Rose rust is a disease caused by the parasitic fungus Phragmidium tuberculatum and some other closely related species which specifically infect roses. The disease is often introduced into a field on infected planting materials and causes significant losses if left untreated. Infection with Rose rust can weaken and distort the plant growth or even kill the rose bushes.
The fungus causing rose rust is a biotroph, i.e., it infects the host tissues for extended periods without killing it while feeding on the living cells. It cannot survive on dead plant material, so must either alternate with a different, perennial host, or produce a resting spore to pass the dormant season.
The first formed spores infect young stems, causing distortion and the production of bright orange pustules. These in turn infect the leaves to produce dusty orange spores which are spread by wind and initiate further infections.
In adverse conditions, the pustules produce dark, tough resting spores which survive the winter often adhering to stems or trellises. Rose rust is able to survive winter and complete its life cycle on one host. The pathogen thrives in cool, moist weather, especially in rainy, foggy or misty conditions. The disease develops on leaf surfaces that remain wet for 4 hours (as can occur during fogs, heavy dews or extended rains).
Infection often starts on lower leaves, but eventually an entire plant can be affected, with the typical symptoms being general yellowing of leaves followed by eventual leaf death. Affected rose stems/canes can become curled and distorted.
The disease causes the young foliage to curl and distort while the older leaves are not deformed but display the same coloured spots on the top and underside of the leaf lowering photosynthetic efficiency which results in reduced plant growth and vigor.
As the disease progresses, powdery orange or black, circular spots called pustules, which contain spores of the fungus that causes the disease form on the undersides of leaves, with corresponding yellow spots visible on upper leaf surfaces. Pustules may also form on stems and green flower parts (sepals).
If left untreated the foliage eventually falls off which sets a rose plant back. Infected plants can be severely stunted if they are heavily infected early in the growing season.
For an effective control fungicides with preventive and curative properties are used. These include the following;
- DUCASSE 250EC 20ml/20L,
- SPARROW 500EC 20ml/20L
- DOMAIN 250EC 10ml/20L
- JUPITER 125SC 15ml/20L
- MILESTONE 250SC 10ml/20L
- MONUMENT 400EC 50ml/20l
- ABSOLUTE STAR 400 SC 10ml/20L
- Plant rose varieties that are less susceptible to rose rust whenever possible.
- Prune out affected canes and remove leaves as symptoms develop to prevent the spread of rust fungi to other rose shrubs.
- Use disease-free planting materials.
- Proper planting and pruning promote good air circulation which facilitate rapid drying of leaves and canes, thus making the environment less favorable for rust development.
- Fertilize and water roses appropriately because nourished plants are less susceptible to disease.
- Avoid overhead or sprinkler irrigation as these spread rust spores and wets leaves and canes, thus providing a more favorable environment for rust infections to occur.
- Destroy the infected materials by burning or burying them.
- Remove and destroy any remaining dead leaves and other rose debris to eliminate places where rose rust fungi can overwinter.
- Practise rotations with non-host plants.
- Whenever spraying, mix the fungicide with INTEGRA 3ml/20L, which improves the efficacy of the chemical by acting as a sticker, spreader and penetrant.
- Repeat fungicide treatments every seven to 10 days, and DO NOT use the same active ingredient for all treatments, in order to help minimize potential fungicide-resistant strains of rose rust fungi.
- Read and follow all label instructions of the fungicides that you select to ensure that you use these products in the safest and most effective manner possible.
- Ensure a proper crop nutrition.