Gardeners’ queries

Press, 24 February 1978, Page 6

Gardeners’ queries

This leaf is from a rhododendron which we planted last autumn. It has had plenty of water and regular dressings of a mixture of superphosphate potash and sulphate of ammonia. It appears to be growing quite well, but this brown discolouration is now occurring on the older leaves. Advice would be appreciated.— E.S.M.N. (Chch). Unless pathogenic symptoms are very specific it is generally not feasible to make a diagnosis from single leaf specimens. This applies particularly to certain problems and also those associated with some plants, such as rhododendrons. There should be no need for regular feeding of these plants unless a soil analysis has indicated otherwise. Rhododendrons tend to display various leaf symptoms when conditions do not suit them, such as too dense shade too hot or too exposed to wind. Again, odd leaves may tend to show blemishes irrespective of where the plant is growing. Enclosed are samples of several conifers in our

garden. W e have sprayed with pyrox and all season spraying oil and also with kelfhan'e which is specific for red spider, but none of these have appeared to help. Already two trees hav e died and it is heartrending to think the others will too. Even the yew is affected. I should be most grateful for any advice you can give.— M.T. (Chch). I think the reason for the browning of so many of these conifers is due predominantly to physiological causes and would suggest dryness and sunshine for this. Yews are very prone to the effects of such prevailing conditions and other conifers tend to vary in their susceptibility, much depending on the situation in which they ar e growing. Some of the golden foliaged forms are inclined to display some discolouration during the warmer, drier months but unless conditions become extreme effects are rarely fatal. Please could you tell me what is wrong with my four-year-oid grapevine. It has a heavy crop. All bunches are becoming affected as per sample. Ten days ago I sprayed with Carboral SO for roller leaf grub, the fruit was becoming affected before spraying. The vine has had adequate water. Your advice would be appreciated. —"Worried" (Ashburton). The problem is one of disease and not pest, therefore carbaryl, which is a suitable control for eaterpillars, is ineffective. Botrytis rot is responsible for the condition of your grapes. This disease eventually covers all infected bunches of grapes with a feltlike grey mould. Attack tends to follow some damage to the developing fruit and remedial measures are by then too late. Tile use of thiram or benlate from mid-January on is recommended where this disease has been a problem. Protection from damage and good ventilation are also important Diseased fruit should be promptly removed.