(March 2002)

 M.Mirza, Crop Diversification Center North, Edmonton

What a grower described as brown spots on the leaves of geraniums which may have been attributed to a lack of watering turned out to be a fungus related diseases called RUST.  Rust on geraniums is caused by a fungus called Puccinia pelargonii-zonalis.  It was reported in 1998 in Alberta.  Here is some relevant information on this rust.  Pictures are also posted so that you can see how it looks like.

What is this rust?

This is a fungus which grows specifically on geraniums. From the disease management viewpoint it should be remembered that this rust is autoecious, meaning that it completes its life cycle on one host. The fungus produces spores continuously which can re-infect the same plant or spread to other plants. The spores need free water for germination. The optimum temperature for spore germination is between 16C to 21C. The spores of the fungus germinate and penetrate the leaves through the stomatoe on upper and lower leaf surfaces. Eventually the rust establishes itself on the leaves.

How to recognize the rust infections?

The symptoms can be confused with odema on geraniums. If you look closely the symptoms are different and distinct when compared to oedema. On the upper surface of leaves you see light yellow, circular or irregular spots with a brown dot in the centre. On the underside of leaves one can see distinct brown/tan lesions which appear to be raised. Under a 10 x magnification one can see these raised spots and if shaken rust colored spores might be seen flying in the air. The infections I saw on one cultivar did not appear to have a pattern of concentric rings. There could be scattered spots between the veins and they could be on the leaf edges. Severely infected leaves become yellow and drop prematurely. Infected plants are not marketable. Remember the cost of one geranium plant at sales time is over 5 dollars.

How is the disease spread?

The fungus spores spread by splashing water. It means if you hand water your plants then the chance of disease spread is greatest. Air currents, workers handling the plants and moving the plants will also spread the spores. The spores can re-infect new leaf surfaces of the same plant or infect other plants. The disease can spread very rapidly especially under optimum temperature and relative humidity conditions.

What are possible control measures?

• Notify the supplier and insist on a crop inspection.
• The fungus only lives on geraniums so get rid of infected plant material. Dispose of the plants by putting them in a plastic bag and don't drag them for a long distance.
• If one cultivar is showing infection, then isolate those plants even some plants are not showing the symptoms.
• Avoid hand watering so that spores don't splash to other plants.
• Many fungicides are available to control the spread of the disease. It will be advisable to spray the entire geranium crop.
• Follow good sanitation practices for next year crop.