Several different species of this common insect feed on lilies. Aphids cause their damage by sucking plant sap; this feeding produces leaf curling, yellowing, and distortion. The aphid is unable to digest fully all the sugar in the plant sap, and it excretes the excess in a fluid called honeydew. The honeydew often drops onto the leaves below. Ants feed on this sticky substance and are often present where there is an aphid infestation. Aphids can also spread viral diseases. In the process of feeding, certain aphids can infect lilies with mosaic virus disease.

Leaf scorch is a condition that may develop in lilies when they are growing in acid soil with a pH lower than 6.5. Toxic amounts of aluminum and manganese salts become available and are absorbed by plant roots in acid soils. Leaf scorch is most likely to occur when the plant is not receiving adequate or balanced supplies of nutrients, such as nitrogen and calcium. During the rapid part of the growing season, significant temperature changes can also cause leaf scorch.

Lilioceris lilii This beetle is a recent introduction to the United States, having first appeared in Boston in 1992. It has now spread throughout the Northeast, and may eventually spread to much of the country. It favors true lilies (Lilium), but sometimes feeds on fritillary, solomon''s seal, or lily-of-the-valley. It does not attack daylilies. The bright red adults spend the winter in the soil or plant debris, emerging in April. They begin to feed on lilies as soon as they appear in the spring. Females lay eggs in rows on the undersides of lily leaves. The larvae emerge in late April or May and feed for 3 or 4 weeks. The slug-like larvae carry their excrement on their backs, making them repulsive to predators, including gardeners. They pupate in the soil in June to reemerge as adults in early summer, then feed until October. The heaviest damage is done by the larvae. Researchers are searching for predators and parasites to release in this country. If they are successful, this beetle may be reduced to a minor pest.

Rot problems are common with lilies. These plant diseases are caused by various fungi (Rhizoctonia, Phytophthora, Pythium, Fusarium, and Cylindrocarpon species). These fungi attack and decay the bulbs and roots, causing stunting, wilting, and eventually death of the foliage and flowers. These bulb and root-rot organisms live in the soil and stored bulbs and are favored by wet soil. Sometimes bulbs in storage are lightly infected, but the fungal decay hasn''t progressed far enough to be easily noticed. When planted, these bulbs may rot so quickly that they do not produce any foliage.

Several viral diseases of lilies cause mottling or flecking of the foliage. Mosaic viruses produce leaf mottling and discoloration. Depending on the species or variety, the symptoms of infection can be mild or severe. Fleck is produced if a plant is simultaneously infected by the symptomless lily virus and the cucumber mosaic virus (which may or may not produce mottling by itself). Leaf flecking is usually accompanied by stunting and poor-quality flowers and foliage. The plant is generally disfigured. Viruses remain in infected bulbs year after year, so successive plantings of diseased bulbs will produce only poor-quality flowers and foliage. All of these viruses are spread by aphids, which pick up the virus while feeding on diseased plants and then, at later feedings, transmit it to healthy plants.