Lemon trees, like all citrus trees, are susceptible to at least 50 different plant diseases, according to "Diseases & Pests of Ornamental Plants" by Pascal P. Pirone. Here are common types and symptoms of lemon tree diseases.
There are three types of lemon tree diseases--bacterial, fungal and viral. Most bacteria in soil are beneficial, but there are about 200 bacteria varieties known to cause plant diseases. Lemon trees are vulnerable to some of these diseases, such as citrus canker.
Fungal diseases have two types of their own. There are fungi whose spores live in the soil and threaten roots, and there are fungi whose spores are airborne and infect leaves, fruit and bark. Melanose and citrus scab are two fungal diseases of the latter type that can afflict a lemon tree.
Viruses can attack and infect every life form on Earth, including bacteria and fungi. The citrus tristeza virus is one that can prey on lemon trees.
Symptoms of bacterial diseases include leaf spots, leaf lesions, leaf drop (premature falling of leaves), fruit lesions, and cankers. Fungal infections can display all of these symptoms as well, along with scabs on the fruit and cankers on the bark. Viral infections show mosaics (variegations of light green, yellow or white) on leaves, leaf drop, leaf spots, and leaf curling.
Both bacteria and fungi can spread by rainwater or sprinkler water. The water drops splash bacteria or spores onto the trunk or low-hanging branches, where they enter the tree through pre-existing wounds.
Viruses are carried by insects or other pests. Infected organisms tunnel into the tree and deposit the virus into the sap. Viruses can also spread to healthy trees through deliberate or naturally-occurring plant grafts.
Pruning off infected branches and deadwood can help keep bacterial and fungal diseases in check, especially fungal varieties such as melanose, whose spores can be washed down to healthy leaves and fruit with rainwater. Applying a fungicidal copper spray (see Reference 2) to the soil and then laying down a thick layer of mulch can also thwart soilborne fungal species.
For viral infections, generally the only thing to do is remove the afflicted tree. There are no sprays or other products which can cure a viral infection, and the virus generally spreads throughout the whole plant (as opposed to remaining on one branch or spot of bark). Note: Be sure to consult a qualified arborist about your particular situation before taking any steps to remove any tree.
Make sure the soil drains well to prevent harmful fungi from attacking the roots. Prune low-hanging branches. Some fungi (such as greasy spot) can linger on fallen leaves from the previous season; gather and destroy any suspicious or unhealthy-looking leaves each autumn.
Some fungal diseases like brown rot can spread from fruit to fruit; store clean fruit well away from infected fruit. Keep leaves, fruit and branches as dry and well-pruned as possible to thwart bacteria and fungi.