Lemon trees are susceptible to many of the same diseases and pests as every other citrus tree. Proper care of the trees, as well as regular inspections of the leaves, branches and fruits can help you identify problems early on to prevent them from spreading to other trees or even other portions of the same tree. Proper watering and aeration will help with many of the diseases as well as with a few of the pests.
Citrus tristeza is a virus that causes a number of side effects such as yellowing of the seedlings, extreme pitting of the stem and/or a quick decline of the roots. These all can result in loss of fruit crops or even whole trees. Citrus tristeza is generally spread by aphids and spreads rapidly.
Root rot, also known as brown rot, is usually caused by excessive water pooling around the roots of trees. Dark brown patches appear on the bark of a tree infected with root rot. These patches will sometimes excrete ooze. The rot can become progressively severe, causing the bark to crack and die. Root rot can even spread to the fruit and leaves, effectively destroying the entire tree.
Aphids are small insects that range in colour from a light green-grey or yellow-green, to a black or a brown. The symptoms of the presence of aphids include yellowing and twisting of the leaves, as well as multiple puckered marks on the leaves. Aphids cause the leaves of lemon trees to appear deformed, which inhibits their productivity.
Citrus whiteflies are very small flying insects commonly found eating on the underside of the leaves of a lemon tree. Whiteflies also lay their eggs on the leaves which cause the leaves to curl and appear to be coated in a mouldy, sticky substance. To check for whiteflies, shake the branches and watch for tiny insects flying around.
Snails eat the leaves and fruits of lemon trees, inhibiting growth and causing the fruits to be pitted and rotten. It is easy to detect the presence of snails by watching for their telltale silvery mucus trail on the trunk, branches and leaves of the trees. Also, look for snails on the underside of leave and branches, or in the foliage dropped on the ground.