How to battle rust in hollyhocks

by Helen M. Scott
Tuesday, July 31, 2007 2:04 PM MDT

No doubt about it, hollyhocks (or Althaea Rosea) belong on the list of “old favorites” for cottage gardens along with iris, roses, Baby’s Breath, peonies, Lily of the Valley and nasturtiums -- just to name a few.

Hollyhock plants are interesting plants in that they are extremely enduring through drought, heat, bad soil and passage of time.

The plants we have today were depicted in Chinese art in the ninth century, supposedly making them Asian in origin. Information reveals that the flowers were used medicinally for soothing the skin, for diuretic properties and were useful for some chest complaints. The plant was once eaten as an herb, though not particularly palatable.

It is hard to believe that small three-inch pots of hollyhock plants in early spring will reach 5-6 feet in one season, but they will. They may not spurt up the first year; instead they will make lots of leaves, 10 inches across or so, near ground level.

They will disappear in the winter, but the next spring they will reappear and grow tall flower spikes that open from the bottom, alternating upward, one on this side and then one on the other side.

Hollyhock plants don’t have many disease problems, but the one disease that plagues them is rust. The rust disease can result in serious injury to hollyhocks and is found nearly everywhere they are grown. The surface of the leaves may develop numerous yellow spots.

However, the more obvious symptoms are the orange-brown pustules on the underside of the lowest leaves as lemon-yellow to orange, almost waxy, pustules that turn reddish brown with age. Pustules are small, blister-like formations.

These pustules also may form on the upper side of the leaves, on stems and on green flower parts and spread quickly to stems. The rust tends to become more severe as the summer progresses killing most of the leaves by early fall.

As the disease progresses, large numbers of tiny rust-colored spores develop in the rust pustules. These spores are carried by splashing rain and air to nearby healthy plants and cause new infections.

The fungus over winters in nearby infected debris. The following spring, new infectious spores are formed on infected plant debris that cause infection on newly emerging leaves.

It is important to break the disease cycle, so pick and destroy infected leaves at the first sign of rust. Cut down and destroy infected stocks when plants are finished blooming. In the autumn, removing ground debris helps control the fungus.

Sulfur, applied as a dust or spray, is effective and must be aimed at the underside of bottom leaves. Using sulfur, providing good air circulation and avoiding overhead watering, especially in the evening hours, will reduce the severity of the rust. Some plant varieties are rust resistant, so look for this when buying new plants.

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