Rust
Also known as Rust fungi, Pucciniales
In short
Rusts are fungi that form small, powdery pustules of spores, usually on the undersides of leaves. They can only live on living plants and each rust species attacks a narrow range of hosts; some need two unrelated host plants to complete their life cycle. Infection is favoured by long periods of leaf wetness.
How it looks
Raised, powdery pustules, most commonly on the lower leaf surface, in orange, yellow, brown, black or white depending on the species; a leaf may carry dozens. The upper surface above the pustules often shows pale yellow, purple or brown spots. Rubbing a pustule leaves coloured dust on your finger.
Signs
Spotted, yellowing leaves that fall prematurely, reduced vigour and flowering, and distorted shoots on some hosts.
Life cycle
Rust fungi can produce up to five spore types. Repeating summer spores (urediniospores) spread by wind, water or insects and can build up in cycles of about 10–14 days; overwintering spores carry the fungus between seasons. Some rusts complete their whole cycle on one host, while others alternate between two unrelated hosts, such as European pear rust on pear and juniper.
What favours it
Prolonged leaf wetness, so wet summers and humid, still conditions; mid to late summer and autumn in temperate climates. Soft, lush growth from excess nitrogen is more vulnerable.
Prevention
- Keep leaves dry: water the mix, not the foliage, and give plants space.
- Avoid excess nitrogen fertiliser.
- Choose resistant cultivars where available.
- Inspect leaf undersides of new plants before bringing them home.
Treatment
Start with the first step and move down only if the problem persists.
- Pick off affected leaves early, while infection is light.
- Dispose of heavily infected material rather than leaving it under plants.
- Where the alternate host is known and nearby, removing it can break the cycle.
- Protectant and systemic fungicide groups are used against rusts, though some advisers such as the RHS recommend avoiding them. If used, follow the product label.
Pros and cons
Cons
- Repeated spore cycles can build up quickly in wet weather.
- Premature leaf fall weakens plants over a season.
Look-alikes
- Fungal leaf spot
- Leaf spots are flat dead patches; rust forms raised powdery pustules that leave coloured dust when rubbed.
- Scale insects
- Scale insects are waxy or shell-like bumps that can be scraped off whole and leave no powder.
- Spider mites
- Spider mite damage is fine pale speckling with webbing and tiny moving mites, not pustules.
Affected plants
Roses, fuchsias, pelargoniums, antirrhinums, hollyhocks, heucheras, chrysanthemums, blackberries, cereals, lawns and trees such as poplar, willow, birch, plum and pear.
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.