Question
I have a navel orange tree which has developed leaf curl.  I am
wondering if spraying it with vegetable oil would fix this. Please
advise.  Lots of oranges on it but not ripe yet.  Hope I wont lose
them.

Answer
Orange tree leaves curl for one of two reasons: the tree has an aphid infestation or it has an infestation of citrus leaf miners. Both options mean bugs, which means a good dose of insecticide will cure your leaf problem. Use context clues to determine which pest is bothering your trees, to choose the right treatment.

Aphids leave behind both leaf curl and a slimy residue on the plant tissue, which turns black and molds over time. If you see black fuzzy or clear slimy substances on your tree, you have aphids. Citrus leaf miners lay eggs inside the leaves, which causes larvae to "tunnel" out when hatched. Look for squiggles inside the orange leaves to confirm the presence of this pest.

While leaf miners cause unsightly damage to your tree, they don't harm the tree's fruit production or its overall health. Aphids cause more harm: the black mold that emerges from the honeydew -- the slimy coating -- diminishes fruit quality and tree growth. The black mold coating also shelters scale insects. For these reasons, definitely treat your tree if you determine aphids caused the curl; treatment for leaf miners is optional.

If aphid infestation is mild, spray your orange tree with a blast of water from a hose, in hopes of dislodging the pests. For larger aphid problems, try insecticidal soap. For citrus leaf miners, horticultural oils work well. The University of Florida IFAS Extension warns that horticultural oil can burn orange tree leaves if misapplied, so read the labels before treating your tree. With any pest care product, follow all instructions for use and dosage to give your tree the most effective treatment.

The vegetable oil is not the Horticulture oil and should not be used. The above is more organic treatment I prefer chemical insecticides like Orthene for the aphids control. Make sure both sides of the leaf is sprayed and wash the fruit before eating.  For the leaf miners I would do nothing since thy will not damage the tree.