Leaf Rust (Melampsora medusae, M. occidentalis, M. abietiscanadensis)
Ostry, M.E.; Wilson, L.F.; McNabb, H.S.; Moore, L.M. A Guide to Insect, Disease, and Animal Pests of Poplars. United States Department of Agriculture, Forest Service. Agriculture Handbook 677. January 1989. 118 p.
Importance - Species of Melampsora are found throughout North American. Infection by Melampsora spp. can cause premature defoliation and reduce growth potential by more than 20 percent. Trees defoliated early in the growing season are more susceptible to other diseases and to environmental stresses.
Look For -
- Yellow to orange pustules (uredia) on the undersides of leaves in mid to late summer. Uredia may be present in early summer where the alternate host exists.
Biology - Melampsora spp. require two hosts to complete their life cycle--poplar and a coniferous alternate hosts. The species of the alternate host depends upon the Melampsora species present.
In the spring during wet weather, spores from fallen infected poplar leaves travel by wind to developing pine, larch, or hemlock needles and infect them. In early summer, another spore type (aeciospore) is produced on the conifer host and is windblown to poplar leaves where the yellow uredia pustules develop. The closer poplars are to the conifer alternate host, the earlier in the season they become infected. Urediospores from the uredia pustules are windblown to adjacent poplar leaves, infecting them. The fungus overwinters on fallen poplar leaves.
Monitoring -Examine clones for rust infection in midsummer to late summer. This is especially important for clones thought to be resistance because rust species or races new to an area can develop.
Many species of leaf rust are found on Populus spp. Special mention should be made of two species common to other parts of the world, but not believed to be present in North America yet. The most common and serious rust in Europe is Melampsora larici-populina, whose common alternate host is Larix decidua. Melampsora larici-populina is also common in poplar plantings in Asia, South America, and Australia. Another closely related species in Europe is Melampsora alli-populina, whose alternate hosts are a number of species of Allium and Muscari comosum. These two species in Europe can be differentiated by the more reddish color of the summer spore-masses of M. alli-popluina compared to the more yellowish color of those of M. larci-populina. M. medusae, M. larci-populin,. M. medusae, M. larci-populina, and M. alli-populina are difficult to differentiate in the field. Microscopic examination of summer spores and careful, controlled greenhouse inoculation studies are required for diagnosis.
Careful monitoring of rust diseases of Populus should be part of integrated pest management. Not only can new rust species be imported, but plant rust populations can change and may overcome host resistance over time.
- Plant only clones that are resistant to leaf rust. This is especially important where the conifer alternate hosts are common.
- Do not plant poplars next to conifer alternate hosts because poplars will become infected early in the summer.
- Wide spacing reduces rust severity in moderately susceptible clones but does not protect highly susceptible clones.
For Additional Information -
Ostry, M. E.; McNabb, H. S., Jr. 1985. Susceptibility of Populus species and hybrids to disease in the north central United States. Plant Disease. 69: 755-757.
Ostry, Michael E.; McNabb, Harold S., Jr. 1986. Populus species and hybrid clones resistant to Melampsora, Marssonina, and Septoria. Res. Pap. NC-272. St. Paul, MN: U.S. Department of Agiculture, Forest Service, North Central Forest Experiment Station. 7 p.
Schipper, A. L., Jr.; Dawson, David. 1974. Poplar leaf rust--A problem in maximum wood fiber production. Plant Disease Reporter. 58: 721-723.
Schipper, A. L., Jr.; Widin, K. D.; Anderson, R. L. 1978. How to identify leaf rust of poplar and larch. St. Paul, MN: U.S. Department of Agiculture, Forest Service, North Central Forest Experiment Station. 6 p.
Widin, K. D.; Schipper, A. L., Jr. 1981. Effect of Melampsora medusae leaf rust infection on yield of hybrid poplars in the north central United States. European Journal of Forest Pathology. 11: 438-448.