QUESTION: We are having a real problem with our raspberries. Just as the weather turned warm and the fruit was halfway developed, the leaves wilted, then turned yellow and brown and died. It’s a mystery to us as we have kept them well watered! In addition, the new canes produced this spring look just fine. What’s causing this? Should we spray them with something?

ANSWER: Based on the symptoms you described, I suspect your raspberries are infected with root rot. Unfortunately, this is a common problem on raspberries throughout Western Washington. Plants infected with root rot fungi not only have rotted roots but also lack fibrous roots. After hot, dry periods, older leaves may wither or become bronzed and scorched. Affected leaves flag. Fruit stems usually are shortened and berries, if formed, remain small and often wither before ripening. Once the root system begins deteriorating, new roots may arise from above decayed ones in summer.

Although this makes plants look as though they have recovered, the new roots are often weak and lack lateral development. The new roots in turn will become infected during the cold, wet weather this fall and winter so that the plant progressively declines and is unproductive. No Pacific Northwest red raspberry varieties are very resistant to the problem, however Chilliwack, Meeker, Sumner and Summit are moderately resistant. Young Meeker plants are very susceptible but mature plants seem to have some field tolerance.

Home gardeners can reduce root rot problems by using certified planting stock and setting plants in fertile, well-drained soil which has a three to four foot deep water table in winter. Select a site that has not grown raspberries or other bramble fruit for several years. Place plants in beds raised so that the top of the bed is at least 12 inches above the surrounding soil. Slope soil away from the center of the planting bed to the alleyway between rows.

The fungicides Aliette, Agri-Fos, Fosphite, Phostrol and Ridomil Gold SL are all registered for use to help prevent raspberry root rot. Most require multiple applications and must be used well in advance of fruit harvest.

QUESTION: We have a Colorado Blue Spruce tree and noticed over the weekend that the top has died. The rest of the tree looks perfectly healthy. What caused the top to die?

ANSWER: White Pine Weevils are notorious for attacking Colorado Blue Spruce in our coastal area. The larvae of this insect do the most damage by killing leaders of infested trees. Leaders with expanding buds droop, forming the typical “Shepard’s hook.” The weevils are often hard to find but their damage, dieback of the leader, is easy to spot. The weevils are light reddish-brown with several patches of white on the wing covers. Like most weevils, the adults have a long snout-like beak from which arise knobbed antennae. The larvae, which are found under the bark of the dying leader, are white, legless, C-shaped and have brown heads.

This pest overwinters in the adult stage hiding in the duff under trees. In early Spring, around the middle of April, the adult females climb to the leaders of trees and feed through small holes chewed in the bark. Eventually, the females will insert eggs into these feeding wounds. The larvae hatch in about a week and feed just below the bark. The larvae continue to feed downward, girdling and killing the shoot as they go. The larvae mature and pupate in mid-July. The pupae rest in cells carved into the sap wood and lined with straw like sawdust. The adults emerge in late July and early August. The adults feed on the upper branches, making small puncture wounds. As winter approaches, the adults return to the duff to hibernate.

The easiest method of controlling this pest is to simply prune out and destroy infested leaders. As soon as the infested leader droops, cut the leader out to just below where the bark discoloration stops. Do not throw this pruned leader on the ground as some of the weevils may survive. If the damage is stopped before the first whorl of branches is reached, a new leader can be easily trained.

There are no chemical controls registered for use in controlling White Pine Weevils in home gardens.

QUESTION: This last week when the temperatures reached 85 degrees F. the leaves on our Japanese maple actually got scorched. Is there anything we can do to speed up its recovery?

ANSWER: Japanese red maples are more susceptible to leaf scorch from high temperatures than many other small trees. Scorch damage may appear anytime temperatures exceed 85 degrees F. Conditions causing rapid loss of water in leaves do not alone produce leaf scorch. Usually these conditions must be coupled with other unfavorable growing conditions that might prevent rapid uptake and reduce flow of water to leaves. Drought is the most common cause of this condition.

Less obvious conditions that could cause scorch include shallow soils that overlie rocks or hardpan. Poor, heavy soils as well as soils that do not retain water well are frequent contributors to leaf scorch. Layers of asphalt or concrete over root systems often add to the problem because they tend to keep soil dry and may cause heat buildup.

When leaf scorch is noticed, the tissue has usually dried past the point of recovery, but several steps can be taken to prevent more severe damage and improve the condition in subsequent years. Thorough, deep watering will usually help increase water uptake. Conserve soil moisture by mulching trees and shrubs with rotted leaves, bark or compost. If watering is necessary, be sure to water thoroughly because mulches absorb water from the surface. Light waterings will do no more than wet the mulch. Practices that improve growing conditions usually help prevent leaf scorch. Although your Japanese maple may have lost its aesthetic appeal for this growing season, it should totally recover. If another heat wave is forecasted, use a screen to provide shade to the foliage and consider moving the plant next winter to a cooler (less intense sun) location in the garden.

EDITOR’S NOTE: For answers to local gardening questions, contact Master Gardener Earl Miller at 642-0541 or e-mail him at eymil@charter.net.