Plant Health Problems

Diseases caused by Fungi:

Rhabdocline needlecast, Rhabdocline spp.
The first symptoms of this disease usually pass unnoticed in late summer and early fall as yellow spots on needles of the current year's growth. By late winter and early spring the spots develop their diagnostic characteristic and appear as reddish-brown bands or spots on the needles. These bands have very sharp borders between the healthy green and the brown tissues. Infection often begins on lower limbs. Trees that have been infected over a period of years appear thin and usually have only one year's needles at the tips of the branches.

This disease can be managed by selecting a site with good air circulation and by maximizing tree vigor with good cultural care. It is helpful to prune and remove any symptomatic branches when the bark and needles are dry. Tools are best disinfested with 10% bleach or 70% alcohol between cuts. Control can also be achieved with the use of fungicide sprays applied when new growth emerges in the spring. Several applications may be necessary, especially when wet weather persists. Among the compounds registered for use in Connecticut are chlorothalonil and mancozeb. Consult the label for dosage rates and safety precautions. For more information, see the fact sheet on Disease Problems in Connecticut Christmas Tree Plantations.

Swiss needlecast, Phaeocrytopus gaumanni.
Symptoms are evident in late winter and early spring as needles develop a yellow, mottled appearance. These gradually turn brown and look "dirty" due to the presence of numerous fruiting bodies of the fungus popping out of the stomates. Infected needles eventually drop and trees appear very thin.

Factors which maximize tree vigor help to counteract defoliation. It is also helpful to prune and remove infected branches to minimize spread to healthy limbs. Tools are best disinfested with 10% bleach or 70% alcohol between cuts. Control can also be achieved with the use of fungicide sprays applied when new growth emerges in the spring. Several applications may be necessary, especially when wet weather persists. Among the compounds registered for use in Connecticut are chlorothalonil and mancozeb. Consult the label for dosage rates and safety precautions. For more information, see the fact sheet on Disease Problems in Connecticut Christmas Tree Plantations.

Insect Problems:

Cooley spruce gall adelgid, Adelges cooleyi.
This species of adelgid, Adelges cooleyi, may spend a portion of its life cycle on the needles of Douglas-fir and the remainder of its life cycle forming galls on blue spruce twigs. Some populations may develop entirely on Douglas-fir. Cooley spruce gall adelgids overwinter in the form of immature females on the Douglas-fir needles. They mature in the spring and lay eggs that hatch soon after the new growth begins in May. The young nymphs attach themselves to the needle, and while feeding, cause the needle to become laterally bent. A yellow spot often shows on the upper side of where an adelgid has been feeding. Approved control methods consist of spraying early in April to kill overwintering adults, using a horticultural oil or insecticidal soap, which are among the compounds registered for use against this pest in Connecticut. Horticultural oil can cause yellowing of foliage on Douglas-fir if applied after bud break. Imidacloprid is very effective as a soil-applied systemic treatment. Consult the labels for dosage rates and safety precautions.

Gypsy moth, Lymantria dispar.
Oak is a preferred host of the gypsy moth, and solid stands of oak are subject to periodic defoliation. When fully grown, the caterpillars are between 2 and 3" long, dark gray or brown with prominent light brown hairs. Some have a light narrow stripe along the back and all have two rows of tubercles bearing hairs. From the head, the first five pairs are blue, and the remaining six pairs are brick red. They feed during May and June, and do most of their feeding at night.
Caterpillars pupate in cracks or crevices spinning a very small amount of silk. The moths emerge in about 2 weeks. The female is buff with narrow zigzag lines across the forewings. The wingspread is about 2", and the body is so heavy that the female cannot fly. The male is reddish-brown with variable light gray and dark brown markings and a wingspread of 1 to 1 1/2". The males fly freely.

Eggs are laid on the bark of trees, on stones, or lumber. They are laid in masses of about 400 eggs and covered with buff hairs from the body of the females. Individual shade trees may be sprayed.

A fungus, Entomophaga maimaiga, discovered by Station scientists in 1989, is giving natural control of larvae. Larvae killed by the fungus characteristically remain on the tree with their head hanging down. As these are the source of fungal spores needed to infect future populations of gypsy moth caterpillars, do not destroy them. The fungus originated in Japan and was introduced to the Boston area via infected gypsy moth larvae in 1910. The fungus was never recovered, despite attempts in subsequent years, and so it was thought not to have established. This fungus grows best in warm, humid weather.

Should chemical controls become necessary, sprays can be applied when caterpillars are young, about 1/4" long. Carbaryl, malathion, methoxychlor, spinosad and Bacillus thuringiensis var. kurstaki, which are among the compounds registered for control of this pest in Connecticut, are all effective treatments. Consult the label for dosage rates and safety precautions.

Spruce spider mite, Oligonychus ununguis.
Considerable injury is caused each season to conifers in ornamental plantings by the spruce spider mite, which feeds on the needles and webs them like twospotted spider mites. The trees take on a rusty brown appearance. Ultrafine horticultural oil, which is among the compounds registered for use against this pest in Connecticut, sprayed during the dormant season will control the overwintering eggs. Consult the label for dosage rates and safety precautions.