Lemon trees are often used as landscape trees in warm climates. Homeowners or landscapers may suffer distress if the tree's characteristic dark and glossy foliage begins to fall off. Some leaf drop is normal for citrus throughout the year, and the rate of drop will increase in late winter or early spring as new leaves emerge. Heavy, sudden leaf drop could be cause for alarm, and you may need to adjust some aspects of the tree's care.
Improper Watering or Drainage
Lemon trees may suffer from a sudden leaf drop of they receive too much or too little water. In general, citrus require deep waterings on a weekly basis during hot summer weather and about once per month during the winter. The soil should be moistened to a depth of about 12 inches. Root damage, water damage and foot rot can result in leaf drop. Ensure that drainage is adequate or decrease waterings. Trees that are planted too deep are especially vulnerable to these root rot problems.
A severe freeze will force a lemon tree to shed its leaves and fruit. Avoid pruning any seemingly dead branches until spring growth emerges. In a worst-case scenario, an entire tree can die back. Fortunately a new top often grows from the cut trunk. Container-grown lemon trees are more susceptible to cold damage and should be moved to a sheltered location if cold weather is anticipated.
Over-fertilisation or Salt Burn
Salt burn or over-fertilisation can cause sudden leaf drop. Improper fertiliser use can cause leaf tip or marginal burns followed by leaf drop. Sodium may accumulate naturally over a period of time, particularly if the tree is irrigated with high-sodium water. Mitigate the fertiliser or salt by leaching it out with heavy, thorough watering.
Citrus Mesophyll Collapse
Although grapefruit trees are the most susceptible to citrus mesophyll collapse, any citrus can be affected. Collapse occurs as a result of unseasonable weather fluctuations. For example, below average temperatures in early fall followed by a period of above-average high temperatures can stress the tree as it enters and exits a winter slow-growth period. On an affected tree, the leaves and fruit will dry and drop while stems remain green. To care for an affected tree, remove any fallen fruit to prevent the spread of disease, continue to water the lemon tree with its regular winter watering schedule and wait until the tree has leafed out the following spring to remove any defoliated branches.
Individual Limb Dieback
If the leaf loss is limited to one area or limb of a lemon tree, the drop may be the result of a fungal infection. Diagnose sooty mould by locating a black fungus under the peeled bark. Lemon is also more susceptible than other citrus to fungal wood rots like antrodia, coniphora and nodulisporium. For a fungal infection, remove infected wood at least 12 inches below the visibly diseased portion of the branch and sterilise pruning tools between each cut.
Lemon Trees in Containers
In addition to the factors that cause leaf drop in lemon trees in the landscape, container-grown lemon trees can become root-bound and suffer from twig dieback and leaf drop. To correct this, moderately prune the top to achieve a root-canopy balance or repot the lemon into a larger container.