More troubles – in different shades this week – grey, yellow plus mild. There is more on Mosaic Disease – here and at Cornell University (a useful source on information on pests and diseases). Mosaic diseases confuse due to the many variations on a theme. Some spread by insects and some seemingly not. As well as Tomato Mosaic Virus, I often find mention of Cucumber Mosaic Virus and Tobacco Mosaic Virus – all in relation to tomatoes. Here’s what Mr.Cuthbert has to say:
Grey Mould
This disease of the stem is quite a common trouble amongst tomatoes. It is most liable to occur where leaves or side shoots have been carelessly removed leaving jagged wounds, and in moist, warm atmospheric conditions. It can be recognized by greyish-brown markings which later become smothered with a typical brown fluffy mould.
The maintenance of a buoyant free-moving atmosphere by careful ventilation and careful removal of too thick foliage will do much to prevent the appearance of this disease. The aim should be to keep the air moving freely among the leaves and stems but not in the form of draughts. When “stopping” plants or removing foliage, a sharp knife should be used to make clean cuts, and leaves and shoots should be taken off as close to the main stem as possible. Where infection has occurred , the borwn area should be painted over with a thick paste of liver of sulphur.
Mosaic Diseases
These diseases usually occur at the beginning and end of the year when hours of daylight and intensity of sunlight are reduced and temperatures are normally low. There are two main forms, Mild Mosaic and Yellow Mosaic. The former is a common trouble throughout the country but it can be dealt with and calls for no drastic measures. It usually occurs in plants which have failed to develop a healthy, robust action, and that in turn is often found to be due to wet, stagnant soil conditions.
Mild Mosaic is recognizable by a pale green and greenish-yellow mottling of the leaves which may also become blistered and distorted, sometimes to a degree where only mid-rib and a small strip of flat leaf-blade remains. If, as sometimes happens, the disease affects the flower-trusses, “setting” may be hindered or prevented altogether.
When signs of the disease are noted, steps should be taken to promote steady, healthy growth by care in watering, ventilation and feeding. One or two applications at the rate of 2 oz per sq.yd. of top dressing will usually bring about a quick change and enable the plant to outgrow the disease.
Yellow Mosaic is more serious as it greatly retards growth and the fruits are disfigured by bright yellow, orange and silvery-white patches. The leaves are not always distorted but the mottling is much more noticeable with its blotches of bright yellow than is Mild Mosaic. If this disease is identified, infected plants should be dug up and destroyed, and the soil and house throughly sterilized before another crop of tomatoes is planted.
Both of the above diseases are caused by viruses which attack and spread in the cell sap. They are easily carried from plant to plant on hands, clothes and tools, whilst green-fly and other sucking insects can carry the disease from an affected plant to a healthy one. These viruses can also be perpetuated in the seed, so that only seed from a known reliable source should be used.
