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South Central Gardening: Garden Pests and Diseases

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Practice vigilance in treating garden pests and diseases in the South Central region of Texas and Oklahoma.

Healthy zebrine plant, Healthy daylily

By Susan Albert

Most gardeners know they can expect to face insect problems or disease infections at some time or another. Those combined with adverse environmental conditions can wreak havoc with plants in Texas and Oklahoma.

There’s no need to panic when you observe something amiss, but vigilance is important. Look for signs of problems. Once you detect a problem, you can decide what action to take.

If the problem is not severe, you can wait and let nature take its course. For example, lady beetles are voracious eaters of aphids. With fungal diseases, simply removing and discarding the affected leaves as you see them may eliminate the spread of the spores. Garden cleanup in the fall can reduce overwintering insects and disease.

If the situation escalates, however, it may be necessary to use chemical controls. Insecticidal soaps and botanicals are thought to interfere less with beneficial insects, so they may be a good first choice.

Here are common problems facing the South Central region:

aphids on plant
  • Aphids are soft-bodied sucking insects, generally under 1/4 inch in size. They vary in body shape and color. They secrete honey-dew, which attracts ants and fleas, and also can promote a sooty mold. A sure sign of aphids is their discarded skins, which appear as white grains. You can blast small infestations with strong water pressure. Also, wait a few days and see whether natural enemies intervene. Chemical controls include products such as Garden Safe Insecticidal Soap Insect Killer; Ortho Max Flower, Fruit and Vegetable Insect Killer; and GardenTech Sevin Concentrate Bug Killer.
Fall webworms on tree
  • Tent caterpillars, forest tent caterpillars, and fall webworms, right, feed on foliage. Tent caterpillars build webs or tents around foliage in crotches of limbs, while fall webworms form webbing around foliage at the ends of branches. Forest caterpillars shun the tents altogether but congregate when molting or resting. Experts say the caterpillars may defoliate the whole tree, but usually without lasting harm. You can open the unsightly tents with a pole or high-pressure water spray to give birds access to the caterpillars. Spray insecticidal soap on forest tent caterpillars congregating toward the bottom of the trunk.
Leaf miner
  • Leaf miners form tunnels on leaves of ornamentals, vegetables, fruits, etc. The tiny larvae of moths, flies, sawflies, or and beetles feed on tissue between the upper and lower leaf surfaces. When you notice the larvae, prune the twigs or leaves; the larvae rarely cause much damage. For chemical control use products such as Ortho Max Flower, Fruit and Vegetable Insect Killer; or GardenTech Sevin Concentrate Bug Killer. Shown is an affected leaf, followed by the Zebrina plant, with unsightly foliage removed.
Rose rosette virus
  • Rose rosette virus symptoms include red or yellow leaf mottle, excess thorns, elongated shoots, and flower distortion. The virus is transmitted through grafting or eriophyid mites. The microscopic mites transmit the virus after feeding on infected plants. The mites can move short distances on rose plants, but they can be carried to new roses by wind, or on gloves, clothing, and tools. To slow the spread of the virus, space out roses and interplant them with non-rose species. There is no cure for RRV; you must remove infected plants.
Iris leaf blight
  • Bacterial leaf blight occurs on leaves of several species of iris, narcissus, and certain other ornamentals. The blight causes foliage to die, and reduces plant vigor. The disease begins with very small brown spots surrounded by yellow margins. As the spots grow larger, they develop whitish or grayish centers. There is no cure for the disease, so prevention is key. Splashing water and contaminated tools spread the bacterium. Remove infected leaves. Rhizomes are not affected, so transplanting may help.
Powdery mildew on leaves
  • Powdery mildew resembles a white to grayish film over the surface of leaves, stems, buds, and flower petals. Infected leaves may become distorted, turn yellow, and drop. Certain plants in our region are more susceptible such as ash, crape myrtle, lilacs, oaks, roses, zinnias and monarda. Humid conditions in shaded, crowded plantings promote the fungal disease. To prevent powdery mildew, look for resistant varieties. Remove and destroy infected plants. Water only in the mornings so the foliage is dry by evening. If cultural methods fail, follow a good spraying schedule of a fungicide.

 

Black spot on rose
  • Black spot fungus on roses favors a long, wet, warm period in spring. It begins as black circles with irregular margins, but may grow to include yellow halos. Eventually, leaves turn yellow and drop. Frequent defoliations weaken the plant. Buy resistant varieties, use drip irrigation in early morning, and remove infected leaves and stems. Use a fungicide to aid in prevention, if desired. (Shown is affected foliage, followed by healthy rose foliage.)
Infected daylily leaves
  • Leaf streak on daylilies is caused by a fungus. Symptoms include yellow streaks along the center margin, followed by browning or spots with yellow borders. Infected leaves may wither and die. Isolate infected daylilies from healthy plants. Purchase only disease-free stock, and propagate only from healthy specimens. Apply a fungicide to slow the disease. (Shown is an infected daylily, followed by healthy daylily leaves.)

 

Healthy daylily leaves
  • Whiteflies measure about 1/8 inch, and you commonly find the adults and nymphs on the undersides of leaves. With piercing-sucking mouth parts they feed on plant sap, causing leaves to yellow. When foliage is disturbed, a white cloud flies out. Saturate the underside of leaves with products such as botanical insecticides; Garden Safe Insecticidal Soap Insect Killer; Ortho Max Flower, Fruit and Vegetable Insect Killer; or Ortho Bug-B-Gon Multipurpose Insect Killer Ready Spray.

Practicing vigilance in the garden can detect or prevent many problems before they become severe.

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