Causal Agent and Disease Cycle
Cassava geminiviruses are transmitted in a persistent manner by the whitefly Bemisia tabaci, by vegetative propagation using cuttings from infected plants and occasionally by mechanical means. Cassava produces its first leaves within 2-3 weeks of planting; these young leaves are then colonized by the viruliferious whiteflies. This is the key infection period for CMD geminiviruses, as they cannot infect older plants. As the genome of the viruses has two components, DNA A and B, that are encapsidated in separate geminate particles, it requires a double inoculation to cause infection.
Generally, Whitefly requires 3 hours feeding time to acquire the virus, a latent period of 8 hours, after which it requires 10 minutes to infect the young leaves. There is variation in the literature on this score, however, with other sources citing a 4 hour acquisition time and 4 hour latent period. Symptoms appear after a 3-5 week latent period. Adult whiteflies can continue to infect healthy plants 48 hours after initial acquisition of the virus. A single whitefly is sufficient to infect the host, however, successful transmission increases when multiple infected whiteflies feed on the plant.
After entering the plant via the leaves, virus remains in the leaf cells for 8 days. As it is a single stranded DNA virus, it needs to enter the nucleus of the leaf cells in order to replicate itself. After this initial period, the virus then enters the phloem and travels to the base of the stem and out into the branches. Travel to the branches of the plant is much slower than travel through the stem, so cuttings of branches from infected stems may be free of disease. Some literature has indicated that infection is limited to above ground tissue, but it is not clear why this would be the case.
Read more about this topic: Cassava Mosaic Virus
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