PSI: The case of the parasitic rust

Breakdown:

Rust can coat your old pipes, or colonize your new crops — the rusts we are considering here act as parasites, and grow directly upon the flowers you produce.

Rust fungi are more sophisticated than many of the fungi that you combat in the greenhouse. They are actually close relatives of mushrooms — and they are very tuned into their hosts because they are “obligate parasites,” meaning they are closely tied to their host plant and cannot live off dead organic matter. Some rusts, though, have a very fancy life scheme wherein they require one plant for part of their life cycle and another plant for the rest.

Nurseries cope with many kinds of rust — growers of herbaceous perennials may see them on veronica, campanula, heuchera, daylily or ornamental grasses, for example. Biennial lovers will be familiar with hollyhock rust, and mum growers know to beware of brown rust and white rust. If you grow indoors, there are primarily two to be alert to: geranium rust and fuchsia rust. Both of these are known to cause greenhouse crop losses.
 

Known hangouts:

Rust will be more of a problem in a wetter environment. It thrives in the same conditions as Botrytis cinerea, the notorious gray mold. Rusts on geranium and fuchsia make a ‘repeating’ spore stage (able to reinfect the same plant) that requires 5 to 6 hours of leaf wetness to germinate and infect. The symptoms and signs of geranium rust and fuchsia rust are found on the leaves.

In the case of fuchsia, rust is often noted during propagation under mist in the spring. Fuchsias will develop large leaf spots, often purple-rimmed.Turning the leaf over will show the apricot-orange masses of rust spores. When conditions strongly favor rust, spores may even form on the top of the leaf, and fuchsias may be defoliated. Geraniums also will develop symptoms in spring. At first, these appear as tiny round yellow or reddish spots on the top of the leaf. These can easily be confused with “ghost spots” made by unsuccessful Botrytis infection attempts. Key to rust identification is always to turn the leaf over: on geraniums, the spores are reddish brown, just like rust on pipes. The first rust pustule forms directly opposite the spot on the top of the leaf, and then later spore pustules are produced in concentric rings. They split open and release conspicuous masses of brown spores.
 

Monitoring:

1. Watch for yellow to red spots on geranium leaves.

2. Watch for large (up to dime-size) spots on fuchsia leaves.

3. Turn over spotted leaves and check for colorful (brown or orange) spores.
 

Prevention:

1. Keep leaf surfaces dry as much of the time as possible.

2. Water early in the day.

3. Scout rust-prone crops regularly during the spring.
 

Treatment:

1. Carefully bag any infected plants for removal by bringing the bag to the plants. Avoid spreading spores — they can be splashed about, or moved from plant to plant by handling or air currents.

2. Adjust watering and ventilation systems to reduce the length of time leaves stay wet.

3. Treat the remainder of the plants with fungicides to protect them.

4. Rotate among contact and systemic fungicides, and avoid using materials within the same mode of action group repeatedly. Mancozeb and chlorothalonil materials are effective contact protectants against rust. Systemic fungicides for rust management are strobilurins (such as Heritage or Insignia) and demethylation inhibitors (DMIs, such as Strike, Strike Plus or Terraguard).

Margery Daughtrey is a plant pathologist specializing in ornamentals at Cornell’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases.