The fungal pathogen Uredo dianellae attacks flax lily, resulting in what gardeners know as rust. Rust presents itself as reddish, purple, orange, brown or yellow masses of spores or pustules developing on leaves. Infected leaves can yellow or brown and begin to wither, curl and fall off the plant prematurely. Sooty mold is a black substance with a soot-like appearance on the foliage of infected plants. This annoying fungus requires honeydew -- which is secreted by sap-sucking pests -- in order to grow, and typically doesn’t threaten the life of the flax lily. Powdery mildew is most easily recognized by the white powdery growth that forms on the leaves of the infected plants. This extremely contagious fungal disease doesn’t require moisture to grow and water can actually inhibit the growth and spread of the fungal spores.
Fungicides won’t get rid of sooty mold and control requires managing the insects secreting the sticky honeydew. Insecticides are generally not recommended since they can cause more harm than good. Instead, attract beneficial predatory insects to prey on the honeydew-producing insects infesting the flax lily. Ladybugs and lacewings -- which feed on aphids, scales, whiteflies, mealybugs and thrips -- can be attracted to your garden by planting California poppy (Eschscholzia californica), cilantro (Coriandrum sativum) and parsley (Petroselinum crispum). Once you have controlled the insects, wash the sooty mold off the flax lily by spraying the plant with a water hose. If you don’t wash the sooty mold off the plant, it will slowly disappear on its own.
Neem oil, horticultural oil and potassium bicarbonate are three fungicides that help control fungal diseases such as rust and powdery mildew. Each type of fungicide lists specific instructions, warnings and application rates that you should follow to improve its effectiveness and prevent damage to the flax lily. Unless otherwise stated on the fungicide label, the chemical should be applied on a day when the wind is calm, the temperatures are above 40 degrees Fahrenheit but below 90 degrees and when rain will not occur 24 hours after the application. If possible, spray the flax lily with the chosen fungicide at the first signs of infection.
Most fungal diseases can be kept at bay by implementing proper cultural control methods in your garden. Proper spacing between the flax lilies and surrounding plants help to ensure good airflow. Air circulation dries leaves more quickly and reduces fungal growth. Also avoid overhead watering since it favors the germination and spread of fungal diseases such as rust. Instead, water at the base of the flax lily early in the day to allow the plant to dry before the sun goes down. Good sanitation also helps control flax lily problems; remove all plant debris that has fallen to the ground and prune infected plant matter off the flax lily.