Why do varieties with tolerance to TYLCV (Tomato Yellow Leaf Curl Virus) sometimes show symptoms of the disease?
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TYLCV is a virus disease transmitted by the whitefly Bemisia tabacii. The adult whitefly acquires the virus from diseased plants and the virus accumulates in its lymphatic system but does not multiply. The longer the whitefly ingests then the more virus it accumulates.
When infecting plants, the virus infection in the plant is cumulative. This means that the severity of the disease depends on the quantity of virus within the plant – resulting from the quantity donated by the whitefly and the multiplication of the virus within the plant. Furthermore, white fly feeding on plants with high virus levels, easily absorb large quantities of the virus and pass these high levels on to other plants. Susceptible plants exhibit the disease symptoms even when there is a low quantity of virus in the plants.
Tolerant tomato varieties also often show symptoms of the disease, such as yellowing heads and leaf curling. A complex of 3-5 genes controls tolerance to the virus and in effect there are 5 levels of tolerance on a scale of 1-5 with 5 representing the highest level. Since infection is cumulative, then, depending on the tolerance level, the plant may continue to grow or may even stop. Even when the whitefly transmits the |
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