UCCE Tulare County, Citrus Notes ( September 1999)

Learn from more than a dozen leading experts in irrigation management at this one-day symposium. The program is designed for growers, farm/vineyard managers and consultants interested in applying up-to-date methods regarding irrigation practices.

The enrollment fee is $60 and includes course materials, lectures and a box lunch. Enroll in section 992E160.

For more information call the University Extension Agriculture Unit at (530) 757-8899, fax (530) 757-8639 or e-mail aginfo@unexmail.ucdavis.edu.

Weed Management Meeting

October 13, 1999
Holiday Inn, Visalia Airport

Look for announcement and registration information in mail.

Ant Damage to Young Trees

Damage to young trees continues to be a problem resulting from ant feeding on the bark underneath the tree wrap. The ant activity and feeding goes unnoticed until there is a change in the appearance of the tree, either a color change in the canopy or the development of wilt. In some instances the damage is so extensive that the tree is completely girdled and must be replaced. A program of routinely removing a few wraps and checking for ant activity each time someone is working in a block of young trees is a good management policy. If left unchecked, a significant number of otherwise vigorous young trees can be partially or completely girdled before the ant activity is detected. Treatment options include sprays applied to the trunk and soil around the base of the tree; spray material can also be applied to the ant nests if they can be detected. Ant bait is also now available which can be spread on the orchard floor and which the foraging ants will take back to the nest and will eventually eliminate entire ant colonies.

Late Pruning

Frost/Freeze damage is not a subject we want to dwell on too long but the issue is related to late pruning. Following the freeze of 1990 there was evidence that trees that are pruned in the fall are more susceptible to low temperatures during a cold episode. Mature orange orchards that had been topped in mid to late October had massive damage to large scaffold branches, which led to their removal later that year. This increased susceptibility to freeze damage from late pruning has been reported following freeze episodes in the past. Pruning results in a higher state of activity in the tree than would normally be the case with the arrival of lower temperatures. This normal (resting) state of activity allows the tree to tolerate low temperatures better than a tree still in a more vigorous condition as a result of the response to the pruning operation. Canopy surface also absorbs heat during the day, which is released back into the orchard at night. Pruning reduces the canopy and surface available for heat absorption. Pruned material that is subsequently shredded and left as a mulch can result in a lower minimum temperature on a cold night compared to a bare orchard floor condition.

Iron Deficiency

Although one of the more abundant of the elements in the earth's crust, iron deficiency in citrus is not uncommon. Although present in ample quantities the element may be in a form that the tree cannot assimilate. Frequently soil that is kept too wet will also result in a condition in which the iron is unavailable to the tree. Where free lime is present in the soil a condition known as lime induced chlorosis may develop. In this situation the rootstock is unable to take up the iron in sufficient quantities to meet the tree's requirements. Rootstocks vary in their tolerance to the lime and their ability to assimilate iron under that condition. Trifoliate orange and citrange rootstocks (troyer, carrizo) do not tolerate the presence of lime and may likely develop typical iron deficiency symptoms. Where the chlorosis results from excessive soil moisture, adjustment of the irrigation program is often helpful. Reduction of the soil pH has had some limited success; however, the soil mass involved that must be adjusted may make the process slow. Use of inorganic sources of iron amendments has had limited success because the iron is rapidly converted to an unavailable form in the soil. Iron chelates have provided more consistent positive responses although treatments may be considerably more costly. Research is currently being conducted to identify new rootstocks that would be tolerant to soil conditions with high pH.

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Revised: September 17, 1999