How to recognise it
Initially appears as a vague yellowing of the leaf and stem. This soon turns into the more familiar elongated, raised brown pustules on the underside of leaves from which brown dust (the fungal spores) are emitted when rubbed.
Why it’s a problem
As well as being unsightly, it can lead to defoliation and a reduction in flower numbers.
Where you are likely to find it
On carnations (although the related species of pinks and sweet williams are rarely affected – Puccinia arenarie being more common on sweet williams). It is spread by wind currents (infecting leaves through the stomata in damp conditions) and can overwinter in the soil. It is generally associated with glasshouse grown carnations, but can occasionally be found on outdoor plants.
Rust diseases can often be introduced by weeds hosting the fungus, particularly groundsel and dock.
How to prevent it
Use more resistant carnation cultivars – most modern cultivars have a level of resistance.
Nevertheless, good hygiene should be adhered to, including:
- Ensure that cuttings taken are from uninfected plants.
- Sterilize border soil to destroy any overwintering spores.
- Ensure that glasshouses are well ventilated to prevent damp/humid patches occurring which can encourage infection. Space plants out to encourage good air flow.
- Don’t wet the leaves when watering.
The use of the fungicide mancozeb can provide protection from infection and, used fortnightly from the first signs, can prevent spread to some extent.
How to get rid of it
Remove and destroy infected leaves (do not put on domestic compost heaps). Ensure that hands and implements are cleaned before using on uninfected plants.
Use of a high potash fertiliser can help plants recover from an attack.
A myclobutanil based fungicide can be effective on infestations. Alternatives to myclobutanil are penconasole, moncozeb (also used for prevention, see above) or triticonazole.
Is it good for anything?!
Other useful information
Rust fungi have a very complex life cycle, in which they have five possible spore-forms within the same species. Where more than one spore form occurs on the same host this can lead to very high infection rates. Therefore in some North American forested regions they do now allow the planting of blackcurrants near five-needle pines, since they are both susceptible to the rust Cronartium ribicola and combining different spore types can lead to severe infestations.
Uromyces dianthi was introduced into England around 1890 on plants imported from Europe.