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Pathogen Biology

The white pine blister rust pathogen is a typical heteroecious, macrocyclic rust that produces five distinct spore stages on two different hosts to complete its life cycle. The pycnial stage consists of pycniospores, or spermatia, which are haploid spores that fertilize compatible receptive hyphae. The two sexes are not distinguishable and are simply designated plus and minus. This is the stage where genetic recombination can occur that may lead to development of races of the rust. However, the nuclear cycle (i.e., dikaryogamy, diploidization, meiosis) of the blister rust fungus has not been fully determined, but is assumed to be the same as for other better known rust fungi such as Puccinia graminis. The aecial stage develops in host tissue occupied by pycnia the previous season (Figure 6). The fungus is perennial in the pine host and aeciospores are produced annually as long as the host tissues remain alive. Aeciospores are disseminated by wind over long distances, and Ribes spp. as far as 480 km (300 miles) from the nearest known white pines have been infected.

Figure 6

Within a few weeks after infection of Ribes, the uredinial stage (Figure 7) appears and urediniospores continue to be produced until the end of the growing season . Uredinia comprise the repeating stage of the fungus, and urediniospores infect other Ribes in the area. Teliospores, which make up the telial columns (Figure 8), germinate to produce a four-celled promycelium on which four haploid basidiospores (sporidia) develop. The basidiospores are small, thin-walled and relatively short-lived. They are wind-borne to pines to start the disease cycle.


Figure 7	Figure 8

Pathogenic races are common in rust fungi, and early European pathologists suggested the occurrence of races of the white pine blister rust fungus. The occurrence of pathogenic variation, specifically physiologic races, has been based on several criteria but not yet by the traditional method of inoculating differential hosts and determining if infection occurs.

The blister rust fungus is believed to have originated in Asia on pines native to that region but did not become a serious problem until after the highly susceptible American white pines were introduced into Europe as early as the 16th century. Eastern white pine seed from America was planted at "Longleat," the estate of Lord Weymouth at Wiltshire, England, and soon was planted throughout Europe where this pine is known as Weymouth pine. The disease on white pine was considered to be the same as a stem rust that occurs on European pines such as Scotch pine, and the fungus was called Peridermium pini. In the late 1800s Heinrich Klebahn separated this fungus group into three species and renamed the white pine blister rust fungus Peridermium strobi. The fungus on Ribes spp. was first reported in Europe in the mid 1800s, and named Cronartium ribicola. "Ribicola" means living on Ribes. Klebahn proved the connection of the two fungal stages in 1889 by dusting Ribes leaves with aeciospores from pine and completed the cycle a few years later by placing healthy white pine seedlings in a bell jar with infected Ribes bushes. Peridermium refers only to the stages on pine, and Cronartium ribicola is now the name used for the heteroecious, macrocyclic rust pathogen found on both pines and Ribes.

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