Answer: The most common problem symptom seen at this time of year is curling or rolling of the leaves. This usually develops first on the newer or outermost leaves. It can be caused by insufficient water, or insect feeding. If the curling leaves seem to be concentrated on the south or west side of the tree, then the problem is likely insufficient water. If the curled leaves are in various locations around the tree, the problem is likely insect damage.
The feeding of sucking insects such as aphids and thrips cause leaf curl on citrus. Aphid are small green or yellowish green insects. Their presence causes leaves to curl and yellow. They also secrete a clear sticky substance called "honeydew" which creates a varnish-like appearance on leaves.
Other common insects attacking citrus are thrips. These tiny flea-like pests feed on developing leaf buds of citrus, by tearing and slurping up the juices. As a result, mature leaves are cupped or curled and marked with distinctive whitish lines.
If the curling is caused by thrips or aphids, the resulting curling of leaves is merely cosmetic and causes no actual harm to the tree. No control is possible for thrips; but as they appear, aphids can be knocked off the leaves with a strong spray of water.
Leaf curling from insufficient water is a problem that must be dealt with! Water the tree deeply and from the trunk out to at least the edge of the branches. If you are using a drip system and have only a few drip emitters under the tree, add some more. A mature citrus tree is likely to need ten or more drip emitters. If your basin irrigating you tree, make sure the basin extends out to the edge of the branches. Fill the basin with three inches of water. This will soak the soil down to the required 3 foot depth. If you problem is a lack of emitters and you can't add more for some time, then water temporarily with a soaker hose. Wrap the hose around the tree, spiraling it out from the trunk to the branch ends. Then turn the water on low and let it run for several hours. If the soil is very hard, it may be necessary to water the better part of a day to get it to soak in.
Sometimes citrus leaves will become sun-scorched. If some leaves are curled and others have bleached-out patches of yellow, sun-scorch is occurring. Shading the citrus will help temporarily. Increasing the frequency of watering may also help. Shade cloth can be purchased at local garden centers.
Citrus trees are generally grafted onto a compatible root stock. It's possible, over the years, that buds from below the graft grew into new branches and what you're harvesting isn't from the original cultivar. Check the trunk of the tree andtry to find the graft. Then make sure that any branches below that graft are removed. The remaining branches should be from above the graft, and should produce sweet fruit.
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