Grapefruit trees are susceptible to a variety of diseases, including curly leaf disease caused by the citrus leaf miner (Phyllocnistis citrella). According to the University of California, this small moth is not native to the United States but entered through Mexico. It is not fatal to grapefruit trees, but severely affects fruit production and foliage appearance if left untreated.
The adult citrus leaf miner is 1 1/2 inches long and appears silvery white, with tan and black markings over its body. They live for a few days and commonly remain unnoticed on the grapefruit tree because of their small size. These moths lay eggs singly on the underside of young leaves on the host citrus tree, usually in early morning or late evenings. The eggs develop into tiny translucent, green larvae that measure 1/6-inch long and are usually found inside trailing leaf mines visible on infected leaves.
Infected grapefruit trees exhibit the characteristic curly or distorted leaves. The University of Florida states that the adult leaf miner selects young, fresh or immature leaves to lay eggs over instead of old ones, which is why only the new growth appears curled as the larvae tunnel through these. Leaf mines (shallow tunnels) appear only on the underside of leaves, but are seen on the upper surface in case of large infestations. Usually there is one leaf mine per leaf, but serious outbreaks cause larger leaves to have up to 15 leaf mines.
According to the University of California Integrated Pest Management Program, besides grapefruit trees, the citrus leaf miner infects all types of citrus trees, including oranges, lemons, mandarins, limes and other closely related plants.
Grapefruit trees older than 4 years tolerate curly leaf disease without it affecting fruit production or tree growth. The growth of young or new plantings is retarded. However, even in case of heavy infestations on young grapefruit trees, they are unlikely to die.
The Urban Harvest website suggests immediate spraying of neem oil on the upper and lower surfaces of curling leaves, and repeating the application a week later to remove or significantly reduce the miner infestation.
The pest responsible for leaf curly disease is attracted to new growth, so prune the grapefruit tree once a year to reduce chances of pest occurrence.
The University of California Integrated Pest Management Program suggests applying Imidacloprid through irrigated water or over the soil of grapefruit trees younger than 4 years as it provides citrus leaf miner control for up to three months.
As an alternative, use foliar sprays that provide control for a shorter period of time, usually up to several weeks. It does not control eggs laid or hatched after the spray wears off. Apply diflubenzuron to control eggs and larvae.