Citrus normally drops many small fruit beginning shortly after the blossoms fall and continuing for about 1 month. Drop of small fruit can continue until fruit is 1/2 inch in diameter or somewhat larger. Older fruit may drop later in the growing season.

Identification

Small fruit dropped beneath the tree canopy is generally easy to see. The drop of older, larger fruit is especially obvious.

Disorder Development

Extensive drop of small citrus fruit is normal. Citrus naturally adjusts the amount of fruit carried on the tree, partly to achieve a balance between the amount of fruit relative to the amount of foliage. This is because in the presence of sunlight along with carbon dioxide and water, foliage produces carbohydrates via photosynthesis. The carbohydrates are needed to create and grow the fruit. Excessive drop of small fruit sometimes occurs due to stressful growing conditions, such when hot temperatures occur suddenly at fruit set or shortly after.

Excessive drop of larger, older fruit can be caused by drought stress, freezing weather, inadequate nutrition, heavy or improper pruning, windy conditions, and high temperatures or sudden changes in temperature.

Certain pest problems may cause fruit drop. For example fruit with Botrytis blight (Botrytis rot or gray mold, Botrytis cinerea) or brown rot (Phytophthora species) may drop, but by the time this occurs decay is often obvious on the fruit. Citrus greening, or Huanglongbing disease (Candidatus Liberibacter asiaticus) can also cause fruit to drop. But before this occurs other tree symptoms develop and the aphidlike psyllids that vector the pathogen may be observed feeding on foliage and green shoots.

Damage

Drop of small citrus fruit is normal and generally unavoidable. Small fruit can drop anytime from late March to early July. June to early July generally is when most of the drop of small fruit occurs. Older fruit may also drop. This is generally related to cultural practices, environmental conditions, or pest problems, some of which can be modified to reduce the drop of older fruit.

Solutions

It is not always possible to avoid excessive fruit drop. But it can be kept to a minimum by giving the trees the best possible growing conditions and cultural care so they grow vigorously. This results in a dense canopy with abundant leaves that can support the growth of more fruit. Appropriate irrigation is especially important.

Avoid excessive pruning because reducing the amount of foliage reduces the amount of fruit a tree can support. Shortly before or after blossoming is generally the best time to prune citrus. This timing allows the tree to balance the amount fruit retained or dropped while small according to the amount of foliage. Provide trees proper nutrition, primarily adequate but not excessive nitrogen. Manage pests of citrus because some of these can cause fruit drop.

References

Adapted from Integrated Pest Management for Citrus, University of California Statewide Integrated Pest Management Program (UC IPM).