Introduction

Ludwig’s Roses on: Rust is a disease caused by the parasitic fungus Phragmidium tuberculatumand and some other closely related species. It is specific to roses, and appears in spring and persists until the leaves fall.

Susceptibility to rust varies widely among rose cultivars, and most modern roses are resistant to rust. Rose rust is the least serious of the common rose diseases; black spot and powdery mildew are far more prevalent.

Description 

Rust appears as its name implies, as red-orange spots (raised looking like warts) on undersides of leaves and yellow blotches on top surfaces. Infected leaves may fall early.

Generally prevalent during cool moist weather.

Damage

Rust causes the young foliage to curl and distort  – older leaves are not deformed but display the same coloured spots on the top and underside of the leaf  lowering photosynthetic efficiency that results in reduced plant growth and vigour.

If left untreated the foliage will eventually fall off which will set a rose plant back considerably if infestation takes place in spring.

Plants can be severely stunted if they are heavily infected early in the growing season. Rose tissue becomes more resistant to infection as it ages.

Environment

Rust thrives in cool, moist weather (18 to 21 degrees Celsius), especially in rainy, foggy or misty conditions. This disease will develop on leaf surfaces that remain wet for 4 hours (as can occur during summer fogs, heavy dews or extended rains).

Life Cycle, Survival and Dispersal 

The fungus causing rose rust is, like all rusts, a biotroph:  it infects the host tissues for extended periods without killing them, feeding on the living cells. Like all rusts, it is not able to survive on dead plant material, so must either alternate with a different, perennial host, or produce a resting spore to pass the dormant season.

Phragmidium tuberculatum and several other very similar species which infect roses do not have an alternate host; that is, they only attack roses and pass the winter as resting spores.

The first formed spores (spring spores) infect young stems, causing distortion and the production of bright orange pustules. These in turn infect the leaves to produce dusty orange spores (summer spores) which are spread by wind and initiate further infections. In late summer, the pustules producing summer spores switch over to produce the dark, tough resting spores. These spores survive the winter often adhering to stems or trellises. And then the infection starts over again in spring.

Reproduction of rust spores occurs every 10 to 14 days throughout summer.

Infections may be severe enough to cause serious damage, but this is relatively rare and most infections are light enough not to require control.

Environment

Rust thrives in cool, moist weather (18 to 21 degrees Celsius,), especially in rainy, foggy or misty conditions. This disease will develop on leaf surfaces that remain wet for 4 hours (as can occur during summer fogs, heavy dews or extended rains). Reproduction of rust spores occurs every 10 to 14 days throughout summer

Treatment and Prevention

NATURAL PREVENTION 

Good soil drainage is essential for moisture control; adding organic matter, double digging beds or planting raised beds are effective means in providing a good healthy environment for roses. Also it is best to avoid working in a wet rose garden so as not to help spread rust spores (this is also sound advice regarding minimizing the spread of blackspot and mildew). In humid regions, try to limit wetting the foliage on rose plants while watering and provide good air circulation between plants by spacing them well apart from one another. Prune to keep the centers of rose bushes open for air circulation as this will assist in keeping them drier.

FUNGICIDES

Dithane                     mancozeb

Rose Protector/Rosecare         Propiconazole

Resources

http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=269

http://www.rosemagazine.com/articles02/pages/rust.asp