Hosts: Rosa Eglentaria Mill. (= R. rubiginosa L.). On leaves, stems, petioles, and fruits. Herb. No. 373. I. York Bay (Wellington), E. H. Atkinson ! 3 April, 1921. Blenheim, E. H. Atkinson! 3 Nov., 1922. Mapua (Nelson), G. H. C. 17 May, 1922. Rosa sp. cult. II, III. Nelson, W. C. Hyde! 7 Jan., 1922.

0. Spermogones caulicolous, sparse, flattened, immersed, honey-coloured.
I. Caeomata hypophyllous, caulicolous, petiolicolous and fructicolous, on leaves scattered or crowded, orbicular, pulvinate, 0.25-1 mm. diam., on stems confluent, up to 20 mm. long, forming large inflated distortions; reddish-orange, pulverulent; paraphyses present and as a rule encircling only the smaller sori; incurved, clavate, hyaline. Spores elliptical, obovate, or subglobose, 22-30 x 15-23 mmm.; epispore hyaline, finely and densely verruculose, 2-2.5 mmm. thick, cell-contents oily, reddish-orange.
II. Uredosori hypophyllous, scattered, seated on pallid-yellow spots, orbicular, 0.25-0.5 mm. diam., orange, pulverulent; encircled by a layer of incurved.hyaline, clavate paraphyses. Spores elliptical, obovate, or subglobose, 22-28 X 15-20 mmm.; epispore hyaline, closely, finely and bluntly echinulate, 2 mmm. thick, cell-contents pallid-orange; germ-pores scattered, numerous (6-8), obscure.
III. Teleutosori hypophyllous, scattered, orbicular, 0.25-0.5 mm. diam., somewhat pulverulent, black, naked, with numerous spores in each sorus. Spores 5-9-celled, commonly 7-8, oblong-terete, 72-98 X 28-35 mmm.; apex obtusely rounded, not thickened, crowned with a prominent papilla, hyaline at the tip, coloured below, not continuous with the upper cell-wall, up to 12 mmm. long, base rounded or truncate, spore narrowed slightly above and below; not constricted at the septa; wall dark chestnut-brown, opaque, 5-7 mmm. thick, unevenly covered with coarse hyaline warts which are more numerous apically; pedicel persistent, continuous with the spore, tinted above, hyaline below, hollow, up to 150 mmm. long, 8-10 mmm. thick, slightly (15 mmm.) swollen at the base, lower third minutely and closely verruculose, central area filled with an oily orange-coloured matrix; germ-pores 3-5 in each cell, commonly 3, conspicuous.

Distribution: Europe; western Asia; North and South America; Hawaii; Ceylon; Australia.

Both hosts are introduced, the former being common throughout. The caeomata form conspicuous reddish-orange inflated areas on the stems of sweetbrier. The mycelium of this stage is perennial in the host-tissues, so that once the plant has become infected the rust appears on it season after season. Spores so produced cause local infection of the leaves, and these sori in turn give rise to uredo- and teleuto-spores.
This rust is prevalent throughout the Nelson and Marlborough districts, and in the spring becomes conspicuous on account of the brilliant colour of the caeomatospores.
Ramsbottom (1913) has shown that the name of this species should be as above.

Type: Rust and Smut Fungi; Description: Spermagonia in small groups, flat, pale honey-yellow, 0.1–0.2 mm in diameter; on upper surfaces of leaves and on twigs. Aecia on lower surfaces of leaves scattered or crowded, roughly circular, bright reddish orange, 0.25–1.0 mm in diameter; those on twigs confluent, forming large inflated distortions, bright reddish orange, up to 20 mm long; also on fruits. Aeciospores ellipsoid, 22–30 × 15–23 μm, finely and densely verruculose, pale yellow. Uredinia scattered or in groups, roundish, orange, 0.1–0.3 mm in diameter, in pale yellow spots; on the lower surfaces of leaves. Urediniospores globoid to ellipsoid, 22–28 × 15–20 μm, finely and bluntly echinulate. Telia scattered or in groups, rounded, black, 0.2–0.5 mm in diameter; on the lower surfaces of leaves. Teliospores ellipsoid to subfusoid, 6–7-septate, 72–98 × 28–35 μm, base rounded, apex with an almost hyaline, prominent papilla, chestnut brown, coarsely verrucose; pedicel persistent, longer than the spore.

Distribution: Auckland, Wanganui, Wellington, Nelson, Marlborough, Kaikoura, Mid Canterbury, Central Otago, Dunedin.; 1st Record: Kirk (1901).

Significance: Rose rust causes premature defoliation and reduces plant vigour. It is therefore of concern to commercial rose growers.; Host(s): Rosa rubiginosa, Rosa sp. cult.