Bougainvillea is a tropical vining plant, with some varieties growing up to 30 feet. The attractiveness of the plant is not just in its showy foliage, but in its easy care. It takes to pruning, it takes to pots and it can support itself on its woody growth or climb on supports provided. It blooms profusely in proper conditions; its small flowers are not nearly as noticeable as the large, colorful bracts (leaves) surrounding those blooms. The plant is generally disease and pest free.
Yellow or tan leaf spots can be a sign of watering too much or of a deficiency (provide bougainvillea fertilizer). Red-brown spots, on the other hand, which get larger larger and darker, are a sign of fungal or bacterial leaf spot disease. If not treated, your foliage will die off. The main help is to keep foliage dry by pruning so that leaves receive good air circulation. Get rid of infected leaves and infected plants.
Leaves that turn black or sooty have been infested by aphids, whiteflies or scale insects. Aphids secrete a nectar ants eat---a nectar that creates mold on the plant. Meanwhile, ants begin protecting the aphids; make sure you rid the environment of ants. Fight fire with fire by buying aphid predators---ladybugs and lacewings---or grow plants these helpful insects like such as spearmint, Queen Anne's lace, sweet clover and sweet fennel. Get rid of infested foliage and treat the rest with a natural or other insecticide. Scale insects, meanwhile, can be scraped off--an advisable treatment, since they are pesticide resistant. You can also put predators of scale insects---Chilocorus nigritus an Lindorus lophanthae---right on the bougainvillea. Isolate infested plants. Whiteflies are very hard to control since they become resistant to pesticide. Besides mold, they carry plant viruses. They are best controlled by their natural enemies, which, again, include lacewings and ladybugs.
When the bougainvillea drops its leaves, you might be watering too much, too little or not giving it enough light. The plant likes a lot of sun and won't bloom without it. Another cause of leaf drop, one easy to diagnose, is exposure to cold. The plant is a child of the equator, after all, at its best with nighttime temperatures that stay above 65 degrees F and daytime temperatures up to 95 degrees F. If you don't live in Plant Hardiness Zones 9b through 11, you'll have to take measures to protect the roots and leaves from frost. Consider growing bougainvillea as an indoor plant or an annual.
Root or Stem Rot
Bougainvillea plants have very fine roots that like well-draining soil. They don't like soggy soil or to be overwatered; it makes them susceptible to root rot or stem rot, which will kill the plant. This being the case, prevent root and stem rot through planting in proper soil at the start and by using a fungicide in the area where you will plant the bougainvillea.
Yellow leaves are a sign of nutrient deficiency. Bougainvilleas need regular feeding to sustain their vigorous growth and blooming. Standard balanced fertilizers aren't the best choice. Instead, about every month use fertilizer specially formulated for bougainvilleas or for hibiscus. Besides yellowing, signs of deficiency include older leaves turning pale with reddish veins (nitrogen lack), the whole plant taking on a purplish cast (phosphorus lack), browning leaf tips and purple leaf edges (potassium lack) and dead leaves among new leaves (calcium lack).