Features

• The Phragmidium fungus causes rust in roses. The pathogen spends the winter in diseased plant tissue. It produces spores during the spring, and the wind blows the spores onto healthy plants to create new infections. The pathogen enters plants through the stomata, or tiny pores on the surface of leaves, when the leaves remain wet for two to four hours. The disease is most active when the temperature is mild and humidity levels are high, according to the Oregon State University Extension.
  
  

Effects

• Rose rust infections are characterized by red or yellow circular spots on the lower leaves of the plant that contain masses of fungal spores. The infected leaves turn yellow, become distorted and fall from the plant, causing defoliation. The disease attacks the leaves, stems and twigs, but does not affect the roots or petals.
  
  

Prevention/Solution

• The Clemson University Cooperative Extension recommends watering plants early so that they will have time to dry during the day. Because the fungus overwinters in leaf litter, raking and removing leaves limits the number of spores that can infect plants. Planting disease-resistant cultivars and spacing plants so that air can circulate helps to prevent infections. Preventative fungicides also keep healthy roses from becoming infected.