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Plant protection: Pests and diseases to watch out for

June 27, 2013
What’s your offensive garden strategy? For winning plants, keep an eye out for these pests and diseases this July. The Plant Clinic offers this advice on late-summer garden pests.
Japanese Beetles
Adult Japanese beetles are now emerging. Typically, adult beetles feed in large groups on the upper leaf surface, leaving only a lacelike skeleton of veins. They can also devour flowers and ripening fruit, and if large populations are present, they can defoliate a large tree.
Feeding damage caused by beetles usually results in leaves turning brown, dying, and eventually falling off. Hand-picking the beetles off isolated plants or knocking them into jars of soapy water will reduce populations. Before applying any pesticide, get a recommendation from the Arboretum’s Plant Clinic at 630-71-92424 or
Gypsy Moths
Gypsy moths emerge in July. Examine your trees, outdoor furniture, firewood, and vehicles for pupae and egg masses. Scrape egg masses off with a putty knife. Remove other life stages by hand, and dispose of them in a container of hot water, household bleach, ammonia, or kerosene. Be sure to wear gloves, protective clothing, and a dust mask.
Scale Insects
Scale insects feed on plant sap with their long, thread-like mouthparts. You can see signs of scale insects in the yellowing of tissues, reduced plant vigor, and branch dieback. During feeding, excess plant sap can be excreted as a sweet, sticky material, which often attracts ants, bees, wasps, and flies as well as a dark fungus called black sooty mold. Check with the Plant Clinic to determine your type of scale insect and how to treat it.
As individuals, aphids do little harm to a host plant, but large infestations can produce severe damage. An aphid feeds on its host plant by sucking plant sap through a beaklike feeding tube inserted into plant tissue, thereby weakening stems and leaves. Check the undersides of leaves for small groups of aphids, or you may find them clustered on the new buds, stems, and young leaves of flowers. An aphid problem may often be a symptom of too much nitrogen fertilizer or an overuse of pesticides. Changing to organic, slow-release fertilizers and nontoxic pesticides and encouraging natural aphid predators such as ladybugs are important steps in reducing aphid populations.
Fungal diseases
If your plants experienced any fungal diseases this spring, you will continue to see signs of the fungi at work. Fungi cause about 85 percent of plant diseases, which spread easily by wind, water, soil or plant debris, and even from gardening tools. Powdery mildew is one common example. If your plants are showing signs of fungal disease, be sure to remove affected leaves as soon as they drop off and do not compost or mulch them. Dispose of them in the landscape waste.
Dutch elm disease
The elm bark beetles will be hard at work this month, spreading Dutch elm disease once again. The disease is most easily detected during early summer when the leaves on an upper branch curl and turn gray-green or yellow and finally brown. Valuable trees should be inspected weekly this month. An infected tree may be saved by pruning out the diseased branch promptly after seeing the first signs.
For more information about plant pests and diseases, contact the Arboretum’s Plant Clinic at 630-71-92424 or