Cascades of nearly neon, violet-blue flowers have made jacaranda trees (Jacaranda mimosifolia, Jacaranda acutifolia) garden fixtures in U.S. Department of Agriculture plant hardiness zones 10 through 11. Even when not buried in blooms, jacarandas dazzle with lovely twisted branches and feathery green leaves. Only a few insect or cultural problems and one fatal disease prevent jacarandas from being year-round dazzlers.
In the mild-winter climates where jacarandas grow, bean aphids (Aphis fabae) may be year-round pests. In large numbers, piles of the black insects cover the leaves' stems and backs, draining sap and giving birth to as many as five live young a day. The pests also cover the trees and surrounding objects with gooey transparent waste, called honeydew.
Glassy-winged sharpshooters also drain jacaranda leaf fluids. The fluid waste they excrete dries to form a white layer on the branches and nearby surfaces. Besides making a mess, the 1/2-inch, transparent-winged bugs transmit the deadly Xylella fastidiosa bacteria. Infected jacarandas develop oleander scorch disease, an incurable condition.
Organic insecticidal soap controls both these pests, but complete control on large jacarandas requires patience. Choose an overcast day when the temperature is expected to remain below 90 degrees Fahrenheit to treat the tree.
Water the tree well.
Add 5 tablespoons, or the label's recommended amount, of the soap concentrate to the 1-gallon container. Top up the container with water.
Dress in the protective clothing and safety goggles and pour the soap solution into the sprayer.
Spray the tree until the soap drips from all the leaves -- including their undersides -- and branches. Shake the sprayer frequently to keep the soap suspended. Mix more solution if it's needed to complete the job.
Repeat the application every one to two weeks, or as often as the label suggests, until the insects are gone.
Oleander Leaf Scorch
Killing the sharpshooters on a scorch-infected jacaranda does nothing to stop the disease's progress. It begins with wilted yellow, dark-margined leaves dropping from a few branches. The bacteria move inward from the tips of the leaves to their bases, and eventually through all the branches. As they plug the water-transporting xylem tubes, the tree slowly dies of thirst.
Jacarandas stressed from too little water have yellowed, wilted prematurely dropping leaves. Those getting too much water have undersized leaves; their small branches die back and their large ones may drop. Excessive watering also leaches essential minerals from the soil.
From spring to fall, water jacarandas slowly and deeply once every two weeks. During their winter dormancy, watering just once or twice is enough. Water around the dripline, where rain falls to the ground from the outermost branches. Keep the trunks dry to prevent fungal infections.
Excessive fertilizing may lead to damaging soil levels of minerals or salts, causing dead leaf tips and yellowing leaf edges. If a tree has these symptoms, the best way to pinpoint the mineral responsible is with laboratory soil and tissue tests.