Looking into the rusts

Photo by Jill Thomson Wild rose with extensive rust infection of the stem.

by Jill Thomson

The fungi that cause rust diseases are an interesting group. Although they disfigure their host plants, they rarely destroy them, finding a way to feed off the hosts without killing them, while producing different spore stages, sometimes as many as five, sometimes on two different hosts, in order to perpetuate their life cycle.

Rusts have historical significance, going back to at least Roman times. There are records of a festival, called the Robigalia, where sacrifices of red animals (such as foxes, dogs, cows) were made to the rust god, Robigus, to protect the wheat crops from destruction.

More recently cereal rusts have caused epidemics that destroyed the wheat crops planted by European colonists in North America. Stem rust of wheat was found to have 5 spore stages and the barberry plant was required as a host to complete the life cycle. In 1918 the United States created a program that required the removal of barberry in wheat growing areas.

Eventually a barberry testing program was developed to ensure that susceptible barberry species were not grown in areas where wheat stem rust was a problem. I now grow one of those barberries in my yard. Data also showed that spring infection occurred in Canada because spores were being blown north from infected wheat fields in the USA, so barberry is not so important for providing spores in the spring.

In Saskatchewan, we frequently see infection of roses, saskatoons, hollyhocks, and sunflowers. All are caused by different rust species, with varying complexity in their life cycles.

Rose rust can be caused by several species belonging to the genus Phragmidium, and only roses are infected. The disease begins in early summer, when you will see round, blackish-brown spots with orange, tan, or yellow centers on the leaves. Infected leaves turn yellow, and on the underside of the leaf you will see clusters of tiny, orange spores. These spores may also be formed on the stems, as you can see on the infected wild roses I photographed at a Rosewood park in mid-June this year.

Infection often starts on the lower part of the plant probably because rust likes the humid conditions found in the lower foliage. Usually, foliage on the bottom of the plant succumbs first, and then the disease moves its way up the plant. In the fall, the leaves or stems will develop black spores.

Rust spores travel in air currents and water splash. The spores cause infection when temperatures are around 20°C and moisture must be present on the leaves for at least 2 hours. This information helps you control the disease: avoid crowding your plants so they dry out quickly after rain, water below the foliage so moisture does not stay on the leaves.

Photo by Jill Thomson
Wild rose with rust infections on leaf and stem.

Remove and destroy any lesions you see early in the season, and if black cankers develop on the stems in the fall prune them out and destroy them, as they contain the spore stage that will overwinter.  Rugosa roses are very resistant to rust and hybrids are more susceptible, so check which to plant if you are having rust problems. There are also fungicides that can be used if the disease is problematic.

Sunflower rust is also very host specific. The rust pustules only occur on sunflowers, on the underside of lower leaves first. They are cinnamon-brown and dusty, as they contain the spores that spread disease.

Weather and microclimate play a large role in determining disease development. Longer dew periods and dense canopies both encourage infection and development of disease. So avoiding over-crowding and long periods of leaf wetness will minimize disease development. Also removal and destruction of infected leaves helps prevent spread.

Place infected leaves in bags in the garbage, and if infection is severe remove stems and petioles that also may be infected. Black pustules containing overwintering spores will be found at the end of the season , these should be bagged and destroyed.

Hollyhock rust is caused by the fungus Puccinia malvacearum and it can infect all members of the Alcea family, which includes Mallow and Lavatera. It begins as yellow spots on the leaves, with rusty pustules containing spores on the undersurface.

The spots may grow together and cause large sections of leaf to die and ultimately the leaf will fall off. Stems may also develop spots and the disease can spread to other hollyhocks, plus any mallow weeds nearby. In fact these weeds can act as a reservoir of disease and they should be destroyed.

The conditions conducive for disease are hot and humid, so make sure there is good air circulation around the plants and do not let leaves remain wet in the evening.  Spread mulch under the plants to prevent last year’s infection from producing spores. Cut down the hollyhocks at the end of the season, and bag and remove the debris. There are fungicides that can be used if necessary. 

The 3 rusts just discussed are relatively simple diseases, affecting single host types and producing 1 or 2 spore types. The rust you find on saskatoon plants is much more complex. Saskatoon-Juniper rust alternates its growth on saskatoon plants and juniper plants, and it produces 5 spore stages to complete its life cycle. It can be extremely destructive on the saskatoon plants, destroying leaf tissue and causing malformed berries that are unfit for consumption.

Infection of saskatoon bushes occurs in the spring, when tiny, windborne spores are produced on juniper shrubs. The shrubs are infected with the fungus several years previously, and they cause the plant to grow hard, woody galls.

In damp weather at the end of May, long orange, jelly-like masses (called telial horns) grow quickly out of the galls. The tiny spores present in these jelly masses are splashed around when it rains and the spores are then blown by the wind to saskatoon bushes that are just leafing out.

The spores can travel several km, so it can be very difficult to isolate the two hosts, particularly in the wild. When the spores land on saskatoon plants they infect the new leaves, forming small orange or yellow “rust” spots on the underside of the leaves. Tiny spiny projections are then formed on the leaves, and on infected berries. The berries are unsightly and although they can be used to make juice or jelly they are not acceptable for fresh fruit consumption.

In July the rusty spots on leaves and fruit produce a different kind of spore, which is blown to juniper bushes, and it can infect, and cause the gall formation that takes several years to mature, to complete the cycle.  Control of this disease is difficult, the telial galls can be pruned out of junipers that are close by, but spores can travel easily on wind currents. Many commercial growers have to resort to a protective fungicide spray in the spring, to prevent infection of the bushes.

Regardless of their complexity, rust diseases do not usually cause complete death and destruction of their hosts, which probably explains their success historically.

Jill Thomson is a retired Plant Pathologist who lives in Saskatoon, where she enjoys gardening with her family, including the dogs. This column is provided courtesy of the Saskatchewan Perennial Society (SPS; saskperennial@hotmail.com). Check our website (www.saskperennial.ca) or Facebook page (www.facebook.com/saskperennial) for a list of upcoming gardening events.