British distribution: Widespread introduction.
An autoecious rust, with all stages on Oregon-grape (Mahonia) species. In Britain it is predominantly on Mahonia aquifolium, but it has also occurred on M. japonica (Wilson & Henderson, 1966). M. aquifolium itself is by far the most common Mahonia species in cultivation and as an introduction into the wild.
Aecial cups form on conspicuous swellings on the undersides of the leaves in May and June.
Uredinia and telia develop on the undersides of the leaves - especially on overwintered leaves early in the season but also on new leaves later in the year. In this author's experience there is no clear distinction between uredinia and telia, though telia tend to be darker. The sori may consist predominantly of urediniospores but with a few teliospores present, of equal amounts of each, or predominantly of teliospores with urediniospores still present. Old, infected leaves more readily develop black and red colours on the upper surface. Urediniospores are thick walled and frost-tolerant.
This rust was first recorded in Europe near Edinburgh in 1922. The following year it was found to be well established in the Tweed Valley and this may have been the original site of introduction. Within the next decade it was recorded in Wales (1926), in England (1930) and in a number of countries throughout Europe.
Infected bushes show a degree of leaf distortion and early defoliation but this rust does not seem to cause such severe infestations as sometimes seen with other introduced rusts. Since its main host here is also introduced from North America, it presumably has a greater degree of resistance than may happen when an introduced rust infects a native species.
Cumminsiella species characteristically possess teliospores with two, rounded, more or less equal cells, on a long pedicel ('stalk'). Urediniospores are pear-shaped, thick-walled and minutely spiny.
All photographs: Paisley, Renfrewshire, April/May 2000, on Mahonia aquifolium.