I am often asked about the black coating on leaves called sooty mold. Indeed it is a fungus, but the black mold itself is not the problem; no fungicide need be applied. The black sooty mold growing on leaves points to the fact that there is an insect infestation on the plant, most likely aphids, mealy bugs, soft scale or whitefly.
To control the mold, simply control the insects.
The sooty mold fungus is not harmful to the plant but is actually growing on a sweet, sugary substance called honeydew. The honeydew is secreted by insects that are sucking out the plant fluids. As the insect feeds, a clear, sugary liquid is excreted onto the leaf below. It is on this honeydew that the mold grows. In most cases the insect population can be controlled with an oil or soap spray.
Occasionally, a high population of insects will create so much honeydew that all the leaves will be coated with the black sooty mold. This can harm the vigor of the plant by not allowing enough sunlight, and the plant will suffer. Usually rain will eventually wash the leaves clean.
For a University of Hawaii recipe for an oil/soap spray solution, visit my website, www.gardenguyhawaii.com, and search “Using soap to kill insects.”
There are three common pests that will cause azalea foliage to have a silvery, whitish or bleached look. They are mites, thrips and lacebugs.
Of these, the most common are the lace bugs.
Lace bugs have piercing-sucking mouthparts which remove the plant’s sap as they feed from the underside of the leaf. This feeding reduces vigor and detracts from the plant’s beauty.
The lace bugs are about 1/8- to 1/4-inch long and have a brownish, black coloration to them. But most notable are the highly sculptured, transparent wings which give them a lacy appearance. The immature stages are flat and oval with spines projecting from their bodies in all directions. A good identification of the lace bug is the presence of dark droplets of excrement on the undersides of the leaves, along with the old skins of the immature stage as the adults molt.
The control for lace bugs, thrips and mites, is similar. Although there are general predators for these pests such as spiders, predatory mites and ladybird beetles, they are often not efficient enough to keep these pests in check. Sometimes the pest can be knocked off the plant with a heavy spray of water. Applications of insecticides such as horticultural oils, neem oil and insecticidal soaps can be effective temporarily to reduce the pest population. It is important though, to thoroughly cover both the top and the bottom portions of the leaves. It is best to apply these oils or soaps when the pest population is low. Repeated applications at five to 10 days apart are usually required for good control.
The following azalea cultivars have resistance to the azalea lace bug: Dawn, Pink Star, Ereka, Cavalier, Pink Fancy, Dram, Seigei, Macrantha, Salmon Pink, Elsie Lee, Red Wing, Sunglow and Marilee.
For more detailed information on common pests such as mites, thrips and lacebugs, visit my website, www.gardenguyhawaii.com.
Does citrus fruit get sweeter once it is picked? Someone told me to leave my sour tangerines on the counter and they would get sweeter. I tried it and they didn’t. What are your thoughts?
Citrus does not ripen once it is picked. Citrus fruit has no starchy reserves in storage that are converted to sugar. After picking, fruit can lose some acidity, as well as the vitamin C content. For some, the less acidic fruit can be interpreted as slightly sweeter.
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at email@example.com.