ISSUE 5 June 1, 2006
A couple of weeks ago, the Plant Diagnostic Lab received a sample of rose that was infected with a disease known as rust. On roses, rust diseases are caused by species of fungi in the genus Phragmidium. Rose rusts are autoecious, meaning the fungi can complete their entire life cycle on one host (other rusts, such as cedar-apple rust, or wheat stem rust, are heteroecious, meaning two hosts are needed to complete the life cycle of the fungi).
There are several species of this fungus that attack roses, with subtle differences in their life cycles. The orange pustules contain spores (urediospores) that are able to reinfect the rose plant. Later in the summer, the orange spores will give way to a black spore stage that will allow the fungus to overwinter. Depending on the species of rust, the black spore stage can appear on the leaves or on the stems or on both. On the sample submitted to the lab, the black spore stage occured on the canes, and they looked like small, black colored galls or scabs. The second rose in the sample, a different variety from the first, had a different spore stage from those listed above, and it was determined to be a different species. The spore stage will eventually give rise to the orange spore stage if left unchecked.
To manage this plant disease, affected leaves and other plant parts should be pruned out and destroyed. Leaves that fall from the plant should be raked up and also destroyed. For ‘organic’ management of the disease, applications of lime sulfur or copper-based products may help protect against additional infections. For ‘non-organic’ management, fungicides such as triforine (Orthonex Insect and Disease Control - formerly known as Funginex) or chlorothalonil (Dexol) applied every 7-10 days during the growing season when conditions are favorable for infection, such as during moderate temperatures when leaf surface wetness continues for prolonged periods, may prevent additional infections. Regardless of treatment (whether organic or not), be sure to read the label instructions. And remember - more is not necessarily better. Applying too much or too frequently will likely be a waste of money and could possibly injure the plant.
NDSU Plant Diagnostic Lab
206 Waldron Hall, PO Box 5012
Fargo, North Dakota 58105