Damage. Several strains of virus cause this disease. In addition to cucumber, the following may also be affected: spinach, celery, tomato, Pelargonium and Petunia. On cucumbers, a mottling of young leaves occurs (see Figure 15.20) followed by a twisting and curling of the whole foliage, and fruit may show yellow sunken areas. On the shrub Daphne oderata, a yellowing and slight mottle is commonly seen on infected foliage, while Euonymus leaves produce bright yellow leaf spots. Infected tomato leaves are reduced in size (fern-leaf symptom).
Life cycle. The virus may be spread by infected hands, but more commonly an aphid (e.g. peach-potato aphid) is involved. Many crops (e.g. lettuce, maize, Pelargonium and privet) and weeds (e.g. fat hen and teasel) may act as a reservoir for the virus.
Control. Since there are no curative methods for control, care must be taken to carry out preventative methods. Choice of uninfected stock is vital in vegetatively propagated plants, e.g. Pelargonium. Careful control of aphid vectors may be important where susceptible crops (e.g. lettuce and cucumbers) are grown in succession or next to other susceptible species. Removal of infected weeds, particularly from greenhouses, may prevent widespread infection.
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