We focus on spotted wilt and lettuce mosaic virus. Both can be controlled, and even avoided, with the correct agronomic practices.
I first encountered spotted wilt after the western flower thrip arrived in this country.The insect is a vector for the virus and entire districts were soon infected.
Spotted wilt attacks a wide range of crops and weeds, but once the functioning of the virus is understood, it can be brought under control and should cease to be a major problem. Only occasionally do I come across the odd infected plant.
Weeds are the reservoir for the virus, and thrips carry the virus from there to the land.
The first precaution, therefore, is to spray a broad leaf herbicide on weeds growing near the vegetable lands. This is a good practice generally, as it reduces many diseases and pests.
Next, remove all plants already infected to minimise risk of the virus spreading. If there are many infected plants, apply an appropriate insecticide to the entire crop, then remove the infected plants. As its name implies, the virus causes spotting on the leaves. This is especially noticeable when the plant is infected at a later stage. The lettuce also turns a lighter colour and becomes stunted and unmarketable.
Unlike spotted wilt, lettuce mosaic virus (LMV) is seed-borne. Some years ago, each batch of seeds had to be certified to contain no more than one infected seed per 30 000. This phytosanitary requirement was later dropped as it is in the interest of a seed company anyway to ensure that its products are completely disease-free or it will lose sales.
Infected plants develop a yellowish colour and the edges of the leaves bend down. If you hold an infected leaf up towards the light, you will see a mosaic pattern. Aphids are the chief vector. As with spotted wilt, weeds can carry the virus through winter, and aphids will spread it into the lands in spring.
From time to time, I’ve had to investigate cases of infected lettuces on behalf of a seedling nursery. Farmers who experience an outbreak will invariably blame the nursery for supplying infected seedlings. But if other growers do not experience problems from the same batch of seed, the nursery can be assured that its seed is clean.
On one farm, I noticed that the mosaic was more severe on one side of the land and seemed to be spreading from there. I scouted amongst the weeds near the edge of the land and soon identified two species with LMV symptoms.
Plastic bag treatment
As with any virus, you should chop out plants with symptoms to reduce spread of the virus. If you have already sprayed for aphids, it may not be necessary to remove the chopped-out plants from the land, If not, put them into clear plastic bags and seal the openings to prevent any aphids from escaping. A day in the sun will destroy any life in the bag.
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