HOLLYHOCK RUST Puccinia malvacearum   F A C T S H E E T

Introduction

This rust disease can result in serious injury to hollyhocks (AIthaea spp.) and is found nearly everywhere they are grown. The rust also occurs on mallow (Malvs rotundifolia), a common weed, which can serve as a disease reservoir and contribute to infection of hollyhocks.

Hollyhock rust is caused by the fungus Puccinia malvacearum. It is classified as an autoecious rust because it has only one known infective stage (III, teliospore) and is not known to have an alternate host.

Symptoms

The surface of the leaves may develop numerous yellow spots (Fig. 1). However the most obvious symptoms are the orange­brown pustules on the undersides of the leaves (Fig. 2) 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.

Figure 1:Small yellow spots are obvious on the leaf surface. (provided by Dawn Dailey O'Brien, Cornell University)

Figure 2: Orange-brown pustules on the under surface of the leaf. (provided by Dawn Dailey O'Brien, Cornell University)

Disease Cycle

Large numbers of tiny rust­colored 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.

Control Strategies
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. If found in the vicinity, the weed mallow should be removed and destroyed.

For maximum protection begin fungicide treatments in early spring when the first leaves are expanding. Homeowners in New York State may apply fungicides containing chlorothalonil, sulfur, neem oil, or myclobutanil, if needed, according to label directions. Be certain any formulation(s) of pesticide(s) you purchase are registered for the intended use. Sulfur may damage leaves if air temperature exceeds 30°C (85°F) within 24 hours of spray application. When plants are dry, pick off and destroy any leaves or other plant parts as soon as signs of rust infection are noticed during the growing season.

Created, KLS, 05/02; Updated, SLJ, 3/09