One of the things we like most about Chelsea, and indeed all RHS flower shows, are the visitors who only rarely visit Wisley and take the opportunity while at the show to meet the advice team face-to-face.
Even better they often bring a collection of samples and photographs of all the problems in their gardens that perplex them. Not only do these samples help us but also provide useful teaching material as well as the opportunity to get more pictures for our archives.
Raspberry rust has been very common this year. Our disease experts think that recent wet years have led to a build up of this disease.
The fungus actually produces a number of different spore types during the growing season of the raspberries, each of which causes a slightly different symptom. In the spring, small clusters of orange pustules are visible on the upper surface of the leaves. As summer progresses, these are replaced by numerous small orange pustules on the underside of the leaves. Towards the end of summer the orange pustules are replaced by very conspicuous, black pustules similar to those in your sample. These black pustules produce the resting spores of the fungus which can overwinter on fallen leaves. There are no chemical controls available to amateur gardeners for this particular disease, although protecting young growth with copper fungicide in spring may have some effect. Fortunately, even heavy infections do not seem to have a great deal of effect on the vigour of the raspberries. It may be worth moving and disposing of any leaves which fall prematurely, together with the rest of the leaves as they fall in the autumn. This will reduce the amount of the fungus capable of over-wintering in the vicinity of the canes, and may reduce the disease levels next year.
Here a grape vine is being damaged by a microscopic type of creature known as a gall mite (Colomerus vitis). This feeds by sucking sap from the undersides of the leaves, causing the affected parts of the leaf to bubble upwards. The undersides of these areas are covered with many fine creamy white or pink hairs, amongst which the mites live and feed during the summer. Later in the summer, these hairs dry up and turn brownish, at that stage the mites leave the foliage and seek overwintering sites on the stems and around the buds of the vine. Although the damage caused by the mite can appear alarming, this is a pest that has little real impact on the growth of the plant or its ability to produce fruit. This is fortunate as none of the chemicals available to amateur gardeners will give effective control of gall mites! On lightly infested vines it may be worthwhile picking off affected leaves in order to stop the infestation building up. However, if a large proportion of the leaves are already affected, you would do more damage by removing them than by leaving them on.
We look forward to more interesting and unusual problems for Friday and Saturday.