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Symptoms and signs

White pine blister rust causes a range of symptoms on the white pine hosts. The earliest symptoms are indistinct chlorotic spots on needles shortly after infection. As the fungus moves into the branches, spindle-shaped swellings form (Figure 1) and eventually develop into cankers. The bark at the margins of these cankers often has an orange color (Figure 2). When the fungus girdles a branch, that branch dies within a few years and the dead needles on the affected branch turn red. This is called a blister rust "flag" (Figure 3) and often is the first indication of disease in the forest or plantation. If the branch is not killed, the fungus may continue to grow into the trunk, or bole, causing a canker that can kill that portion of the tree upward. Cankers often exude large amounts of resin (Figure 4), another conspicuous diagnostic symptom. Sometimes, rodents eat the bark at the canker margin, as seen in Figure 4.


Figure 1	Figure 2

Figure 3	Figure 4

Signs of the disease on pine occur as the fungus develops and produces two of the five spore stages characteristic of the rust. Pycnia develop at the margin of a canker a year or two after the canker forms. This stage is apparent for a short time in the spring and appears as small honey-colored droplets on the surface of the bark, consisting of masses of pycniospores (spermatia). Aecia develop in this region of the canker the following year. This sequence of pycnial and aecial development occurs progressively each year as the canker grows and continues as long as the fungus remains active. The aecial pustules erupt through the bark of cankers and are covered at first by a white membrane, the peridium (Figure 5). The peridium soon ruptures to release masses of yellow-orange aeciospores (Figure 6). This stage gives the disease the name "blister rust."


Figure 5	Figure 6

Symptoms on the alternate hosts, currants and gooseberries (collectively referred to as Ribes spp.), are not as distinct as on the pines and usually consist of chlorotic spots on the upper leaf surface. If leaves are severely infected the leaves may drop prematurely, but usually there is little damage to these host plants. Signs of the disease on leaves of Ribes are orange pustules that develop on the underside of leaves below the chlorotic spots (Figure 7). This is the uredinial stage of the fungus in which masses of orange urediniospores are produced. As the season progresses, brown, hair-like telial columns (Figure 8) develop from the uredinial pustules.


Figure 7	Figure 8

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