Rust fungi are biotrophs: they feed on the living cells of the host plant over an extended period without killing them. They are not able to survive on dead plant material, so must either alternate with a different, perennial host, or produce resting spores to pass the dormant season.
Poplar rust, caused by several species of Melampsora, appears in autumn and although spectacular, causes little damage in gardens. The alternate hosts are Larix (larch), Mercurialis perennis (dog’s mercury) and Allium spp. (wild onions).
Willow rust is also caused by several species of Melampsora, not the same as those infecting poplars. The alternate hosts include Euonymus, Larix (larch), Ribes, Saxifraga and some orchids. There is one Melampsora species that carries out its whole life cycle on Salix (willow).
Birch rust (Melampsoridium betulinum) alternates its life cycle with Larix (larch), but since both are deciduous it is also thought to overwinter in buds, as well as producing resting spores.
Plum rust (Tranzschelia pruni-spinosae var. discolor) alternates with Anemone coronaria and A. x fulgens. Like poplar rust, it develops late in the season and does little damage. Fruit are not affected.
White pine blister rust is a very serious disease of commercial five-needled pines in Europe and the USA. Pinus flexilis, P. lambertiana, P. monticola and P. strobus are susceptible. This rust is a damaging pathogen because it forms girdling cankers on the stems. The alternate hosts are currants, particularly blackcurrants, and gooseberries, which are not seriously damaged.
Some other trees are affected by rusts, but seldom to the same level as those discussed here.