Rusts are easily recognized by the dry, brown, orange, purple, reddish, or yellowish spore masses or pustules that form on the lower leaf surfaces. Rusts infect many plants, including aster, carnation, geranium, iris, lily, pansy, primrose, snapdragon, sunflower, and sweet pea. The upper surface of heavily infected leaves can become spotted or turn yellow or brown, and infected leaves may curl, wither, and drop prematurely. Severely infected plants may be stunted. Some rust species cause tissue swellings, galls, or cankers, especially on woody plant parts. These rusts can cause stem dieback and, rarely, can kill the entire plant.
Solutions
Rust fungi infect under mild, moist conditions. Reduce infections by minimizing the length of time that foliage is wet. Avoid overhead watering, which favors rust spore germination and spread. Alternatively, water early in the day so that plants dry more quickly. Use good sanitation. Remove and destroy affected plants or plant parts as soon as they appear. Prevent excess humidity, provide good air circulation, and don’t crowd plants. Some plants are resistant to rust. Consider using these to avoid rust problems. Fungicides, including neem oil, applied at the first signs of infection can also prevent serious damage from most rust fungi.
