Lemons are an almost universally popular fruit, from the tropics to the Himalayas, and lemon trees are perhaps the most cultivated citrus in the world. They are subject to the same diseases as other citrus species, but they are usually an easy tree to grow.
To avoid problems, examine your lemon trees frequently for diseases. Very few lemon tree problems are life-threatening, especially if you recognize the symptoms. This will enable you to take action to protect your lemon tree. Most diseases can be controlled by spraying with chemical or organic products.
Citrus greening, or yellow dragon disease, is bacterial and originated in China in the early 1900s. It is a serious problem, especially for professional citrus farmers. It is primarily spread by two species of psyllid insects and has harmed trees in Asia, Africa, the Arabian Peninsula and Brazil. It has been present in Florida since 1998. There are three strains of the citrus greening bacteria: the Asian, the African and an American strain recently found in Brazil. This disease causes infected trees to yellow and weaken. Fruit can become misshapen.
Citrus canker is a bacterial disease that causes premature leaf and fruit drop. It is highly contagious and is spread rapidly by tropical storms, mowers and other equipment, and by people carrying the infection or moving infected or exposed plant material. Citrus canker causes brown leaves and fruit, raised areas surrounded by an oily, water-soaked margin and a yellow circle. Leaves may have holes. Citrus canker causes the lemon tree to decline in health and drop in fruit production. Eventually, the tree will stop producing fruit.
Brown rot, or root rot, is a fungal disease caused when Phytophthora spores from the soil are splashed onto the tree trunk during rainstorms. Infections develop when wet conditions continue. Brown rot is first seen as brownish patches of hardened bark on the trunk. Discolored sap may seep from the infected area. Leatherlike lesions are seen on almost mature and mature fruit and appear water-soaked until they become soft and brown. They will have a strong odor, and the infected fruit eventually drops. Twigs, leaves and blossoms also may decay and brown.
Citrus tristeza is a virus affecting citrus worldwide. It is spread by aphids and is most serious in trees grafted onto susceptible rootstock. Trees may be reduced in size, vigor and fruit yields and trees with a severe strain can decline and die rapidly. Symptoms include wilting and dieback, curling and yellowing of leaves, heavy fruit set, honeycombing and bud union staining. The disease causes blockage of the tree's circulatory system, effectively starving the tree to death.
Greasy spot is a fungus caused by airborne spores reproducing in decomposing leaves on the ground below the tree. Warm, damp conditions are the most hospitable. The spores will transfer from the fallen leaves to the tree, penetrating through the lower leaf surfaces. Incubation takes several months, then there will be black spots on the leaves, followed by increasing leaf loss.