The Agent's Corner May 1993

QUESTION: When my roses bloom they have brown and black petal edges and are deformed. Also the leaves are sticky. Some of the leaves are covered with yellow spots mixed with the green color of the leaves. What is causing these problems and what can I do?

ANSWER: Your roses have two insect problems and a virus. The flower petals are brown or black because of a very small insect called the western flower thrip, Frankliniella occidentalis (Pergande). Adult thrips are about 1/8" (2 mm) in length, usually tan-to-dark brown-bodied, with four feather like wings. The young or nymphs are creamy white and wingless and develop into adults in about two weeks. The adults enter a rose bud and lay eggs inside the immature flower. The eggs hatch and the resulting nymphs and adults injure the plant by rasping the bud, flower and leaf tissue of host plants and then suck the exuding sap. This causes petal tissue to die and results in brown or black petal edges. Thrips also effect other flower, fruit and vegetable plants. These include apples and peaches which result in surface damage to the fruit. Onions, snap beans, chrysanthemums, gladiolus and iris are also damaged by other thrips species. There has been many more thrips the last couple of years because of the above normal rainfall which has provided abundant wildflower and weed crops for the thrips to live on and thus increased populations. The other insect problem is aphids. These small insects are yellow to green in color and suck sap from plants that they infect. The "sugars" which they do not metabolize are excreted and fall onto the leaves of the plant. This is the sticky, shiny substance that you see. Sometimes ants and flies will "milk" aphids for this exudate and feed on it. So if ants are spotted on plants there is a good chance that aphids are present. The yellow marks mixed with the green color of the leaves is a virus or a complex of several viruses. The spotted yellow-green leaf color is known as mottling and is very symptomatic of viruses. These viruses generally do not kill the plant but can weaken it.

Control: Several inlets are predators of thrips and aphids. These include ladybird beetles and their larva, minute pirate bug and lacewings. Thrips have alternate hosts of weeds and wildflowers. By controlling host plants thrip populations will be lowered. Because thrips do damage inside the rose buds a systemic insecticide should be used. There are several products on the market which control thrips and aphids systemically. Sometimes disystox, a systemic insecticide, is included in rose fertilizer. Always follow label directions when applying pesticides. To reduce the problems of viruses in plants purchase virus-indexed or certified virus free plants. Virus infected plants can be a source of infection that can be transmitted to healthy plants by aphids or other inserts. Therefore, control the aphids and other insect vectors to control the spread of virus diseases.

Rob Call
May, 1993