Tree rusts are fungal diseases causing dusty orange, brown or black spots (pustules) on the leaves of poplar, willow, birch and plum, and cankers on the stems and branches of five-needled pine.
Scientific name Various
Plants affected Mainly Populus spp. (poplar), Salix spp. (willow), Betula spp. (birch), plum and Pinus spp. (five-needled pine)
Main symptoms Orange or brown pustules
Caused by Fungi
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) and plum, and cankers on the stems and branches of 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 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: There are usually small yellow or brown spots on the upper leaf surface, corresponding to the pustules on the lower surface. In severe attacks there can be 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
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
- There is normally no need to control these rusts in gardens, because they do little damage
- Removal of alternate hosts (see 'biology' section below) 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 also growing nearby
- Remove pine branches with rust cankers, particularly if dieback occurs
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. The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of rust diseases on ornamental plants.
The following products contain a combination of both insecticide and fungicide, enabling the control of both damaging invertebrates and disease: triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if invertebrate damage is not a problem on the plants treated.
Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Rust fungi are biotrophs: they feed from the living cells of the host plant over an extended period without killing them. They are not able to feed from dead plant material, so rusts affecting deciduous trees must either alternate with a different host plant or produce resting spores in order 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 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 (Cronartium ribicola) 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.
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