Yellow leaf curl virus has swept through the warm tomato-growing areas of the world, and the effects have been devastating. There are various strains and ours has been named curly stunt, but is called tomato yellow leaf curl virus elsewhere, and sometimes whitefly-transmitted geminivirus.
There are genes used for control of the virus which work for many strains and some which only work in certain places. Fortunately, our curly stunt responds to the common ones. resistant genes come from a wild species of tomato L. chilense. he bad news is that these genes confer resistance, not immunity. There’s a vast difference between susceptible and resistant varieties, and between uninfected plants and inoculated resistant plants. Expect some yield loss when a resistant variety is infected. Much depends on the stage of infection and amount of virus received.
The virus is transmitted by the Bemisia whitefly, which is very common worldwide. An infected whitefly can retain the virus for some weeks and infect a plant within 15 to 30 minutes of feeding. Symptoms develop up to three weeks after inoculation. An infected land can look very uneven according to the stage at which each plant became infected and the amount of virus received. Leaf symptoms look similar to bunchy top virus, but plants are less stunted. The leaf margins become a lighter colour and curl upwards. hough the vector is widespread, this virus is more prevalent in warm climates, suggesting a break in its life cycle due to a lack of host crops and/or reduction in whitefly populations during the cold season.
There are many host weeds and some host crops. Beans (Phaseolus vulgaris) and plants of the nightshade family can be hosts. Datura (Thorn apple or Stinkblaar) is a common host. Farmers in areas prone to outbreaks should tackle the problem on multiple fronts. Potential host weeds should be cleared out well before an intended planting. Plants should be treated with a suitable insecticide with a drench of Confidor or something similar. Applying this to the planting hole before planting will help control whitefly. plants should preferably be grown in an area which is clear of host species. It’s often impossible for a nursery to prevent a whitefly infecting a few plants, so inspect plants regularly for symptoms after transplanting and remove immediately. After planting, the accent is on whitefly control.
Much less damage is caused if plants become infected later on. In high-risk areas, plant different lands as far apart as possible. Resistant varieties and the above practices can take the sting out of this feared disease. – Bill Kerr ((016) 366 0616 or e-mail [email protected]) |fw
