Q. The top of my oleander is turning yellow and dying. Will the plant recover if I prune out the affected branches?
A. Removing the dying branches from your oleander will improve its appearance for a while, but this will not restore the plant’s health. Your oleander will probably die in a few years if it has oleander leaf scorch. This fatal disease is caused by the bacterium Xylella fastidiosa. The bacteria grow inside and soon plug the plant’s xylem tissue, which transports water from the roots to the leaves. The upper foliage initially turns yellow and may droop on one or more branches. Unfortunately, by the time these symptoms appear, the bacteria have already spread to other parts of the plant that appear healthy. As the disease progresses, more branches are affected and the plant eventually dies. All oleanders (Nerium oleander) are susceptible to the disease although some cultivars may express symptoms to lesser degrees and live longer when infected.
Oleander leaf scorch is spread primarily from plant to plant by an exotic sap-feeding insect, the glassy-winged sharpshooter. Spraying oleanders with insecticide does not adequately protect plants from sharpshooter feeding and possible infection. The bacterium does not persist in the soil when a plant dies. Another oleander could be planted in the same location, but this is not recommended since the new plant could also become infected when sharpshooters feed on the plant.
The best permanent solution is to replace dying oleanders with a different ornamental. Some possible substitutes are lemon bottle brush (Callistemon citrinus), purple hop bush (Dodonaea viscosa ‘Purpurea’), American arborvitae (Thuja occidentalis), toyon (Heteromeles arbutifolia), and lemonade berry (Rhus integrifolia). Additional plants suggested by four landscape designers are listed in an article, “Good Bets for Hedges to Replace Dying Oleanders” at:
Q. My oranges look ripe, but they taste sour. Do I need to fertilize the tree to make the fruit sweet?
A. Fertilizing an orange tree in winter will not make the fruit sweeter. To develop sweet fruit, orange trees need a full canopy of foliage, a sunny location, warm weather and time.
You cannot tell when oranges are ready to eat by the color of their rind. Both navel and Valencia oranges color in fall as the temperature cools, but this does not mean the fruit has enough sugar and is ready to pick. You may have to wait until February for navels to be sweet enough. Valencias are not usually ready to pick until July. Both oranges will hold on the tree for several months after they are ripe.
Mature citrus (5 years and older) should be given a fertilizer high in nitrogen in February and again in April. Nitrogen should not be applied in summer since this will make the rinds thicker and cause Valencias to re-green. Do not apply nitrogen in fall because too much can delay fruit maturity and increase the acid content of the juice.
Vincent Lazaneo is an urban horticulture adviser emeritus with the University of California Cooperative Extension. Send questions for “Plants & Pests” to firstname.lastname@example.org