Calla lilies comprise multiple species, subspecies and cultivars in the genus Zantedeschia, most of which were originally native to South Africa. The lily is successful in other parts of the world, and its spread can be somewhat attributed to the attractiveness of its large solitary blossoms and its ease of care. However, the calla lily can be afflicted with a number of issues that cause its leaves to turn yellow, which is symptomatic of a larger problem.
Pest Issues with Calla Lily Leaves
Calla lilies are a food source for many pest species. Sucking pests like thrips, aphids and spider mites feed off the lily sap found in the leaves and cause them to yellow. The pests also produce a byproduct called honeydew that attracts ants and encourages moulding. Chewing insects that afflict callas include various caterpillars and grasshoppers. Heavy feeding by pests of any sort will cause not just yellowing, but also black and/or brown splotches, holes and wilting of the lily leaves.
Environmental Issues with Calla Lily Leaves
Quite a few environmental issues will cause calla lily leaves to yellow. If deficient in magnesium or iron, there will be a yellowing of the leaves known as chlorosis. Sometimes leaves will yellow because of an excess of water, as the calla lily needs a lot of moisture but does not do well in poorly drained earth. Conversely, lily leaves will yellow and wilt if there is not enough water.
Fungal Disease Issues with Calla Lily Leaves
Calla lilies can be afflicted with many types of fungal disease that can infect the leaves. Examples include anthranose, leaf blight, and powdery mildew. Powdery mildew's primary symptom is a white coating over the leaf surface, while leaf blight and anthranose symptoms appear as black and brown spots on the leaf. Root rots are also a problem, with verticillium wilt and amarilla rot being the most common types. Amarilla rot is observed by the white matting of fungal growth that appears at the base of the stem and roots. Verticillium wilt is evidenced by yellow leaves and severe wilting of the stem.
Prevention and Treatment
Treat infestations of pests with insecticidal sprays. However, to prevent pests from causing issues in the first place, introduce predatory insects like the predatory mites. For magnesium or iron deficiencies, treat the soil with the appropriate commercial chelated product. Ensure that you are giving the lily the appropriate amount of water. The soil medium should be damp, not wet or bone dry. If the soil is too wet, switch to a better-draining pot and add sand to the potting mix. If it is too dry, water more often. Fungal issues can be both treated and prevented with the regular application of a fungicidal agent. Do not plant in soil that has had issues with root rot fungi previously.