What is Rust?
Rusts are a group of common and highly visible fungal diseases that get their name from the rust-colored, powdery pustules they form on plant leaves and stems. If you've ever seen an orange or reddish-brown powder that rubs off onto your fingers from a leaf, you've likely encountered a rust disease.
These diseases are caused by complex fungi from the order Pucciniales. One key thing to know about rust fungi is that they are "obligate parasites," which is a scientific way of saying they can only survive on living plant tissue. They are also incredibly specific; the rust that infects your hollyhocks will not be the same one that infects your lawn. The primary environmental condition that fuels a rust outbreak is prolonged moisture. Several hours of uninterrupted leaf wetness from dew, rain, or overhead watering is all it takes for rust spores to take hold.
Rusts can affect a wide variety of plants in the home garden. Some of the most common victims include:
- Perennials: Hollyhocks, daylilies, snapdragons, and irises.
- Shrubs: Roses and azaleas.
- Vegetables: Beans, corn, and asparagus.
- Trees: Crabapples and hawthorns (affected by cedar-apple rust).
- Lawn Grasses: Kentucky bluegrass and perennial ryegrass.
Life Cycle
The life cycle of rust fungi can be one of the most complex in the plant kingdom. The powdery spots you see are actually masses of tiny spores. These spores are easily carried by the wind, which is how the disease spreads from leaf to leaf and plant to plant, sometimes over great distances.
Here's where it gets interesting: many rust fungi require two completely different, unrelated plant species to complete their full life cycle. One is the "primary host," and the other is the "alternate host." A classic example is cedar-apple rust, which must spend part of its life on an Eastern Red Cedar (or other juniper species) and the other part on an apple or crabapple tree. Spores produced on the cedar can only infect the apple, and spores produced on the apple can only infect the cedar. Other rusts are simpler and complete their life cycle on just one type of plant.
Rust fungi overwinter on infected, living plant tissue (like the leaves of an evergreen host) or on plant debris left in the garden. When temperatures become mild (typically 60-80°F) and moisture is present in the spring, the fungus produces new spores to start the infection cycle all over again.
Symptoms
While the classic orange color is a giveaway, rust symptoms can vary slightly by plant and fungus type.
Early Stage
- Pale Spots: The first sign is often the appearance of small, pale yellow or whitish spots on the upper surfaces of leaves or on stems.
Middle Stage
- Pustule Formation: On the undersides of the leaves, directly below the pale spots, raised pustules (called sorior uredia) will form. These look like tiny blisters.
- Spore Release: The pustules rupture and release masses of powdery spores. The color is most often orange, yellow, or reddish-brown, but can sometimes be purple or black. This powder will easily rub off.
- Stem Galls: In some cases, like cedar-apple rust, the disease forms strange, orange, gelatinous galls on the alternate host (the cedar tree) in the spring.
Late Stage
- Severe Yellowing: As the infection progresses, the leaves will turn yellow, become dry and brittle, and may become distorted.
- Premature Leaf Drop: The plant will shed its most heavily infected leaves. Severe defoliation weakens the plant, reducing its vigor and ability to flower or produce fruit.
- Stunted Growth: Heavily infected plants may be stunted and generally appear unhealthy.
How to Control Disease Progression
Once you see rust, your main goal is to prevent it from spreading to healthy new growth.
- Remove Infected Leaves: As soon as you spot the first signs, carefully pick off and destroy the infected leaves. Do not put them in your compost bin, as the spores can survive.
- Keep Foliage Dry: Immediately switch to watering at the base of the plant with a soaker hose or watering can. Avoid overhead sprinklers, especially in the evening.
- Improve Air Circulation: Thin out dense plantings to allow more air to move through the leaves, helping them dry off faster after rain or dew.
- Clean Up Debris: At the end of the season, meticulously clean up and destroy all fallen leaves and plant debris to reduce the number of overwintering spores.
Treatment Options
Cultural controls are your first and best line of defense. Fungicides are supplementary and work best as a preventative measure to protect uninfected leaves.
Organic Treatment Methods
- Sulfur-Based Fungicides: Wettable sulfur is a long-used organic control for rust. It works as a protectant. Be careful not to apply it when temperatures are high (above 85°F) as it can burn leaves.
- Neem Oil: Neem oil has fungicidal properties and can help prevent spore germination. It must be reapplied regularly.
Chemical Treatment Options
- Targeted Fungicides: Look for fungicides with active ingredients like myclobutanil or propiconazole. These are highly effective against rusts. Always check the label to ensure the product is approved for use on your specific plant.
Step-by-Step Treatment Procedure
- First, perform cultural controls: remove infected leaves and ensure good air circulation.
- If you choose to spray, do it on a calm day when rain is not forecast.
- Mix the fungicide according to the label directions.
- Apply the spray to thoroughly cover all parts of the plant, especially the undersides of the leaves where pustules form.
- Reapply every 7-14 days as directed on the label, particularly during periods of cool, wet weather when the disease is most active.
Expected Timeline for Recovery
Spraying will not make existing rust spots disappear. Success means that new leaves emerge and remain clean and that the disease does not continue to spread. The plant's overall health should improve as it retains its new, healthy foliage.
Prevention Strategies
Preventing rust is far easier and more effective than trying to cure it.
- Choose Resistant Varieties: This is the single best strategy. Many modern cultivars of roses, daylilies, snapdragons, and other plants have been specifically bred for rust resistance. Check plant tags and descriptions when you buy.
- Break the Life Cycle: For rusts that require two hosts, like cedar-apple rust, avoid planting the two hosts near each other. If you have apple trees, consider removing any nearby Eastern Red Cedars if rust is a persistent problem.
- Practice Good Sanitation: A thorough fall cleanup of all garden debris is critical to reduce the source of infection for the following spring.
- Promote Airflow and Dry Leaves: Space plants properly when planting. Water in the morning so leaves have all day to dry. Prune to open up dense canopies.
