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Disease Cycle and Epidemiology

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Figure 9

The blister rust cycle (Figure 9) starts in the fall when pine needles are infected by basidiospores from the alternate host (Ribes spp.) (Figure 9A). The basidiospores germinate, and the fungus mycelium enters the needles through stomata and grows down the needle into the branch. The fungus continues to develop between the cells of the inner bark, and nutrient absorbing haustoria penetrate into the phloem cells. The mycelial network continues to develop both lengthwise and laterally in a spindle-shaped pattern to produce a swollen canker (Fig. 9B). In the spring of the second year, pycnia are produced in the margins of the infected area. The following year aecia are produced and erupt through the bark where pycnia were produced the previous year (Figure 9C). This disruption of the bark leads to drying and death of the inner bark and this dead area enlarges over several years to become a blister rust canker. The two or three year latent period between initial infection and appearance of symptoms or signs allows the disease to become established in a forest stand before it is detected. Aeciospores (Figure 9D) can survive for several months and are wind-borne for long distances where they infect the alternate hosts (Ribes spp.).

A few weeks after the Ribes are infected, light yellow spots develop on the upper leaf surface (Figure 9E) and uredinia (Figure 9F) develop on the undersides of infected leaves. This is the repeating stage of the rust, and urediniospores infect other Ribes bushes in the general area as far as 1.6 km (one mile) distant, perhaps farther. Telial columns also develop on the underside of leaves (Figure 9G), usually from uredinial pustules, and may start forming soon after the uredinia appear, increasing in number as the season progresses. The individual teliospores that make up the telial column germinate (Figure 9H) to produce basidiospores that infect the pine hosts, completing the disease cycle.

Blister rust infection is favored by cool, moist conditions, and the disease tends to be more prevalent in low-lying areas such as creek bottoms, swampy depressions and openings in timber stands. Besides providing conditions favorable for infection, air currents carrying basidiospores are more likely to flow into and settle in these depressions. Studies in the Great Lakes Region and elsewhere have recognized rust hazard zones based on topography that are now used to formulate disease management systems in different areas.

Another epidemiological factor is the relative susceptibility of the different pine species and of different age classes of the same species. Of the three commercial species, sugar pine (Pinus lambertiana) is the most susceptible, western white pine (P. monticola) intermediate, and eastern white pine (P. strobus) least susceptible. Of all the white pines, whitebark pine (P. albicaulis) is most susceptible.

Young white pines, both saplings and pole size trees, are more severely infected and damaged than older trees for several reasons. On young trees the branches are closer to the ground, where conditions are more favorable for infection, and the needles are closer to the main trunk so that less time is needed for infections to get into the main stems and kill the trees. Mature trees can be infected, but these trees are not killed as rapidly and usually can be harvested before they die and deteriorate.

Factors important to development of wild Ribes include any soil disturbance, such as fire, logging, and snow and wind damage, which stimulates dormant Ribes seeds to germinate. Birds do not appear to be important in spreading Ribes seeds as they are for barberry, the alternate host of stem rust of wheat.

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