Controlling Soil-Borne Diseases

Press, Volume CIX, Issue 32251, 20 March 1970, Page 20

Controlling Soil-Borne Diseases

Soil-borne diseases are I amongst the more difficult to control of those which attack; plants. Checking or eradicat-l ing them is often an involved or slow process and such [ methods as crop rotation, soil sterilisation, and the use I of special chemicals or fertilisers may be necessary. One of the most damaging' and widespread of such! diseases is scelerotinia rot which has a very wide 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. Attack usually commences at ground level and particularly leaves (and fruit) which are in contact with the soil are likely to be launching points for the disease. At the onset] of the disease, which can attack at any time until the ; plant is fully developed, wilting occurs. High humidity and wet i soils, in association with poor drainage, too close planting and lack of suffi- , cient aeration are predominant factors conducive to development of scelerotinia rot, particularly amongst vege- \ tables. To avoid the formation of ■ spores which can remain viable in the soil for at least two years under the most ! plants should be lifted and adverse conditions, infected ' tion of the disease has been ; ourned as soon as identifica- 1 verified. On no account] should suspect crop remains be consigned to the compost heap. Damping Off Damping off is widespread I, and although it can seriously , affect crops which have been ,

Iplanted out it is far more; in the glasshouse I where infected soil can cause I major and repeated losses. Attack may take place be- [ fore the seed reaches the surface which is symptomatic of | pre-emergence damping off. Seedlings may become infected shortly after germinai tion and collapse soon after (they emerge from the soil, the result, of post-emergence 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. Besides heating the soil it is most vital to treat al) receptacles and tools involved in i holding or handling the soil, i Verticillium wilt is another | disease caused by two closelyrelated species of fungus, and again it is one which has a wide host range, including both ornamentals and plants of economic importance. The first noticeable feature of the disease is a wilting i of the terminal growths if it occurs in summer. In the : 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 col- , 'lapse and death frequently , follow quickly. The disease itself is not , ; noticeable externally, but stained tissue, noticeable if the stem is cut open, indicates its presence. Seasonal , carry-over in the soil can only

oe counieraciea outside to any degree by good crop rotation, burning of diseased plants, destroying weed carriers such as fatben and by preventing growth checks. Disinfection is the most important single factor in glasshouse controls and it should be remembered that spread can be facilitated by soil on tools, footwear and through transplanting infected plants. Club Root Club root is a fungus disease peculiar to cruciferous plants only, such as the brassicas which are variably susceptible to it. The problem manifests itself most usually in more acid soils. The first symptoms may be undue wilting of advanced seedlings and yellowing of the leaves through lack of sufficient nutrients. However one of the most obvious of symptoms occurs in the root region which become swollen and distorted. Although this disease can kill the affected plant, retardation of growth resulting in stunting is more likely. Spores are released into the soil from decaying root systems and these can remain viable for more than five years and survive to a depth of 12in. Spores can be spread by cultivation, surface drainage water and on infected seedlings. Susceptible weeds can provide a continuous source of re-infection. Club root can be greatly restricted by increasing the soil pH by means of liming. Acidulated mercuric chloride or colomel dust used as a direct root treatment has proved successful and so also has a newer material PCNB which has the advantage of being non-corrosive. There are other soil

I borne diseases such as potato scab and powdery scab, various leaf spot diseases, many of which are carried over on in, fected plant refuse left after harvesting. Still others are induced through bad drainage, water logged conditions or too deep planting, such contributory factors being far more prevalent than may be realised. These problems and others not dealt with here emphasise clearly the necessity to observe a few rules to avoid trouble. They are summarised briefly. Hygiene Essential Hygiene is absolutely essential in the glasshouse and not just to reduce the incidence of soil borne diseases. Continuous cropping of glasshouse soils is certainly not recommended. While soil replacement may seem the best way out it is as well to ponder over the fact that to do even a reasonably satisfactory job it is necessary to excavate to a foot in depth. This means removal of about three tons of soil from a glasshouse of area 12ft by 6ft. Soil sterilisation is therefore to be preferred. Rotation should be practised in the vegetable garden as much as possible, a feature which serves a two-fold purpose—reducing the build up of over wintering diseases and in giving the soil a rest! from a particular kind of j crop A crop rotation plan is simple to devise as there are a number of variations possible and one can be adapted to suit. Good drainage is necessary

to avoid water-logged conditions, thus giving plant roots a chance to develop and fulfil their functions adeo.uately. Few subjects are capable of withstanding wet feet for any length of time. In the home garden, with its intensive culture of plants, spacing is important ;to allow adequate room for ! development and to provide I satisfactory circulation of air leurrents, thus avoiding stagnation. Diseased plants or crop residues suffering from troubles should preferably be incinerated and not dug into the soil or used in composting. Many of these suggestions have a genoral application and are not necessarily confined to the reduction of soilborne diseases.