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Index : Citrus and Other Fruits
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- 18. Correcting Summer Citrus Tree Problems - Top
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 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.
Be in caused by thrips or aphids, the resulting curling of leaves is mealy 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 have only a few drip emitters under the tree, then 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. It should be positioned to screen out the mid-day and afternoon sun. When cloud cover from the monsoons arrive, shading can be curtailed.
Yellowing of leaves on citrus during the summer can also be caused by a lack of certain nutrients. Leaves may turn yellow from insufficient levels of nitrogen, iron or zinc. A deficency in nitrogen causes an overall yellowing of the leaves. If iron is lacking, most of the leaf yellowing will be on the outer and younger leaves. Many of the individual leaves will show a condition called chlorosis. Leaf veins will remain green, but the areas between the veins will turn yellow. This creates a fish-bone pattern in the leaf. Zinc deficiency also causes chlorosis, but unlike iron in which leaf veins are narrowly green, the veins of zinc deficient citrus are broadly green set against a background of yellow.
A nitrogen deficiency can be easily 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 any case, after applying nutrients, water them 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.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161 - Updated: June 30, 2002
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