Diseases of concern to the Christmas tree industry are primarily caused by fungal pathogens and water molds. Most of the diseases of Christmas tree are caused by spore-forming fungi. To infect plant parts, spores require high humidity and wet leaf surfaces which are common in both the spring and fall in the Great Lakes. Because there are several different species of conifers grown for the Michigan Christmas tree industry, there are many diseases that the growers must attempt to manage. The most common way to categorize tree diseases is by the tissue-type or part of the tree subject to infection. Some of the most common diseases in conifers include needlecasts and rusts, which usually cause needle loss or discoloration; root rots, which can kill roots and impact water and nutrient uptake; and stem and branch cankers that can kill individual branches or entire trunks of trees. Needle diseases can occur in one or more tree species throughout the growing season. Needle disease management is important for three reasons. First, needles provide the tree with photosynthetic energy for growth and vigor. Second, needle color and density are primary characteristics od tree quality for consumers.; Finally, diseased needles will cast (fall off) prematurely reducing needle retention and tree marketability. Severe needlecast problems occur with Douglas-fir (Rhabdocline and Swiss needlecast), Scotch and red pine(Lophodermium needlecast and needle rust), Austrian pine (Dothistroma needle blight), spruce (Rhizosphaera needlecast) and true firs (needle rust).
The fungi that cause these diseases are not related and represent different taxonomic groups (ascomycetes and basidiomycetes). Diseases of shoots and branches are caused by several different fungal pathogens infecting all species of conifers grown by Christmas tree producers in Michigan. Some of these diseases will kill trees or reduce quality to the point that the trees are unmarketable. For example, pine gall rust on Scotch pine will not only slow the growth of trees, it will kill branches and ultimately kill trees. Leucostoma canker of spruce, Scleroderris canker, and white pine blister rust are all diseases of Christmas trees that can infect and girdle several species of conifer grown for Christmas trees. In addition, Sphaeropsis tip and shoot blight, commonly associated with pine, is also isolated on spruce in Michigan. This disease will cause shoot death and stems cankers, and it also causes the death of nursery plants. Diseases of roots and lower stem are less common but just as devastating. Phytophthora root rot of Fraser and less commonly Douglas-fir, is caused by several species of the water mold, Phytophthora. Found in both nursery beds and plantations, this disease limits the sites on which Fraser fir can be grown thereby limiting the amount of fir that can be produced in Michigan. Armillaria root rot is caused by a fungal pathogen that can infect all known conifers (and hardwood tree species) used in the Christmas tree industry. In most cases, the fungus is native to the soils in which Christmas trees are planted.
- Needlecast Diseases
- Lophodermium needlecast
- Rhabdocline and Swiss needlecasts
- Rhizosphaera needlecast
- Diplodia tip blight
- Phytophthora root rot