Garden Guy: White residue on plants might be indicator of insects

I just discovered in a struggling Pakalana vine, clumps of white residue on some of the vines. I was unable to find a picture of what it might be. Whitefly came closest, but the few leaves on the plant are clean. Do you have any idea?

The good news is that “whitish residue” is a good, partial description of a plant pest. Unfortunately, it is the description of many plant pests including insects, some diseases and even some naturally occurring plant phenomenon.

A whitish reside is a sign of a number of insects: whitefly, mealybug, scale insects and even some wooly aphids. In addition, rather than observing the insect itself, you may be noticing insect residue, such as the cast skins from the process known as molting; this happens when an insect sheds its outer “skin,’ or exoskeleton, in order to grow.

A whitish residue can also describe the white cottony fungal growth known as mycelium. This is typical of the common disease, powdery mildew. Finally, this residue could truly be a secretion from the plant itself. An example of this is seen in avocados. When an avocado tree is wounded, the bark will weep a sap which when it is dry will leave a sugary white residue. The key is diligent observation. If only small amounts of the mystery substance are found, it is not worth risking a nonessential spray treatment.

In many cases, the residues are temporary and/or minor. If it truly is the beginning of a pest population, as they increase, better identification can be made and a suitable treatment applied.

In dealing with a fungus, such as powdery mildew, early treatment is important. But I would still observe and wait for proper identification.

In your particular case, thanks to the picture you sent, the problem is a scale insect. With light infestations, simply rubbing off the pest may work or washing with a heavy stream of water. Alternatively, an oil/soap spray should be effective. Remember, biological control will frequently take care of the problem.

Dear Garden Guy, You probably know Big Island bananas are so delicious. Well, I have some young keikis of a couple different kinds. I don’t know much about caring for them. When can I expect fruit and how do I look after the trees? Mahalo — glad for any info you have.

The banana plant is actually a perennial herb. The fruit is harvested 11-15 months after planting. Individual bananas turn light green to yellow and angular corners become rounded. Bananas require a well-drained soil, with a pH range of 5.5-6.5. They also need rainfall of 100 inches or more, well distributed throughout the year along with full sun, wind protection and an average temperature of 81 degrees. In general, banana plants need a fertilizer with high amounts of potassium (K), along with nitrogen (N) and phosphorus (P). Soils vary around the Islands, some being richer than others. The University of Hawaii recommends applying 2 pounds, three times a year of a fertilizer similar to 10-5-20 (N-P-K). This amount is applied per mat. When referring to bananas, a group of plants is known as a mat. A properly maintained mat will have one plant in fruit, one plant half-grown and one plant just emerging from the soil.

Applying compost to the surface of the banana mat is beneficial, but most likely will not supply enough nutrients, especially nitrogen, for optimum growth. Green bananas can be stored up to seven days at room temperature or up to 20 days under refrigeration. Neither green nor ripe bananas, however, should be stored at temperatures lower than 58 degrees. Cooler temperatures will cause surface damage. You can read more about the care of bananas and other tropical fruit trees at

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 Also, visit his website at


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