Leaf spot must be arrested or it will kill

Press, 24 October 1980, Page 10

Leaf spot must be arrested or it will kill

GARDENING

M. Lusty

A reader complains about the health of his iris plants. My Evansia iris which grow under trees have this spring developed badly affected foliage. Some of the fans are rotten where they emerge from the cane-like stalk and then the leaves are dying from the opposite end. The spots appear and gradually the leaves become papery in feel and colour. Not all the leaves are affected but those that are have not had any flowers. Some were affected in this way last spring. They are growing in a cold position in winter and receive little sun from now on — H.C. (Winchester). The characteristic of this particular disease is the development of irregu-

lar eye-spots which vary in colour and size until they more or less coalesce, the disease being known as leaf spot. Although this fungus will not kill the rhizome it has a weakening effect through the death of the foliage, and repeated attack of the trouble can eventually lead to the death of the plant. Reinfection occurs from wintering spores on dead or decaying foliage and is stimulated by wet conditions which, over the last few winters, have been conducive to build-up of the problem. Suggested remedial action includes periodic spraying of the plants with Bordeaux mixture plus wetting agent, or zineb. All old leaves should be stripped from the rhizomes and burned.

Over many years I have collected a number of books written to help the home gardener. However, in more than 50 years of gardening I have yet to know what type of vegetables and flowers are lime lovers or those that prefer alkaline conditions, or is it better to aim at a neutral factor. Is there any source of information where this knowledge can be obtained, or must trial and error be the only way? —G.B. (Chch).

There are a number of books and other publications which make reference to the tolerance of various plants to soil acidity or otherwise. I will list some references later. Most plants prefer a slightly acid medium and very few indeed will tolerate either very acid soils or alkaline conditions. Rhododendrons, azaleas, erica, camellias, pittosporums, pieris and kalmia are some of the ornamentals which grow happily in acid soil. Kumara, potato, parsnip, peppers, endive and sweet corn all prefer slightly acid to acid conditions for optimum development. One should be careful not to be become too engrossed in changing the pH (soil acidity rating) too much as there are other factors involved which can have

an influence on the prevailing situation. As a general guide if your 'plants are thriving they are happy. A pH test could be helpful in determining specific requirements, if necessary. Could you please advise what to use to remove the black deposit covering much of the leaves of my lemon tree. How do you stop this disease from getting on to the tree; it is even spreading on to some of the fruit. No spraying has been done but we did feed ■ with citrus food according to recommendations. — T.T. (Kaiapoi). The black deposit is a fungus but not a . disease, which manifests . itself cn your citrus tree by the presence of honey dew. The honey dew is exuded by various species of both scale insects and aphids, but in this case it is more than likely to be the former. Control of scale can be achieved by spraying with one of the proprietary spraying oils other than winter or red oil; use according to maker’s recommendations. It is essential to be very thorough with the application

and to make sure that the branches as well as the leaves, undersides as well as surface, are covered and to repeat the application after three to four weeks. I recently came across a rather unusual’ (to me) plant which is growing in rather poor soil close to a north-facing wall. The plant has rather drooping rush-like leaves, about two feet in length, and flowers of red green and blue; when fully open they appear tassle-like. I have enclosed some for you to see as this would make it easier , than my explanation. I would like you to identify the plant and also please to advise how it can be grown and where. — “Intrigued” (Chch). The plant which you have discovered is Billbergia nutans, variously called Angel’s Tears, or Queen’s Tears, or Snips friend. This bromeliad is one of the very few of the genus which is hardy enough to survive in a sheltered position outdoors and even this, species is more likely to be seen as a pot plant around this

part of the country. Good drainage is essential and although resistant to dry conditions, the addition of water during the growing season will pay dividends. Propagation can be from suckers or side-shoots, or division of old plants, late autumn or early spring usually being the best time for this — but not until the character of the parent plant has been assumed.