Cooler weather, dew on leaves longer in the morning, and the first real rains of the season, can bring with it a bane of the rose garden – rust.
This fungal disease with orange, powdery pustules on the undersides of leaves makes rust easy to identify. They start out yellow to light orange and deepen to reddish-brown. Over time, the upper sides of the leaves may discolor and drop. If leaf drop is severe, it can weaken the plant, reducing flower production and overall vigor.
Rust needs free water to infect plants. Their spores are spread to newly expanding leaves and stems by air currents and splashing water from rain, heavy dew, fog, or irrigation. For spores to germinate, they need cool temperatures of 65 – 70°F, and only two to three hours in free water.
Preventing infection is your best bet. Start with disease-resistant rose varieties. If you currently have a rose that seems to be a magnet for this disease, dig it up and toss it! Replace it with a variety that is known for overall good health and resistance.
Healthy plants can generally withstand a bout of rust, so keep your roses vigorous. Plant them in a location with good drainage, that gets at least six hours of sun each day, and has plenty of air movement through the bush. With water as the main conduit for spreading the disease, take care when doing any overhead watering; do it early in the day so plant surfaces have time to dry before temperatures cool in the evening. If possible, switch to alternate methods of irrigation that don’t wet leaf surfaces.
Good garden sanitation is vital for disease management. Remove any infected leaves, stems and canes from plants and the leaf litter on the soil around them. Prune out infected, unhealthy, or dying stems and canes whenever you see them. These infectious agents can survive through winter on dead leaves and debris and attack the new leaves as they emerge in spring. And don’t add the infested material to your compost pile unless you know it gets hot enough to kill these nasty pathogens.
By Nanette Londeree, Master Rosarian