University of Maryland Extension

July Tree and Shrub Tips

damage on azalea leaves from lacebugs
Photo: Lacebug damage on azalea

(More tips from HGIC)

Insect problems

  • Lacebug feeding (see photo above) is coming to a peak on hawthorn, serviceberry, oak and sycamore as well as rhododendrons, azaleas,Japanese andromeda, and mountain laurel. Look for small white or yellow spots on the upper sides of leaves and small black fecal spots on the undersides. Isolated shrubs grown in full-sun are more susceptible to lace bug damage. They are more of a problem on stressed plants on exposed sunny sites. Monitor the feeding damage to catch it early. Insecticides are only necessary on trees three years or younger. (PDF HG 95)
  • The hemlock woolly adelgid is an aphid-like sucking pest that can severely damage hemlock trees, particularly when they are stressed. Adelgid nymphs are feeding now and can be sprayed with ultra-fine horticultural oil throughout the summer. Follow label directions carefully. Oil sprays should be applied to dry foliage only when temperatures are below 80 degrees F. Research has shown that excess nitrogen may enhance adelgid populations on hemlock, so it is advisable not to fertilize hemlocks that are infested with the hemlock woolly adelgid. (PDF HG 2)
  • Bagworm caterpillars are now very active. Look for the little bags crawling around on evergreen trees and shrubs and be prepared to spray infested trees with the microbial insecticide, Bt before late July. After late July the older bagworms are not well controlled with Bt They are best collected by hand and destroyed or sprayed with insecticides containing spinosad. (PDF HG 32)
  • Fall webworm nests may be noticed on various shrubs and trees. Webworm is easily controlled by pruning out and disposing of the nests in the trash.
  • Soft scales may be found feeding on a variety of shade trees. Soft scales are insects related to aphids but they do not move once they are adults. They have a white waxy covering. Control them with a summer-rate application of horticultural oil. Oil sprays should be applied to dry foliage only when temperatures are below 80°F.
  • Spruce spider mite usually becomes very active this month feeding on evergreens, especially dwarf Alberta spruces. Their feeding damage is favored by hot dry weather and can be very devastating. Control them with a strong stream of water or an application of ultra-fine horticultural oil. Be aware that oils will discolor blue spruces; the oil temporarily removes the wax coating, turning the trees green. Oil sprays should be applied to dry foliage only when temperatures are below 80°F. 
  • Boxwood mites produce small white spots, known as stippling, on leaves. Often predators will keep them in check but if the mite damage starts to be increasing spray with horticultural oil at the summer rate to kill mite eggs and adults. Another pest of boxwood that also appears in July is the boxwood psyllid which causes new leaves to cup inward. It is usually not a serious problem unless so severe that the new growth and overall health of the plant is damaged.
  • Rose slug sawfly larvae are voraciously feeding on rose leaves at this time. These look like small bright green caterpillars that eat small holes in the leaves. They can be handpicked or use a systemic, all-in-one, rose product. Aphids may also be active on roses but predators and parasites will soon control them. 

Diseases

  • Volutella stem blight can be observed on boxwoods. This is a very destructive fungal disease that kills the entire stem or branch of boxwood. Look for stem discoloration and salmon colored fruiting bodies on leaves and stems. Prune out damaged wood and open the shrub up for better air circulation with selective pruning.
  • Apple scab is a fungal disease that produces olive colored lesions on crabapple trees and will cause some leaf drop. Plant resistant varieties to eliminate this problem. There is no cause for alarm.

General

  • If you haven't applied mulch to your landscape now is the time to discourage weeds and help conserve moisture. Mulches should be applied 1-2 inches deep and kept away from tree and shrub trunks. Mature trees do not really benefit from being mulched except that mulch keeps lawn mowers and string trimmers from damaging the trunks.
  • Problem branches (very low or storm damaged), may be removed from trees and as needed. It is not necessary to wait until fall for this type of (PDF) pruning.

 

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