Citrus trees are among the most popular of all the home garden fruit trees in the Stanislaus County area. This is true even with the problems we have some years with growing citrus under adverse weather conditions. While we haven't had much frost this winter, we have had plenty of cold, wet weather. These conditions have apparently contributed to a fruit drop problem, especially in navel oranges.

There normally is a heavy drop of small fruit beginning shortly after the blossoms fall and continuing until the fruit is about 1/2 inch in diameter. This is nature's way of adjusting to the environment the amount of fruit carried by the tree. Once the initial drop has taken place, the remaining fruit holds on the tree fairly reliably.

If your orange tree is dropping its fruit now, there are a couple of things to check in trying to figure out why. Check the stem end of the fruit for the small calyx or "button." If the button is still attached to the fruit, it's likely the fruit was blow off the tree by wind. If buttons are not attached to the fallen fruits, the cause of the drop is more difficult to determine.

Trees that are stressed by harsh weather, cold wet soils, nutrient deficiencies, insect pests or certain diseases may drop fruit when a single layer of corky cells develops between the button and the fruit in an area called the abscisson zone. The word abscisson means "to cut off," which is exactly what happens, causing the fruit to fall. Freezing temperatures in December, followed by weeks of cold and wet weather are likely to blame for much of the fruit drop at this time.

The abscisson zone develops naturally in mature ripe fruit, so that eventually a good deal of the fruit would drop from the tree anyway. If the fruit on your citrus tree is dropping earlier than normal, try to figure out what may be the cause. Is your tree being weakened by an infestation of scale insects? Is the soil around the tree waterlogged? Is the tree suffering from a nutrient deficiency? Any one or a combination of these problems may stress your tree to the point where fruit begins to drop abnormally.

In many cases it is not possible to prevent citrus fruit drop. This is especially so if weather conditions are causing the problem. You can keep fruit drop to a minimum by giving your trees the best care possible. It's especially important to water carefully during the summer months, fertilize properly and control pests.