Soil-Borne Diseases Difficult To Control

Press, Volume CVI, Issue 31166, 16 September 1966, Page 9

Soil-Borne Diseases Difficult To Control

Some of the diseases which are more difficult to control are those which are soil-borne, and can only be checked or eradicated by treating the soil, perhaps by sterilising, crop rotation, the use of special chemicals or application of fertilisers.

Damping off is widespread and although it can seriously affect crops which have been planted out, it is far more troublesome in the glasshouse where infected soil can cause major losses. Attack may take place before the seed reaches the surface of the soil which is symptomatic of pre-emerg-ence damping off.

However seedlings may become infected shortly after germination and collapse soon after they emerge from the soil, the result of postemergence damping off. The only efficient means of control is by thorough disinfection of the soil. This can be done with chemical application to the soil of materials such as chloropicrin or formalin, or by means of steam, and should include all receptacles and tools used in holding or handling the soil. Verticillium wilt is another disease caused by two closely related species of fungus, and has a very wide host range which includes both ornamentals and plants of economic importance.

The first noticeable feature of the disease is a wilting of the terminal growths if it occurs in summer. In winter the more usual primary symptom is a yellowing of the lower leaves. Wilting may only be temporary, giving the impression of recovery, but subsequently the incidence returns with a pronounced increase of the gradual discolouration of the lower leaves which eventually turn brown. Plant collapse and death frequently follow quickly. The disease itself is not noticeable externally but stained tissue, noticeable if the stem is cut open, shows its presence. Seasonal carry-over in the soil can only be counteracted outside, to any degree, by good crop rotation, burning of diseased plants, destroying weed carriers such as fathen, and by preventing growth checks. Disinfection is the most effective control in the glasshouse. It should also be remembered that spread can be facilitated by soil on implements, footwear and by transplanting infected plants. Club root is a fungus disease confined to brassicas which are variably sus-| ceptible to it. The first symptom may be undue wilting of advanced seedlings. Yellowing of the leaves through lack of sufficient nutrients, and swollen and distorted roots will also become visible on closer examination.

Retardation of growth, resulting more or less in a

stunted plant, is more usual than ultimate death which however, can occur more readily to those attacked early in the season. Spores are released into the soil from decaying root systems and can remain viable for more than five years and survive to a depth of 12in. Spores can be spread by cultivation, surface drainage water, on infected seedlings and kept perpetually in the vicinity by susceptible weeds.

More active in acid soil, club root can be greatly restricted by increasing the pH (of the soil by means of limi ing. Acidulated mercuric i chloride or calomel dust used as a direct root treatment have proved successful and also a newer material, PCNB, ! which has the advantage of i being non-corrosive. ' One of the most damaging , and widespread diseases of ‘vegetables in New Zealand is isclerotinia rot which has an i exceptionally large host range. Small round hard black pustules characterise its presence in advanced stages of infection and a cotton wool like mass of growth is also evident on some plants such as beans and lettuce. At the onset of the disease, which jean attack at any time until :the plant is fully developed, (wilting occurs. Attack usually commences at ground level : but leaves and fruit which are in contact w-ith the soil, and higher regions too, are just as likely to be a launching point for the disease. High humidity and wet soils in association with poor drainage, too close spacing and lack of sufficient aeration, are factors conducive to the development of sclerotinia rot. To avoid the formation of spores, which can remain viable in the soil for at least two years under the most adverse conditions, infected plants should be lifted and burned as soon as identification of the disease has been verified. On no account should suspect diseased crop remains be consigned to the compost heap. There are also two other soil-borne diseases which could be mentioned and they are potato scab and powdery scab but as they concern a specific plant only they are dealt with separately. The presence of soil-borne diseases emphasises clearly the necessity to observe a few rules to avoid trouble. They are summarised briefly. Rotation of crops is important not only for the soil, i but to reduce as much as possible the build-up of particu- ( lar over-wintering diseases. Good drainage is necessary to avoid water-logged soil conditions because few plants are capable of withstanding long periods of wet feet. Adequate spacing is required so that the plants have

room to develop naturally and satisfactory aeration is provided.

Diseased plants, or crop refuse should be adequately disposed of. preferably burnt but definitely not composted.