Tree rusts are fungal diseases causing dusty orange or brown spots (pustules) on the leaves of poplar, willow, birch, plum and five-needled pine.
What are tree rusts?
Tree rusts cause eye-catching infections on the leaves of some trees, particularly Populus spp. (poplar), Salix spp. (willow), Betula spp. (birch), plum and Pinus spp. (five-needled pines). Tree rusts may be seen from spring until autumn for trees that lose their leaves in winter (deciduous) and all year on evergreens.
These are the rust fungi involved:
- Poplar rust is caused by several species of Melampsora
- Willow rust is also caused by several species of Melampsora, but not the same as those infecting poplars
- Birch rust is caused by Melampsoridium betulinum
- Plum rust is caused by Tranzschelia pruni-spinosae var. discolor
- Five-needled pines are infected by white pine blister rust, caused by Cronartium ribicola
You may see the following symptoms:
On poplar, willow, birch and plum
- Poplar, willow and birch: Dusty orange spots (pustules) on the undersides of the leaves
- Plum: Dark brown pustules on the undersides of the leaves
- For all: In severe attacks these can be very close together with many pustules on each leaf. Sometimes the orange pustules are intermingled with black ones. Affected leaves fall prematurely
On five-needled pines
- Small yellow swellings appear in spring at the base of the needles. In following years these develop as white blisters which liberate dusty orange spores
- Cankers develop on the stems and if they girdle the stem, it dies
- There is normally no need to control these rusts in gardens, because they do little damage
- Removal of alternate hosts may be useful, but only if these are not also found in neighbouring gardens, because the spores are airborne
- Destroying fallen leaves in autumn, by burning or composting, may reduce the amount of resting spores available to reinfect next year. This only works if neighbouring gardens do the same, or if the relevant tree is not growing nearby
This is not needed in mature trees and in any case most gardeners do not have the right equipment to spray large trees.
If young trees suffer heavy attack while establishing, control may be justified. Fungicides containing difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are available for the control of rust diseases on ornamental plants.
Some formulations also contain insecticides but these are best avoided if no insect pest problem is specifically identified. For example, some formulations of myclobutanil (Westland Rose Rescue) contain cypermethrin, some formulations of tebuconazole (Bayer Garden Multirose Concentrate 2) contain deltamethrin, and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
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.