Rust on plants is a common fungal disease caused by species belonging to the order Pucciniales, a large group of specialized plant pathogens. These fungi are obligate parasites, meaning they can only grow and reproduce on living plant tissue, which they infect through spores. Rust diseases affect a wide variety of plants, including ornamental flowers, vegetables, and fruit trees, and can cause significant damage. Early action is important for managing an infection and stopping the fungus from spreading throughout your garden.
Recognizing the Signs of Rust Fungus
The most distinct sign of a rust infection is the appearance of small, raised, powdery pustules, which are the spore-producing structures of the fungus. These pustules often cluster on the undersides of leaves and can be orange, yellow, brown, or red in color, resembling specks of iron oxide. The presence of these pustules usually corresponds with yellow or pale spots appearing on the upper surface of the leaf directly opposite the infection site.
As the disease progresses, infected leaves may turn yellow, then brown, and often drop prematurely. This loss of foliage weakens the plant by reducing its ability to perform photosynthesis, leading to stunted growth. The powdery pustules easily dislodge microscopic spores when touched, allowing them to be spread by wind, water, or tools.
Immediate Physical Control and Sanitation
The first step in controlling an outbreak is to physically remove infected plant material to reduce the number of active spores. Carefully prune away all leaves, stems, or entire plants that show signs of the powdery pustules, avoiding shaking or brushing the material during removal. This process prevents the fungus from continuing its life cycle and releasing new generations of spores.
It is important to properly dispose of the removed material by sealing it in a plastic bag for trash collection or by burning it, if local regulations allow. Never place rust-infected plant parts into a home compost pile, as the fungal spores can survive the composting process and re-infect the garden later. After pruning, immediately clean all gardening tools, such as shears and clippers, by dipping or wiping them with a disinfectant like a 70% isopropyl alcohol solution or a 10% bleach solution to prevent spreading spores to healthy plants.
Applied Treatments for Active Infections
Non-Chemical/Organic Treatments
Organic treatments can be applied to the foliage to combat the fungus. Horticultural oils, such as neem oil, act as a fungicide and a protectant by coating the leaf surface and disrupting the fungus’s ability to infect new tissue. Apply neem oil according to label instructions, ensuring thorough coverage of both the top and bottom surfaces of the leaves to contact the pustules directly.
Sulfur dust can be used as a preventative or curative fungicide against rust. When applying sulfur, avoid doing so when temperatures are expected to exceed 85°F, as this can burn the plant foliage. Also, avoid applying sulfur within two weeks of using a horticultural oil, as the combination can lead to phytotoxicity, or plant damage.
Commercial Fungicides
For heavy or widespread rust infestations, commercial fungicides may be necessary. Fungicides containing active ingredients like chlorothalonil, mancozeb, or myclobutanil are commonly recommended for rust control. Chlorothalonil and mancozeb primarily act as protectants, meaning they must be applied before infection to coat the leaf surface and prevent spore germination.
Myclobutanil is a systemic fungicide that can be absorbed by the plant and move through its vascular system, providing both protective and curative action against the established fungus. Always select a product specifically labeled for use on rust and for the type of plant you are treating. Strictly follow the label’s instructions regarding mixing rates, application frequency, and safety precautions. Applying a systemic product at the first sign of disease is more effective than waiting until the infection is severe.
Preventing Future Rust Outbreaks
Long-term management focuses on cultural practices that make the growing environment less favorable for the rust fungus. Since rust spores require a film of water on the leaf surface for several hours to germinate and infect, adjusting your watering routine is essential. Always water plants at the base, avoiding overhead irrigation that wets the foliage. Water in the early morning so that any accidental moisture on the leaves can quickly evaporate in the sun.
Improving air circulation around the plant canopy helps to dry leaf surfaces quickly. Ensure plants are spaced appropriately according to their mature size, and perform selective pruning to thin out dense foliage and allow air to move freely. When purchasing new plants, look for varieties that are specifically advertised as rust-resistant, as this is the most reliable long-term strategy for avoiding recurrent problems with the disease.