Cedar-Apple Rust - Solution Guide
Cedar-Apple Rust
Cedar-apple rust is a fungal disease (Gymnosporangium juniperi-virginianae) that requires apple trees (Malus spp.) and cedar trees (Juniperus species) as necessary but temporary hosts for the pathogen to complete its life cycle.
- Cedar-apple rust is generally a cosmetic nuisance and not fatal to infected trees, although heavy infections, left unchecked, can weaken tree health and damage fruit crops.
- It is essential to recognize that once spots are noticed, preventative fungicide applications are too late for that season to be effective.
- The usual threshold for disease tolerance of rust on apples is around 8-10 spots per leaf. If the spots are more numerous than this, pinching off infected leaves to limit infections is no longer viable. Fungicide protocols are recommended if apple leaves become highly infected for multiple seasons.
- Fungal diseases are weather-dependent. All pathogens have specific temperature, moisture, and humidity triggers. This fungus benefits from wet conditions for four or more hours and temperatures between 50° and 75°F. Periods of cool, dry, or hot, dry conditions do not favor this disease, so incidences decline during these times.
Control & Management for Cedar-Apple Rust
- Plant resistant varieties. Rust-resistant apple varieties include 'Liberty,' 'Enterprise,' 'Northwestern Greening,' 'Wealthy,' 'Yellow Transparent,' and most ornamental crabapples. Other fruiting varieties can have moderate rust resistance as well. See our Garden Guide - Rust Resistant Apples
- Pinching or pruning off spotted leaves is the best and easiest initial control. Removing and destroying visibly infected leaves interrupts the life cycle. Put infected material into the trash or burn it as soon as possible. Sanitation is essential to help reduce future infections. Rake and pick up fallen, infected leaves.
- Removing host plants. Cedars (Juniperus spp.), the original and dual host of this disease, can grow a long way away, so physically removing problematic trees may be unrealistic. However, if the trees are in your yard, removing them is best.
- Remove fungal galls. If cedars are nearby, inspect those trees in early spring or fall for the tell-tale fungal globs growing on the foliage and prune those out. During wet weather periods, the fungi appear as distinctive orange gelatinous masses. They appear as dark brown threads during dry periods, which may be more challenging to detect.
- Fungicide applications. Effective preventative fungicides are limited to copper or sulfur products for home gardens. In the Midwest and NE, fungicide applications are typically done in April to May. For the most effective timing, refer to and follow product label instructions and guidelines. Each fungicide product will be slightly different.
- Keep trees healthy. Proper watering, fertilizing, and pruning are critical to promoting healthy, vigorous-growing apple trees. Healthy trees can better resist or naturally grow through infections without adverse issues.
