7.01.2008

Q. I propagate, grow and sell hibiscus (among other perennials)….and every year they get attacked by an insect that eats the green leaf tissue, leaving only the veins. I did some research on this and found out it is called the “hollyhock sawfly.” I’ve tried to control it (foliar sprays) but with minimal success. What do you think. (A. G., Western Maryland)

A. The hollyhock sawfly (Atomacera decepta), also known as hibiscus sawfly or mallow sawfly, can completely skeletonize hollyhock and rose mallow (Hibiscus moscheutos) as well as some other hibiscus species. (Ironically, it is not known to attack rose of Sharon).
Adult female sawflies, which appear like small black flies, about 0.25-inch long, lay eggs in the upper surfaces of leaves in late May/early June. Eggs are laid near the leaf edge, creating blister-like swellings that are the first sign of the pest. Newly hatched larvae initially feed underside of the leaf – and their immature mouthparts only chew the lower leaf epidermis. Monitoring at this stage reveals transparent skeletonized leaf “windows.”
The larva appears pastel green in color, with a dark head and a black spine on each body segment, and about 0.5-inch in length. Like other sawflies, hollyhock sawfly larvae typically feed together in groups. Mature larvae feed on both lower and upper surfaces of the leaves. If not controlled, they can turn a plant into a lacey skeleton within a matter of a few days. They pupate in cocoons spun at the base of the plant.
The biggest problem with this sawfly is that it has up to six generations per year. Adults are active from mid spring until frost, so multiple, overlapping generations occur, three to five generations, depending upon the weather. And a plant with leaves showing only the main veins and petiole left is not saleable.
Control depends upon your vigilance. While control of the first generation typically reduces the numbers of the second, there is still time to win the total battle this season. Handpicking larvae/pruning out affected tissue is the first line of defense. Spraying with pyrethroids such as bifenthrin, permethrin or cyfluthrin or with spinosad (Conserve) is the second line of defense. Good coverage is very important. Repeated applications are necessary at approximately three-week intervals. Consider a fall or late winter soil drench of imidacloprid for next year.
Note : Mississippi reports some resistance in H. acetosella, H. aculeateus and H. grandiflora.

Q. I planted out lots of annual vinca on some commercial sites. It looked fine going in, but now at least 50 percent of the leaves have small black spots and are yellow. Is this a disease that I can control easily? (K. B, Delaware)

A. There are two leaf spot diseases of vinca, caused by the fungi Alternaria and Ulocladium that cause the symptoms you describe. Both can attack vinca in the greenhouse as well as the landscape. Regardless of any potential infection before planting, the rainy spring weather undoubtedly promoted the spread of the fungus, and low fertility soil conditions helped to speed up the infection.
Initial symptoms first appear as small brown to black spots on the lower leaves about the size of a pinhead. Fungal spots soon spread to the leaves, shoots, and petioles; larger spots develop a pale bull’s-eye appearance. Diseased leaves eventually turn yellow and drop off. Infected vinca often has a tufted appearance, since the older leaves drop off first (from the base of the shoot) and the youngest leaves remain.
Vinca cultivars differ in resistance to Alternaria leaf spot. Highly susceptible cultivars include ‘Cooler Grape,’ ‘Cooler Peppermint,’ ‘Cooler Blush,’ ‘Tropicana Pink’ and ‘Little Blanche.’ More resistant cultivars include ‘Tropicana Rose’ and ‘Paraso.’
To control, try to scheduling overhead watering during either the predawn hours (4 to 8 a.m.) or midday to minimize wetting of the leaves — or install a drip irrigation system. Remove infected plants immediately following the first hard frost. Fungicides (both Heritage and Chipco 26019 are labeled) can help protect the plant from new infection, but must be applied every seven to 14 days after symptoms first appear on the lower leaves. (Note that several studies on thiophanate methyl fungicides used on Alternaria leaf spots actually made the disease more severe).

Q. I’ve had it with spruce. They just can’t hack it here. So I avoid them. But now a client insists on a blue spruce… A (colleague) suggested Meyer spruce. Do you know if they have the same issues as Norways/Colorado blues? (T.R., Southern Maryland)

A. Both Colorado blue spruce and Norway can be killed from the lower branches up by Cytospora canker, due to drought. Meyer spruce (Picea meyeri) is indeed reported to be tougher, as it is native to Northern China-Mongolia (semi-arid grasslands) and hardy to zone 3. It is as blue as any high-quality Colorado blue spruce, but the branches are more flexible and the needles are soft (vs. sharp-pointed). It is considered to be insect- and disease-resistant, but time and duration in this country will tell.