Cordyline terminalis plants are also known as ti plants or Hawaiian good-luck plants. The 20 species come in a variety of colors, with leaves of varying shapes and sizes. Although the International Cordyline Society considers them "tough" plants, they are susceptible to a number of problems.
Symptoms of fluoride poisoning include tipburn and localized death of the cells along the leaf margins. The center of the leaves may also look mottled, in severe cases. Using a potting medium with a pH of 6.0 to 6.5 will reduce the amount of fluoride available to your plant. Use treated water containing less than 0.25 parts per million fluoride for cuttings, because cuttings without roots absorb large amounts of fluoride.
Cordyline plants are grown for their colors, which include purple, maroon, rose, pink, yellow and green, but sometimes the colors appear dull. Low light intensity, high temperatures or too much fertilizer can cause this dullness. Increase light intensity, and reduce temperatures and fertilizer levels to improve color.
Cordyline cuttings are susceptible to two types of blight that result in stem rot, and mushy and slimy leaf spots. Cuttings can die but, if they are recut to remove the rot, they will sometimes root. The newly rooted plants will usually develop new symptoms of blight, including black, water-soaked roots. There are no chemical controls, and it's best to destroy diseased cuttings.
Several fungal diseases infect cordyline plants. Symptoms of fusarium leaf spot include the appearance of tan to reddish-brown spots near the tips of young leaves. Lower leaves will wilt and turn yellow when plants have fusarium stem and root rot. In addition, the roots will be black and water-soaked. Symptoms of Phyllosticta leaf spot include the appearance of tan spots on older leaves. Spots have purple borders and yellow halos, and may blend together, causing the entire leaf to die. Phytophthora leaf spot causes brown, water-soaked areas to form on the lower leaves, near the potting medium. Lesions have irregular margins. The fungus that causes southern blight usually attacks the crown first, infecting it with small, white, mustard-seed-sized fruiting bodies that turn brown as they mature. White, branching threads form on the plant and potting medium.
Control fusarium leaf spot and Phyllosticta leaf spot by keeping the leaves dry. Destroy plants with fusarium stem and root rot as soon as you find them and don't let drainage water contaminate other pots, because the fungus is spread through the water. Control Phytophthora leaf spot by growing plants on raised benches so that water doesn't splash onto the lower leaves from the soil, which harbors the infection. Destroy plants with southern blight and use potting media that's free of the fungus.
Fungus gnats, mealybugs, mites, scales and thrips can infest Cordyline plants. Mealybug, mite and scale infestations are usually caused by placing infected plants into contact with plants that aren't infected. Fungus gnats and thrips fly from infested plants to those that aren't infested. Parasitical nematodes can control the fungus gnat larvae that are responsible for causing damage to the roots, leaves and lower stems. Insecticides are used to control mealybugs, mites, thrips, whiteflies and scales, but they aren't effective against shore flies. Reduce the amount of water you give your plants and avoid algae growth, to help with shore fly control.