Campbell Island, boardwalk to Mt. Azimuth, on P. vestitum, March 14, 2000, E.H.C. McKenzie & P.R. Johnston (PDD 87903 – II); Beeman Cove, on P. vestitum, March 15, 2000, E.H.C. McKenzie (PDD 87902 – II).

Uredinia hypophyllous, inconspicuous, ellipsoidal, usually ca. 0.25 mm long, but sometimes up to 0.5 mm, often aggregated on discolored tissue, erumpent, long covered by the epidermis which eventually splits open, surrounded by a rudimentary peridium. Urediniospores (22–)26–36(–49) × (13–)16–24(–27) µm (mean of 65 spores, 32.5 × 20.2 µm), broadly ellipsoid, ellipsoid, obovoid or subglobose, contents hyaline, wall 1–1.5 µm thick, hyaline, echinulate, germ pores not seen.

Uredinia hypophylla, ellipsoidea, usque ad 0.5 mm longa, saepe aggregata, maculis decoloratis insidentibus, erumpentia. Urediniosporae (22–)26–36(–49) × (13–)16–24(–27) µm, late ellipsoideae, ellipsoideae, obovoideae vel subglobosae, interne hyalinae, membrana 1–1.5 µm crassa hyalina echinulata, poris germinationis ignotis.

Notes: Although lacking telia, this rust appears similar to Milesina whitei (Faull) Hirats. f. known on Polystichum species in Europe and Russia. The urediniospores of M. whitei are slightly smaller (Wilson and Henderson 1966; 22–40 × 17– 22 µm, av. 30 × 19 µm). The uredinia of M. whitei are circular, ca. 0.15–0.3 mm diameter, and rupture through a central pore. Milesina exigua Faull occurs widely on Polystichum spp., but it has smaller urediniospores (17.5–32.5 × 19–20 µm) and a thinner wall (0.5–1 µm) (Hiratsuka et al. 1992).
Milesia polystichi-vestiti is the third rust to be recorded on a fern in New Zealand. Milesia histiopteridis (G. Cunn.) Faull (originally described as Milesina histiopteridis G. Cunn.) was described from New Zealand on Histiopteris incisa (Thunb.) J. Sm. (Cunningham 1924), and Hyalopsora polypodii (Dietel) Magnus was recorded on Deparia petersenii ssp. congrua (Brack.) M. Kato (McKenzie 1990). In all cases, only uredinia are known from New Zealand. The urediniospores of Milesia polystichi-vestiti are larger than those of either M. histiopteridis (18–26 × 14–20 µm) or H. polypodii [(20–)23–30(–32) × (11–)14–15(–16 µm)].
Polystichum vestitum is endemic and widespread throughout the New Zealand botanical region. It is morphologically highly variable, but molecular data does not support classification at the varietal level (Perrie et al. 2003).

Campbell Island, Camp Cove, on Polystichum vestitum (G. Forst.) C. Presl [Dryopteridaceae], March 11, 2000, P.R. Johnston (PDD 87904 – II – HOLOTYPE)