Melampsorella caryophyllacearum Schroet.
(=Melampsorella ceratsii (Pers.) Schroet.)
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Basidiomycotina, Uredinales, Pucciniastraceae
Hosts: In B.C., the aecial hosts of Melampsorella caryophyllacearum are amabilis fir, grand fir, and subalpine fir. Elsewhere in North America is is also found on balsam, noble, California red, Pacific sliver, and white firs. The telial hosts include chickweed, sandwort, and starwort (Fig. 36a).
Distribution: This fungus is widely distributed throughout the range of its hosts in B.C.
Identification: This rust is easily recognized by the conspicuous perennial, systemic brooms formed on branches throughout the crown (Fig. 36b). Infected twigs in the brooms are shorter and thicker than normal (Fig. 36c). At the base of the broom, infected branches and stems are swollen, forming an elongate canker or gall. Stem swellings may be observed after brooms have died and are shed.
Needles in the broom are shorter, thicker, and chlorotic. Spermatia are visible in the spring as orange dots, mainly on the upper surface of needles. Orange-coloured aeciospores are produced in pustules on the lower surfaces of needles, giving the entire broom a yellow-orange appearance during sporulation (Fig. 36d). The needles die and are shed in the fall (Fig. 36e).
Uredinia develop on both sides of alternate host leaves, appearing as orange-red pustules, producing yellow-orange urediniospores. Teliospores on the lower epidermis give leaves and orange cast. The rust is systemic on perennial alternate hosts, and can therefore persist in the absence of the fir host.
Microscopic Characteristics: Aeciospores orange, globose, finely verrucose, 14-18 x 16-28 µm. Urediniospores yellow-orange, ellipsoid, sparsely echinulate, 12-18 x 16-30 µm. Telia on lower leaf surfaces, teliospores within lower epidermis, single-celled, cylindric, 13-20 µm wide.
Damage: Fir broom rust has not caused serious damage in B.C., but elsewhere in North America, and in Europe severe growth loss and mortality have been reported.
Remarks: This fungus is systemic and perennial on both aecial and telial hosts. As a result, the rust may be found in areas where one of the hosts is not present. The brooming symptoms on true firs caused by M. caryophyllacearum could be confused with mistletoe infections or brooms caused by physiological abnormalities. However, the chlorotic needles in the summer and complete loss of needles in the winter are diagnostic features of the rust.
Peterson, R. S. 1964. Fir broom rust. USDA For. Serv., Pest Leaf. No. 87. Washington, D.C.
Ziller, W. G. 1974. The tree rusts of western Canada. Can. For. Serv., Publ. No. 1329.
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Figure 36a: Crisp sandwort, one of the telial hosts of Melampsorella caryophyllacearum.