I hear this often.
“There are little white spots on my magnolia leaves. What type of fungus is it?”
Interestingly enough, it is not fungus, but a type of insect called scale.
Scale insects are often mistaken for fungal disease because many cluster together, resembling a crusty or fuzzy mass that often oozes when rubbed. Scale gets its name from the protective fish scale-like covering produced by a tiny insect about the size of a pencil tip. Scale insects anchor to plant parts by their piercing-sucking mouthparts and feed on plant sap. Individual scales look like oval or rod-shaped bumps, ranging in color from white, yellow, grey, brown to black. These insects can be a major problem on many of our evergreen and deciduous trees and shrubs and can occur on leaves, twigs, branches or trunks.
There are two types of scale insects - soft and armored.
Soft scales secrete a thin, waxy layer over themselves, which cannot be separated from the insect’s body. This is a lot like what my 8-year-old does with mud on the banks of the Clarks Hill Reservoir. Because scales consume large amounts of plant sap, soft scales excrete a sticky, sugary liquid called “honeydew” as a by-product of their feeding.
Honeydew provides the perfect source for the growth of a black mold, called “sooty mold.” Large patches of sooty mold that blacken leaves and stems are often what draw attention to a scale problem. Sooty mold growth does not damage the plant but it does look unsightly and in large amounts can interfere with photosynthesis, slowing plant growth. Sooty mold is also a common byproduct from white flies on gardenias and aphid infestations on hackberries. Soft scales range from 1/8 to 1/2 inch in size.
Armored scales secrete a hard, lacquered covering over their bodies. This cover is not attached and can usually be separated from the scale’s body. This is like when my son eats chocolate ice cream. Armored scale typically are smaller than soft scale, ranging from 1/16 to 1/8 inch and do not excrete honeydew. Without the honeydew, no sooty mold is present.
Though the biology differs for various scale species, understanding some general characteristics is helpful in controlling this insect. Immobile, legless females lay eggs under their bodies, then die. Eggs hatch into tiny pinkish-to-yellow mobile immatures called “crawlers” that move around the plant seeking suitable sites to feed, secrete their scale covering, and mature to adulthood.
Some species overwinter as eggs beneath the dead female’s cover and hatch in the spring; others overwinter as fertilized females and resume feeding in the spring, when they lay eggs and die. Adult male scale insects are tiny and winged; do not feed; and live only a few hours. Females of many soft scale species reproduce without mating. Armored scales are more likely to have several generations per year while most soft scales have only one.
Scale insects obtain food by sucking vital fluids from the host plant, causing yellowing and possibly stunted growth of the affected leaves or needles. A heavily infested plant will have extensive leaf yellowing, premature leaf drop, and possibly branch dieback. A plant weakened by a scale population is often more susceptible to damage by a secondary pest that may ultimately kill the plant. Camellias, pittosporum, magnolia, juniper, azaleas and aucuba are some of scale’s favorite plants.
The best defense against severe scale infestations is to monitor landscape plants regularly throughout the year, paying close attention to the undersides of leaves and stems for scale. Avoid over-fertilization. Insects often lay more eggs and survive better on plants that are lush from heavy doses of nitrogen.
Smothering scale insects by applications of horticultural oil is the easiest and often the most effective means of control. There are numerous types of oils that you can purchase. If the scale is on the underside of the leaf, remember the oil has to get on the insect to smother it, so spray under the leaves. Read the label carefully because there are some temperature restrictions that can harm the plant, especially when it gets too hot. Most contact insecticides cannot penetrate the protective covering of the immobile scale nymphs and adults.
When a plant is too large to spray with oil, try using a systemic insecticide with dinotefuran or imidacloprid. Systemic means the plant will absorb the chemical and when the insect bites into it, the bug will get a bellyache and croak. Granular versions are usually easier to apply, but for just a couple of infested plants, a water soluble drench can get the insecticide to the plant a little more rapidly.
Other things happening in landscapes: Get your preemergence out yesterday. Who would have thought it would be 85 degrees on March 1?
Don’t add nitrogen to your grass yet. Wait until late April. Same goes for fertilizing shrubs. Wait until after the Masters Tournament.
If you need to hide, wear yellow. You will blend in well because everything in Augusta is about to be covered in yellow pine pollen.
Reach Campbell Vaughn, the agriculture and natural resources cooperative extension agent for Richmond County, by e-mailing firstname.lastname@example.org.