Answer: Hollyhock rust is caused by the fungus Puccinia malvacearum. The surface of the leaves may develop numerous yellow spots. However the most obvious symptoms are the orange?brown pustules on the undersides of the leaves which are characteristic signs of a rust infection. These pustules may also form on the upper side of the leaves, on stems, and on green flower parts. Hollyhock rust tends to become more severe as the summer progresses, killing most of the leaves on infected plants by early fall.
Large numbers of tiny rust?colored teliospores develop in the rust pustules. These spores are carried by splashing rain and air currents to nearby healthy plant parts and cause new infections. The fungus overwinters in infected plant debris. In the spring new infectious spores are formed on infected plant debris that cause infection on the newly emerging leaves.
To try to break the disease cycle, it is important to cut all hollyhock stalks back to ground level in the fall, and carefully collect all leaves and other aboveground plant parts and destroy them. This autumn cleanup is vital to remove as much inoculum as possible before spring, and it must be done thoroughly. Avoid crowding plants and water early in the day so the above ground plant parts will dry quickly.
You can try using a preventative fungicide spray in the early spring to protect newly emerging leaves. To control infection, pick off and destroy any leaves or other plant parts as soon as signs of rust infection are noticed during the growing season.
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