Branches and stems of jack pine can be infected by five different rust fungi:
Damage to jack pine caused by rust fungi includes growth reduction, cankers, death (by girdling or wind breakage), and creation of entryways for other fungi and insects. Seedlings and saplings are more seriously affected than older trees (Fig. 1).
Rusts are plant diseases caused by a group of fungi that often require two different species of host plants to complete their life cycle. The fungi produce two types of spores (pycniospores and aeciospores) on the pine host and three (urediospores, teliospores, and basidiospores) on the alternate (non-pine) hosts.
All five stem rusts have been found in the Lake States, but only pine-oak and sweetfern rust have been reported in Maine. In Canada, pine-oak rust has been confirmed only in Ontario. The other four rusts are distributed in all Canadian provinces where jack pine grows, with only a few exceptions: stalactiform rust has not been observed in the Northwest Territories, New Brunswick, or Nova Scotia, and comandra rust is not known in Nova Scotia.
Rust diseases are identified by the shape and location of galls and cankers and by the presence of alternate hosts in the area. The following tabulation will aid in field identification.
The spherical galls of the pine-oak and pine-pine rusts are very similar in appearance and can only be distinguished by comparing the germinating aeciospores. After the spores have incubated for 24 hours on 2 percent water agar in the dark at 19°C germ tubes of the pine-oak rust fungus are more than 600 M long with few branches at the tips, whereas those of the pine-pine rust fungus are less than 300M long with basal branches.
The rust diseases are controlled in nurseries by spraying seedlings with a protective fungicide. Alternate hosts growing in or near nurseries should be removed. Rust-infected seedlings should not be planted. Research is underway to develop a jack pine that is resistant to stem rusts.
In the Field
Where infections are scattered or light, trees with main-stem infections or with numerous branch infections should be removed during thinnings or other stand-improvement treatments. If infection in a stand is heavy, conversion to another tree species should be considered.
Kathryn Robbins, Plant Pathologist, Northeastern Area, State and Private Forestry, USDA Forest Service, St. Paul Mn. 55108
Dale K. Smeltzer, formerly graduate student, Department of Plant Pathology, University of Minnesota, St. Paul, Mn. 55108
D.W. French, Professor, Department of Plant Pathology, University of Minnesota, St. Paul, Mn. 55108