Gardeners’ queries

Press, 9 May 1980, Page 7

Gardeners’ queries

I am enclosing g few leaves from my young rhododendron. The plant looks pretty sickly, and I would appreciate your advice. — St Andrew's Hill (Chch). The symptoms displayed on the leaves of the rhododendron are strongly suggestive of a physiological disorder. These shrubs are generally intolerant to poorly drained soil, too-deep planting, drafty positions, and very hot and dry situations. Check these factors out and remedy if possible, otherwise it would be advisable to plant elsewhere.

This weed is encroaching on my lawn In most places and Is also to be found on soil adjacent to the lawn borders. It seems very difficult to remove. What is it and how is it to be eradicated? Also, I have tried to get rid of "mushrooms" on the lawn, using thiram. Will this do the trick?—M.L. (Chch). The weed on your lawn is Hydrocotyl species, a most invasive plant which thrives in moist conditions. It is, unfortunately, virtually impossible to eradicate from a lawn by chemical means at present, although there is supposed to be a material currently under trial. Anyone who knows of a positive method of controlling this weed in lawns without killing the grass could reap a good reward, judging by the numerous areas which are being over-run by it. The best treatment for lawns which

have fairy rings inl’them is to apply a 10 per cent formalin drench. The treated area should remain covered with wet sacks, or similar, for at least one week, after which the soil must be fully aerated before resowing it.

The lettuces in my small glasshouse are rotting off at ground level just as they start to heart up. Could you recommend a suitable spray to use? Should I have been spraying anyway before this rot, to prevent it? Also, how does one go about cleaning the soil in a small glasshouse to ensure the next crop grown is not contaminated?—M.M. (Chch).

It is always desirable to include specimens as well as to give descriptions of problems, especially when they relate to diseases. Sclerotinia root rot is possibly responsible for the collapse of the lettuces. The advanced stages of this fungus disease produces copious white fluffy mycelium and black fruiting bodies. There is no control once the disease has manifested itself on a plant. Liberated spores can remain viable in the soil for

at least two years, so it is essential to pull up and burn infected lettuces and to sterilise the soil before replanting. Formalin, or dazomet, can be used to sterilise the soil but application, in particular to the former, and subsequent follow-up, must be thorough and carefully done.

Could you tell me what has suddenly affected my celery and what I can do about it? —"Wondering" (Chch). The disease known as leaf spot is responsible for the miserable appearance of this celery. This fungus disease is of a highly infectious nature, favours warm, moist conditions, and can remain viable in the soil and on infected celery put on the compost heap. Young and old plants alike are equally susceptible. Generally, it pays to start spraying for control at an early stage In the

development of the crop. Any copper-based material, or zlneb. should be used.

I wish you could give me information about garlic — when to plant, and anything else of importance. — J.Mc. (Chch). Garlic is generally planted In autumn and even now is not too late. It has been found that it produces a better shaped bulb in lighter soils, but can be grown well in most types, providing drainage is good. The cloves can be spaced 150 mm apart, with 300 mm between the rows. Culture is much the same as for onions, weed control and regular moisture availability being of paramount importance to good development.

My parsnip crop has usually been relatively disease free. However, this year the leaves

of most plants have shown a rather greyish appearance, as per enclosed sample, and many of the roots look to have a reddish-brown discolouration about their shoulders. What are the problems and their remedies, if any? — "Grower" (Rangiora). The leaves display Infection by powdery mildew, a disease which has become fairly common. Damage to the foliage is rarely severe enough to jeopardise the crop, although young plants are more likely to suffer. Infection can reduce the individual size of the rqots. Colloidal sulphur is recommended as a control, if warranted. Parsnip canker, again a common occurrence where parsnips are grown, is responsible for the discolouration of your parsnips. Badly infected roots usually suffer from a secondary disease —

soft rot. There are various recommendations for the control of parsnip canker which includes good drainage, crop rotation, liming, deep friable soil and earthing up, and sowing a little later. I think that wet conditions and rotation have as much to do with the incidence of the disease as anything else.