What is pelargonium rust?
Pelargonium rust is a disease that appears on the leaves of zonal pelargoniums, and is caused by the fungus puccinia pelargonii-zonalis.
This particular strain is specific to zonal pelargoniums, i.e. other types of pelargonium are not affected.
Small yellow spots appear on the upper surface of the leaves with corresponding off white blisters on the underside of the leaf.
These blisters develop into masses of brown powdery rust spores.
In severe attacks, the brown spores may also appear on the upper leaf surface causing the leaves to turn yellow and drop off.
As with many fungal diseases, prevention is often the easiest and best option!
Firstly if possible* ensure you purchase disease free plants!
*This disease needs an incubation period of around two weeks so the disease may not be apparent at time of purchase!
As an insurance it may be advantagious to keep newly purchased plants in a seperate area for a few weeks to see if they are disease free!
In the event that the disease is contracted, keep the area around the plant/s open by not crowding plants together, thus increasing the airflow around the plants and reducing humidity.
Do not water plants overhead, try to keep the leaves quite dry!
There are systemic fungicides containing diefenoconazole or triticonazole on the market that will control the disease. (see here)