Gymnosporangium fuscum DC.

SPERMOGONIA (s) in rather compact conglomerations, up to 5 mm diam, hemisphaerical, protuberant, pale brown, blackening with age, 180-225 diam, develop on yellow, later reddish, purple or brown spots on the outer leaf surface.
AECIA (a) develop in more or less compact conglomerations, rarely singly on the inner leaf surface and on petioles. Each aecium partly immersed in a highly protruded tissue outgrowths.

Peridium pale brown, up to 3 mm high and 1.5 mm diam, sharpened at the top. At maturity, the peridium bursts and forms yellowish, filiform strands, and becomes fusiform.

Peridial cells highly irregularly prolate, with inner and lateral walls highly thickened, up to 10 µm, ornamented with cylindrical or prolate warts, distributed in indistinct rows; inner cell walls thin, covered with fine warts.

AECIOSPORES (as) subglobose to elliptical, angular, 25-34 x 22-29 µm.

Wall yellow brown, 3-5 µm thick, ornamented with very fine warts.

Germ pores 6-9, irregularly distributed.

UREDINIA (u) unknown.
TELIA (t) form on twigs in tremellose, yellow brown conglomerations of a different size, up to 5-7 cm, when young, then dry, become brown black, up to 5 mm high.

TELIOSPORES (ts) fusiform to widely elliptical, tapering towards or sahrpened at the top, rounded or more frequently narrowed at the base, constricted at the transverse wall, 37-63 x 20-33 µm.

Wall smooth, yellowish or yellow brown, 1.0-2.5 µm thick.

Germ pores 2-3 in each cell, usually positioned at the transverse wall, without distinct papillae.

Pedicel (p) colourless, dissolving.

PLANT HOST AND DISTRIBUTION. Spermogonia and aecia of G. fuscum develop on members of the genus Pyrus (Majewski 1979). Telia origin on Juniperus sabina L. and some other species of the genus Juniperus.

Gymnosporangium fuscum probably occurs all over the world.

NOTES. Gymnosporangium fuscum is the severe causal agent of rust of pear-tree.

REFERENCES

Agrios 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.

Majewski T. 1979. Grzyby (Mycota) XI. Podstawczaki (Basidiomycetes), Rdzawnikowce (Uredinales) II. Warszawa-Kraków, 462 pp.