GARDENERS’ QUERIES

Press, Volume CXII, Issue 32902, 28 April 1972, Page 11

GARDENERS’ QUERIES

I enclose some leaves from my tomato plants. The disease seems to have come on quite suddenly, and many of the plants are looking quite sickly — limp and yellowish. The tomatoes do not seem to have any deformities or blemishes, but are not of any size. Is there a disease or deficiency involved, and if so, what remedial measures are recommended?—

J.R. (Christchurch). ) Leaves sent in bear the [disease Verticillium dahliae, [the casual organism of Verticillium wilt. This is a soil- ■ borne fungus disease with a wide host range including many ornamental plants such as dahlias. Initially, wilting takes place only in the hot part of the day, but as infection increases in severity wilting increases and stunting of the plant occurs. Light brown staining of some of the tissues within the. stem also becomes evident. The disease is carried over from year to year in the soil. As there is no effective control which can be used in the home garden it is advisable to dig up the infected plants, roots and all, and dispose of them, preferably by burning, but certainly not by composting. It is very strongly advocated to refrain from [growing tomatoes, or such [crops as potatoes and cucumIbers, for some years in i ground where plants have 'been affected by Verticillium I wilt. I have a large conifer

growing in the garden, a piece of which I have sent with this letter. It is developing patches all over it which spoils the golden appearance. Please could you state the trouble and give the remedy, if any?— “Tree Lover” (Oamaru).

The specimen branch from Thuya plicata zebrina is suffering from an infestation of tent caterpillars which are very prevalent on very many different kinds of conifers at the moment. Very thorough spraying is necessary, using D.D.T., or arsenate of lead. If left, these pests can very often defoliate a tree and are a very common cause for the debilitation of conifer hedges. I should be grateful if you could help me with my begonia which used to be a fine healthy specimen but now many of the leaves develop a burnt look, wither and fall off. The plant is not in the sun. I have sprayed with karathane and malathion, with hakaphos as a fertiliser spray and also have given it nitrophoska. The new leaves are green, healthy but much smaller than previously. — “Begonia” (Christchurch).

The trouble here is caused by Botrytis cinera, a disease which causes blotching and grey mould to develop on the leaves. The symptoms which you have described are very typical of this disease. Ail diseased leaves should be removed and burnt but if the ■plant is very badly infected jit would be advisable to get rid of the plant itself. It is important to handle begonias with care to avoid damaging them in any way; they should be given good ventilation, and although they like moisture drainage must be good, and plants should not be damped down too much. Karathane is a specific fungicide for the control of powdery mildew only (a disease to which' begonias are very susceptible) and malathion (maldison) is an insecticide predominantly for use against aphids. Mild attacks of botrytis on begonias can be' controlled by spraying with! zineb. It is not recommended to overfeed these! plants. I have a grape vine which was grown from a slip four years ago. The variety is, however, in doubt except that I think it is Albany Surprise, but it could possibly be a Black Hamburgh. Could you please advise me how I can identify the variety? The vine is a vigorous grower and last year was fruned to the second buds, am wondering if the large quantity of leaf produced this season should have been reduced, as a small proportion of the grapes on each bunch only ripened. The. remainder, in most cases, did not develop, and remained entirely green. I would be grateful if you would comment upon this, and advise me if a change in pruning technique would help in reducing leaf growth, if this is at the expense of evenly ripened fruit.— “Sour Grape” (Ashburton).

The difference between the two grapes has often caused confusion. Albany Surprise is far more mildew- resistant and only develops a sweet flavour when very mature. Black Hamburgh contains larger seeds and has a lighter, softer leaf which is tomentose on the under surface. The foliage of Albany Surprise is inclined to be a much deeper green and is the same colour on both sides of the leaf. Black Hamburgh does not bear as heavily in our climate and probably requires a longer period of warm weather to do well. Early summer pruning is advisable to enable maximum penetration of light, but care must be taken not to prune too much, as the fruit can get scalded. It is usual to prune back to two leaves past the first bunch of grapes when summer pruning. What, if any, fertiliser could be added to wellrotted, pure leaf mould for the production of vegetables in the kitchen garden?—R.l. (Christchurch).

Initially, the addition of a quick-acting nitrogenous fertiliser, such as sulphate of ammonia or urea, should be added to the leaves. When intended for use in a general way lime is beneficial but not if the leaf mould is to be applied around acid-loving plants.

Weed for identification. (G. 8. Christchurch). —Unfortunately, the weed specimen was too small and in far too desiccated condition for identification. There is as yet no effective control of oxalis in the garden and probably the.laborious method of digging out individual plants will have to sulfice in the meantime. It is reported that applications of Weedazol have given some control.