Christmas Trees: Traditions, Production, and Diseases

Saturated soils favor the development of this disease. Phytophthora species produce sporangia that release swimming zoospores. These spores are attracted to nearby elongating or wounded roots where they encyst, germinate, and infect the root. Near saturated soil conditions are needed for germination, and dispersal of zoospores. Root infection is progressive and eventually leads to above ground foliar symptoms. Shoot blight can occur when growers use contaminated irrigation water to over head irrigate trees or when overhead irrigation splashes infested soil onto the lower branches of the tree (McCain and Scharpf, 1986). Some Phytophthora species produce resting spores (chlamydospores and/or oospores) in the infected roots that enable the fungus to survive for extended periods of time in the soil. Phytophthora spores may spread in surface runoff or in irrigation water. (See photo on right showing missing Fraser fir trees resulting from inoculum spreading down hill with water.)

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The high susceptibility of many firs to Phytophthora root rot limits where these trees can be grown. Noble, balsam, grand (A. grandis), red, Fraser, and Shasta firs (A. magnifica var. shastensis) are among the more susceptible species, while Turkish (A. bornmuelleriana), European silver (A. alba),Veitch (A. veitchii), Nordmann, Momi (A. momi), and Korean firs (A. koreana) tend to be much less susceptible (Benson et al., 1997a and b, Cooley et al., 1988, Chastagner et al., 1990b and Hamm and Hansen, 1982). There is also considerable variation in the ability of the different Phytophthora species to cause disease. In general, P. cinnamomi, P. cryptogea, and P. citricola tend to be more aggressive than the other species.

The use of healthy transplant seedlings from nursery beds and site selection are the most important cultural factors affecting the management of Phytophthora root rot and stem canker in plantations (Sidebottom and Jones, 1999, Kuhlman et al., 1989). Highly susceptible firs should not be planted on sites with heavy, poorly drained soils (Hamm et al., 1990). Avoiding the use of overhead irrigation, particularly if the water is contaminated with Phytophthora inoculum will prevent shoot blight. Soil fumigation is commonly used in bareroot nurseries for transplant production to control Phytophthora root rot (Hansen, 1997 and Landis and Campbell, 1991). Fumigation in combination with spring and late-summer applications of selective fungicides has been highly successful in production of healthy transplants when combined with cultural practices and sanitation. Growers can alleviate high soil moisture conditions that favor disease development at some sites by installing drain tiles.

(See photo at right.) Soil fumigation is seldom used in Christmas tree plantations. Applications of selective fungicides have not generally been effective in controlling this disease on noble or Fraser fir in plantations in the PNW or North Carolina.