SPOTTED WILT OF TOMATOES
Most of our tomato growers are familiar with those plants which fail to grow after planting. Instead of developing new healthy green foliage, they tend to become blue in colour and bunchy in growth. This is one symptom of one of the most serious diseases of outdoor tomatoes – spotted wilt.
This stunted growth and unhealthy blue appearance is one of the earliest signs of spotted-wilt. It is a sign of an early development of the virus in your crop, but tomato plants can and often do contract the disease at any stage of growth, and symptoms are variable and sometimes difficult to determine.
The name of spotted-wilt comes from the spotted appearance of the leaves and also markings on the fruit. Plants which fail to develop normally and show curled and bunched leaves should be pulled up and removed immediately they are seen in a crop. They will not recover, and only act as a source of further infection. If the plant is affected later, the leaves show bronze or dark circular markings on newly developed leaves. This marking may be in patches and usually forms at the base of the leaf ﬁrst. The leaf symptoms may only appear as a bronze diffusion.
Plants affected in the early stages of growth often die, or if they live, only send out weak shoots and fail to fruit. If attacked later, the fruit develops discoloured blotchy rings of various shades of red, white and yellow. Affected fruit is often small, mottled, or a pale pinky-white colour. These discoloured areas make the fruit unﬁt for processing.
RECOMMENDATIONS FOR CONTROL
Before considering control, we should consider a few known facts which assist in the spread of this serious disease. It is, of course, a virus, and therefore plants seldom recover. As the virus is present in the plant itself, no spray can be applied to control it. All that can be done is to apply preventive measures.
There are many hosts of the spotted-wilt virus, and these include weeds such as mallow, thornapple and nightshade, or berry-weed, as it is often called locally. Weed control is therefore important. There are many other hosts such as peas, lettuce and tobacco. Many flower plants such as lupin, aster, poppy, dahlia, chrysanthemum, primula and zinnias, and many others also carry the disease.
Spotted-wilt may overwinter on such plants as these and spread to tomatoes by insects, particularly one type of thrip known as the onion thrip.
It may well be that the tomato plants you receive from the nursery may already have virus infected plants among them, particularly if the nursery is producing some of the ﬂower plants mentioned, or is not kept free of weeds, which are hosts to the disease.
Tomato plants should be sprayed in the boxes before being planted out, or shortly after being planted out. It is far easier and cheaper to give them a thorough coverage with an insecticide while still in the seed boxes. Malathion is a good spray at this stage, although its effect is not lasting. New systemic sprays such as Rogor, Metasistox or Phosdrin are even better. A second spray of this type of insecticide should be used three weeks after planting out, when the plant is larger and able to absorb sufﬁcient material to kill sucking insects, such as thrip and aphids, over a long period.
If this course of spraying is carried out, the spread of spotted-wilt from one plant to another can be practically eliminated. It is probable that this point has not been sufﬁciently emphasised in the past. Care should also be exercised in cultivation. Make sure that implements don’t damage plants in any way, which could also be responsible for transmitting the disease from an infected plant to a healthy one.
Spotted-wilt in tomatoes has been responsible for a considerable loss of crop and poor quality fruit. The application of a couple of insecticides, which can be incorporated with most fungicides, is not costly, and anything that can be done to prevent poor quality and loss of crop is well worth while.