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leaf curl[′lēf ′kərl]
a widespread disease of plants, marked by the formation of protruding sections of the tissues of the leaves. These thickenings cause the leaves to curl, twist, and become wavy. The causative agents of leaf curl are fungi of the order Exoascales (leaf curl of peach, the Amygdalus, cherry, pear, black alder, Acer tatarica, birch, and poplar) and viruses (leaf curl of tobacco, cotton, sugarbeet, strawberry, potato, raspberry, and hop). Leaf curl is sometimes caused by insects, such as aphids (leaf curl of black, red, and white currants), or by mites.
The sources of the infectious principle in fungous leaf curl include the endogenous mycelium in branches and shoots and ascospores that have wintered under the glumes of buds and in cracks in the bark. The causative agents of viral leaf curl are transmitted during the vegetative reproduction of the plant and are spread by sucking insects (leafhoppers, aphids, and thrips) and mites.
Control measures include pruning and burning severely affected shoots; culling out sick plants (this method is used with plants that have viral leaf curl); early-spring spraying of fruit trees with fungicides; destroying weeds (reservoirs of viral infections), insects, and mites; and cultivating varieties that are resistant to leaf curl.