White Rust makes its customary appearance in the U.K. over the summer months so it's wise to be on the lookout for this disease. This short (and by no means definitive) article may help growers to recognise and deal with this problem.

So, what is white Rust?
White Rust is a fungal disease caused by Puccinia horiana,  The fungus produces two types of spores - teliospores and basidiospores. Teliospores are produced in pustules and remain in the pustules unless they are aggressively brushed off. 

Under moist conditions (96% to 100% relative humidity) for at least 3 hours, teliospores produce basidiospores. Basidiospores can cause an epidemic if conditions are right. 

They spread from plant to plant by splashing water and they must have a film of water on the plant surface for infection. Infection can occur in as little as 2 hours at the optimal temperature of 17C.

Basidiospores can also travel short distances by wind currents during moist weather. Infections occur when infected cuttings or viable spores are brought into a greenhouse, thereby exposing any uninfected cuttings. Infected cuttings may appear normal even though the fungus is present, waiting to manifest itself when conditions become more favourable.

Chrysanthemum white rust originated in eastern Asia and is now established in Europe, Africa, Australia, Central America and South America. It is a notifiable disease in the USA and Canada.

What does it look like?
Chrysanthemum white rust can be recognized by the small white to yellow spots, up to 4 mm wide, on the upper surface of the leaf. These slightly dimpled spots become brown over time. Pustules form on the underside of the leaf, beneath the small spots. These are buff to pink-colored but become white as they age. Pustules are most common on young leaves but can be found on any green tissue and flowers. Infected plants do not always display symptoms during hot and dry conditions. 

In the early stages it is not so easy to recognise as there are only small pale, slightly raised, spots on the top of the leaves, but these develop quickly and pinkish/ buff pustules, resembling blisters, soon appear on the undersides of the leaves which are easily identified.  These blisters contain thousands of fungal spores, which will soon be dispersed to infect adjacent plants, the complete cycle can be as short as 7 - 10 days. 

How do I deal with it? 
The most likely cause of infection is the arrival of new plants, so it makes sense to keep a watchful eye on all new imports, and if possible quarantine them for several weeks. 

One factor that works for us is that white rust needs the right conditions to spread, high humidity above 96% and temperatures of about 17 °C - these optimum conditions can occur over the summer months (and earlier and later under glasshouse conditions).

Upper surface of leaf

Underside of infected leaf

Prevention not cure
Inspect any newly acquired plants and remove any infected leaves and spray all of the plants regularly for three or four weeks with a chemical spray containing the active ingredient myclobutanil, this is one of the most  effective products available to the amateur grower. 

Maintain good hygiene
Continuing the 'prevention not cure' theme, try to keep a clean site, use a good general hygiene programme, by quickly disposing of old leaves and removed shoots, spray plants regularly, say every two weeks with a suitable chemical. 

In the event that only a small number of plants appear to be affected it may be worth considering removing these plants from the plot as an alternative to a chemical remedy.

End of season treatment
It is possible that while plants are not showing signs of infection they may still be contaminated, and will need treatment at the end of the season if they are to be used for propagation. Continued chemical spraying is one form of ongoing protection.

And Finally ......
Unfortunately white rust is now widespread within the U.K. but fortunately it is now a little easier to deal with than it was a few years ago. If you are observant, identify the problem early and act quickly then it's possible that it can be contained. It only effects chrysanthemums, so it is limited in how easy it can spread. 

Controls

underside of leaf - heavily infested