Melampsora rust, also known as poplar leaf rust, is a foliar rust disease that has been a large problem for trees in the genus Populus and other trees in the family Salicaceae, including various poplars, cottonwoods, aspens and willows, in the United States and throughout the world. The disease is caused by several fungal pathogens in the genus Melampsora, including M. larici-populina (mostly in Europe), and M. medusae and M. occidentalis (in North America). Melampsora spp. are basidiomycetes referred to as a macrocyclic, heteroecious rust fungus. Macrocyclic means that it produces five different types of spores at different times in its life cycle: spermatia, aeciospores, urediniospores, teliospores, and basidiospores; heteroecious refers to the fact that the fungus necessitates two hosts to complete its life cycle. Collectively, the list of alternate hosts is extensive and includes species of fir, hemlock, larch, pine and spruce, in addition to species of Saxifraga (rockfoils) and Ribes (currants and gooseberries).
The disease was originally discovered in the United States in the early 20th century. Today it has a large impact on poplars both in eastern and western regions of the United States. Poplar rust is sometimes known under different names and can be confused with conditions caused by closely related sister species of M. medusae. In addition to its presence in the United States, this disease is present in every other continent with the exception of Antarctica.
Melampsora rust has had negative impacts on trees in both forest ecosystems and in plantation settings. In forest ecosystems, young poplar trees can be drastically impacted. Repeated defoliation of young poplar trees can make them more susceptible to pests and other diseases, and in extreme cases, the disease can even kill them. As such, the healthy regeneration of poplar trees in areas impacted by the disease can be impeded. In poplar plantations, the disease can cause large economic impacts (Figure 1). For example, poplar plantations exposed to Melampsora rust in the north-central United States have reported up to a 65 percent reduction in volume and consequently large financial losses.
|Figure 1. Landscape scale symptoms (yellow foliage) of Melampsora rust caused by M. larici-populina in a poplar plantation in France. Note the different susceptibility of different poplar clones. Photo courtesy of Arnaud Dowkiw, INRA, France.|
Symptoms and Signs
Melampsora rust can be managed effectively if the symptoms are recognized before a large outbreak. The disease is characterized by a few select symptoms on poplars and allied species. One of the first visual symptoms of this disease is the formation of yellow spots on the upper surface of infected leaves (Figure 2). These spots eventually become necrotic and result in the premature death of leaves. The main sign of the disease is orange/rust-colored (hence the disease name) pustules, called uredinia, forming on the undersides of the leaves sometime after the yellow spots appear (Figure 3). These pustules contain masses of spores, the urediniospores, which reinfect the poplars during the season.
|Figure 2. Yellow and necrotic spots on the top of an aspen leaf as a result of M. medusae infection. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org.||Figure 3. Uredinia from M. medusae in the form of orange pustules on the underside of an aspen leaf. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org.|
The leaves dry and massive defoliation will usually occur after leaf yellowing and rust pustule formation; defoliation is another symptom of the disease. On the alternate conifer hosts, yellow pustules (aecia) will form on the underside of needles and occasionally cones. Shortly after the pustules form, the needles may shrivel and die, but generally the alternate hosts only experience minor impacts.
Causal Organism and Disease Cycle
A number of fungal pathogens responsible for a variety of leaf rusts exist in the genus Melampsora but the species M. medusae is the most important in the United States because it has one of the largest economic impacts. M. medusae is readily dispersed by wind as basidiospores, urediniospores and aeciospores.
The symptoms described above coincide with different stages in the complex life cycle of this heteroecious and macrocyclic rust fungus. During the spring, the aecial host (usually a conifer) is infected through its needles and cones via wind-dispersed basidiospores. Within weeks after infection, spermogonia (which produce spermatia) and aecia (which produce aeciospores) form on the underside of the host’s needles. Next, aeciospores disperse from the pustules and infect telial hosts (the poplars and allied species) through germ tubes formed by aeciospores that follow the leaf surface and penetrate the leaf through its stomata. After infection, uredinia form on the underside of infected leaves and produce urediniospores that infect other leaves of poplars and allied species throughout the summer. Telia take the place of uredinia in the fall and the resulting teliospores overwinter on dead, fallen leaves and occasionally on buds and in bark when winters are mild. Finally, in the spring, the teliospores germinate, producing wind-dispersed basidiospores, and the cycle starts over again. M. medusae infection can occur only when temperature is mild and conditions are moist.
The most effective methods of control for M. medusae involve sanitation and host resistance. Chemical treatments are not usually considered an effective form of management in forest or woodlot situations, but can be used to prevent the infection of high value trees. Some of the potential measures for treating Melampsora rust include:
- Removing the branches of infected aecial or telial hosts.
- Removing entire infected aecial or telial hosts.
- Removing the fallen leaves of infected or susceptible trees from the plantation floor.
- Planting trees farther from infection centers and farther from each other, if planting susceptible trees.
- Planting clones of resistant or tolerant individuals (Figure 1).
- Using preventative fungicides such as Banner MAXX, Bayleton 25 WP and RosePride Funginex on high value trees.
- Planting trees outside of the genus Populus and outside of the family Salicaceae.
In addition, experiments have been carried out showing that bacteria in the genus Bacillus, commonly used in biocontrol programs on many ornamental plants, can reduce M. medusae infection on Douglas-fir (Pseudotsuga menziesii), but only in a greenhouse setting; these results can be considered only as preliminary findings.
We thank Dr. Arnaud Dowkiw, INRA, France, for useful suggestions.
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