phaeocryptopus nudus, Phyllosticta abietina, Toxosporium spp., and Rhizosphaera spp. sometimes are also found on symptomatic needles. It is unclear whether all of these fungi cause disease, or whether some of these fungi are endophytes or simply infest necrotic needles that have died from some other factor(s).

Click image for
more detail

Little information is available about factors that affect disease development. The disease appears most commonly on trees in areas of plantations where air movement is poor, and/or in plantations that experience extended periods of free moisture on needles during the spring. Trees adjacent to tall timber or in plantations with poor weed control are examples of sites that appear to favor disease development. Attempts by growers to salvage trees by removing the branches on the lower portions of trees have not been successful.

Applications of chlorothalonil during shoot elongation have provided effective control of this disease, suggesting that infections occur on the new growth shortly after it emerges from the buds (Chastagner and Staley 1995). See below, photos of sprayed (right) vs. unsprayed trees (left). Based on fungicide studies, there also appears to be a very long incubation period with this disease. The benefits of fungicide treatments are not evident for at least 16 to 18 months. Under high disease pressure, chlorothalonil is the only fungicide that provides effective control of this disease. Unfortunately, under some conditions applications of this fungicide can result in needle yellowing and/or the development of necrotic flecks on the needles.

Click image for more detail

For growers to minimize the impact of interior needle blight on the production of noble fir in the PNW, additional studies are needed relating to the etiology and epidemiology of this disease. Studies are also needed to identify alternatives to chlorothalonil based products that are effective in controlling this disease.

The Future
Since the use of Christmas trees plays such an important role in the cultures of many people, demand will remain strong for species of Christmas trees like noble and Fraser fir that have excellent needle and moisture retention characteristics. Even though there are many innovative and successful noble and Fraser fir Christmas tree growers in the PNW and North Carolina, Phytophthora root rot, CSNN, and interior needle blight are currently limiting the ability of growers to meet the demand for these trees. Emphasis on production of healthy transplants coupled with careful site selection will help growers meet the increasing demand for these Christmas trees. Growers may also eventually utilize innovative approaches such as grafting noble and Fraser fir to rootstocks that are resistant to Phytophthora root rot. Long-term solutions to Phytophthora root rot and CSNN will no doubt come through breeding programs for disease resistance such as those that are underway in North Carolina and the PNW.