March 2, 2002

My citrus trees are putting out new growth and buds. Although the buds have not yet opened into blossoms there are whole herds of aphids feeding on the new leaves. If I spray now with Diazinon to control the aphids will I injure the blossoms? I have been told in the past not to spray when the tree is in flower as it will cause the blossoms to drop.
- Jim Matlack, e-mail

Answer: This brings up the larger questions of what one should or should not do to citrus trees while they are in bloom or coming into bloom. Here’s a short list of the does and don’ts:
1. Do not apply Diazinon or insecticidal oil sprays. These sprays can burn sensitive blossoms and cause premature drops of blossoms thereby reducing fruit yields. Wait until fruit set before applying either product. When necessary, April is usually the month to begin applications of these materials.
2. Do not apply a foliar nutritional spray during blossom set. While its application may not cause blossom loss, nutritional sprays are not well absorbed by newly emerging leaves and are basically ineffective this early in the year. Manganese and Zinc, two common components of nutritional sprays, are effectively absorbed by leaves when they are 2/3 to fully expanded. This will occur sometime in April. For most trees, foliar application is not necessary.
3. You may apply a copper fungicide, if necessary, to control disease problems. However, copper application is most effective 2 to 3 weeks after petal fall. Copper application does not decrease yields.
4. You may apply a citrus fertilizer now to assist the tree in fruit development and to increase yield. Flowering and fruit development take up much nutrient reserve which can be replaced by fertilizer applications. Apply 8 pounds of a 6-6-6 citrus or similar analysis fertilizer per each mature tree.