Researching Desert Citrus
A visit to the Citrus Ag Center

The center's first tree were planted in 1990. Twenty-five miles west of Phoenix, near the foot of the White Tank Mountains, the Citrus Agricultural Center (CAC) features research projects specific to growing citrus and other fruit crops in the desert. More than 50 varieties of citrus trees occupy the 40-acre site in Waddell, along with a variety of berries and deciduous fruit and nut trees. The center’s first trees were planted in 1990. Currently, twenty-one research projects focus on variety evaluations, irrigation, nutrient requirements, orchard floor management, increasing yields, internal fruit quality, organic citrus production, and other concerns.

Extension and department faculty from the UA College of Agriculture, including plant scientists, plant pathologists, entomologists and soil scientists, conduct these trials. The results of their projects are disseminated to the public through written reports, field days and seminars. Dean Bacon, director of the CAC, oversees the management of the experiment station and coordinates the projects that are designed to match the researchers’ expertise with growers’ needs. He has sponsored tours of the station for small neighborhood groups and larger organizations, and gives lectures on growing citrus in arid climates.

Highlights of Ongoing Projects

Orchard Floor Management: Weeds compete with trees for water and nutrients, reducing yields. This series of studies on two two-acre blocks of Mineola tangelos and valencia oranges compares yields using three different ground cover systems: cover crops (clover), natural grass and chemically controlled bare ground. The clover serves both as weed control and as a nitrogen source when it’s disked into the soil. Results thus far indicate that yields are higher for trees on bare ground. Temperature checks have shown that there is no temperature difference between bare ground and covered ground during the winter, at least in the desert.

Girdling Trees to Increase Yields: Trees that are cut or girdled around the trunk below the branches tend to flower and fruit more heavily. The method is labor-intensive and can be too costly for large orchards, but has its merits in small groves. Researchers are girdling Fairchild mandarins and has increased their yields; his next step is to find ways to increase their size as well.

Reducing Granulation in Navel Oranges: When citrus fruit granulates, it means the normally succulent fruit lacks juice--the sacs are dry and "ricey." It is thought that calcium binds the moisture and holds it in a different form. Rootstock, environmental conditions, watering methods and fertilizing practices all affect granulation. Only the last two can be controlled once trees are in the ground. This study includes both regular watering and over-watering regimes, and nutrient additions, such as boron and potassium.

Nutrient Studies: Researchers are checking four different regimes for adding nitrogen to 18-month-old Newhall navels--weekly, monthly, three times per year, and the control (none added). The nitrogen is applied to young trees through the microjet irrigation system. They have found so far that young citrus doesn’t need to be fertilized: there is no difference in trunk size or petiole (leaf) sample, in any of the nitrogen regimes.

Variety Trials: Grapefruit--Because color really sells grapefruit, this trial is focusing on red and pink-fleshed grapefruit using Flame, Texas Star and Rio Red varieties. Strawberries: Will include two varieties plus date of planting. Navel orange: about 12 varieties on different rootstocks. A deciduous tree variety trial includes nectarines, apples, peaches, plums, apricots and jujubes.

Organic Citrus: The center has organically grown tangelos and navel oranges. These blocks of trees were designed to develop methods and demonstrate to growers that citrus can be grown organically without a reduction in fruit quality or quantity.

In addition to its research projects, the CAC has taken the initiative regarding two issues facing both commercial and backyard growers: transmitting frost warnings and monitoring trees for the tristeza virus:

Monitoring Systems: An AZMET station at the Center gathers weather data that feeds into a computer database for climate conditions in --- locations in Arizona. In addition, Bacon says that because the national weather service has discontinued its "first warning" system for frost, the CAC has developed its own with the help of a private climatologist.

The automated system dials weather stations in 11 different areas and downloads the data to be read by a meteorologist, who will predict low temperatures for the following day. The reporting area covers acres of citrus, from west of Phoenix, 80 miles east to Queen Creek and south to Picacho. Growers can get the information by calling a recorder at the Center or receive a faxed version upon request.

The Tristeza Task Force: Bacon helped organize the Citrus Tristeza Virus-Brown Citrus Aphid Task Force, a group that surveys all the citrus in the state, both commercial and urban, for signs of tristeza, a citrus virus also known as quick decline. The brown citrus aphid (BCA), which is not yet in Arizona, vectors the disease. "We speculate this vector will arrive in Arizona in three to five years," Bacon says. In some cases the virus can kill citrus trees in about three to six weeks; it can also move more slowly. It affects some rootstocks more than others: for example, sour orange rootstock is extremely susceptible.

"About fifty percent of the citrus in this area (Maricopa County) is on sour orange rootstock," Bacon says. If the brown citrus aphid moves into this area we’re going to be in trouble, not only in commercial orchards, but also with the three million citrus trees that are in back yards all over this county."

The task force is collaborating with the Arizona Department of Agriculture in sampling and monitoring trees across the state for the virus, and in eliminating infected trees before the aphid can feed on them and transmit the virus to healthy trees.

"There are about 113 trees that have tested positive so far in the state," Bacon says. Because emotions can run high regarding cherished backyard fruit trees, education is very important in persuading the homeowners to allow the state to remove the infected trees. The task force has focused on presenting information to community organizations and to individuals in neighborhoods.

Citrus Agricultural Center

Established in 1985 on 40 acres. The first citrus trees were planted in 1990 in one and two-acre blocks of different varieties. The farm has three types of irrigation systems, microjet sprinkler, low-pressure bubblers, and flood irrigation.

  • Standard Citrus: lemons, grapefruit, tangelos, mandarins, valencia and navel oranges
  • Exotic Citrus: kumquat, limequat, pummelo and buddha-hand citron
  • Vines: blueberries, blackberries, grapes, jujubes (a Chinese date eaten fresh or dried)
  • Deciduous trees and nuts: apple, peach, apricot, plum, nectarine, pear, almonds
  • Other: jojoba, olives, strawberries

Article written by Susan McGinley, ECAT, College of Agriculture
This is part of the 1998 Arizona Experimental Station Research Report
This document is located at http://ag.arizona.edu/pubs/general/resrpt1998/citrusthirps.html
Return to index for 1998 report

Researcher:

Dean Bacon, Citrus Agricultural Center
Phone: (520) 255-3316
adbacon@ag.arizona.edu