Summer Ailments of Citrus Trees

Citrus are favorite fruits here in Tucson and other low desert areas of Arizona.  They are well adapted to the hot conditions, but like most plants with the intense heat of summer, problems can develop.

The most common problem symptom seen at this time of year is the curling or rolling of 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 feeding causes leaves to curl and yellow.  They also secrete a clear sticky substance called Ahoneydew@ which creates a varnish-like appearance on leaves.  Thrips are a tiny flea-like insect that feeds on the developing leaf buds of citrus.  As a result, mature leaves are cupped or curled and marked with distinctive whitish lines.  In either case, curling from these insects is mealy cosmetic and causes no harm to the tree.  No control is possible for thrips; but as they appear, aphids can be knocked off 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 have only a few drip emitters under the tree, then add some more.   A mature citrus tree is likely to need fifteen or more drip emitters.  If your basin irrigating you tree, make sure the basin is 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.

Sometimes citrus leaves can 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.  It should be positioned to screen out the mid-day and afternoon sun.  When the monsoon clouds arrive, shading can be curtailed.

Yellowing of leaves on citrus during the summer can be for other reasons than sun-scorch.  Leaves may turn yellow from a deficiency of either iron or nitrogen.  In the case of iron, newer or outer leaves turn yellow first.  They are characterized by yellowing, called chlorosis, between the leaf veins while the veins remain green.  Thus a green  fish-bone leaf pattern is evident.  In the case of nitrogen, the deficiency is more pronounced on the older, inner growth.  Nitrogen deficiency may yellow the entire leaf or just the veins,  resulting in a yellow fish-bone leaf pattern. 

A nitrogen deficiency can be corrected with the addition of ammonium sulfate or other nitrogen containing fertilizer.  Iron can be applied to the soil using a chelated form of iron available at local garden centers.  In either case, after applying the nutrient water it into the soil thoroughly.

When the monsoon rains arrive, citrus that are improperly planted or are in poorly drained soils can be susceptible to fungal diseases.  The most common is foot rot caused by a water mold fungi called phytophora.  Foot rot is so named because it attacks the base of the trunk.  It causes bleeding lesions that weep sap.  This sap forms into gummy patches. When the trunk is partially girdled by foot rot, leaves turn yellow as if nitrogen deficient.  Eventually the tree will die.

To prevent foot rot, make sure that soil is not in contact with the base of the trunk.  Planting citrus too deep results in soil coming in contact with the trunk.  Pull this soil back and keep it back by using an edging product.  Finally, be careful not to over water during periods of abundant summer rain.