05:39 PM PDT on Friday, April 27, 2007
Q: I just noticed that some of my snapdragons have bright orange bumps on the underside of the leaves. Should I be concerned?
A: What you have spotted is a fungal disease that affects snapdragons and many other plants such as roses and hollyhocks. It is characterized by yellow spots on the upper sides of the leaves and orange pustules (spores) on the undersides of the leaves. The spores are carried by the wind and may infect susceptible leaves on which they land.
Moisture and moderate temperatures typical of spring weather are necessary for spore germination.
In the fall and winter, gather and destroy all infected leaves, both those on the ground and those still on the plants. While the weather remains favorable to the disease, you can spray your plants every seven to 14 days with a fungicide specifically labeled for rust. As always, the manufacturer’s directions must be followed carefully to ensure effective protection.
A final tactic for the future is to select one of the newer cultivars that have been bred specifically to resist rust infections. Although their degree of resistance may not be 100 percent, their performance will be a significant improvement over older cultivars.