One project by University of California researchers has demonstrated that girdling of Black Mission fig can increase fruit size and value per tree, while another has found potential replacements for the Calimyrna variety.
Both projects are funded by the California Dried Fig Institute, and reports were made at a recent growers meeting.
Louise Ferguson, pomologist at the Kearney Agricultural Center at Parlier, said her replicated trials from 1997 through 2000 showed that early-season trunk girdling of Black Mission could increase dry weight of fruit per tree by up to 42 percent. Value per tree could increase by up to 57 percent.
Jim Doyle, staff research associate at KAC, reported that hybrid cultivar evaluations he began in 1991 at the center have narrowed to four “elite” selections, including the leader, 6-38W.
The purpose of his trials was to find a cultivar with fruit having qualities of the industry standard, Calimyrna, but not requiring the labor-intensive caprification process for pollination.
Ferguson said the girdling was done, once per tree per season, on dates ranging from May to July, on 14-year-old, commercial trees near Chowchilla. The girdles were made with a grape knife around the trunk with the first season's about 18 inches above ground.
Girdling, Ferguson explained, removes a quarter-inch strip of the phloem layer just beneath the bark and prevents carbohydrates from moving from the foliage down into the roots. The cuts are not deep enough, however, to kill the tree and heal over in about 10 days.
Every other year
“Girdling means that, on a temporary basis, you favor fruit development over storage in the roots. Based on what we saw in this experiment, my recommendation is that the girdling be done every other year,” Ferguson said.
After four successive years of the treatment, she said, the smaller canopies of girdled trees can be distinguished from ungirdled trees.
To measure results, she considered pounds per tree and fruit size. Generally, the earlier girdling, from mid-May to mid-June, gave the best results. An arbitrary base value of 45 cents per pound was assigned for the experiments. Grading of the trial fruit was done to industry standards by Mike Emigh of Valley Fig Growers in Fresno.
In the 1998 trials, for example, yield of the trees girdled on June 4 was 62.5 pounds, vs. the ungirdled control at 43.9 pounds, a 42 percent increase. The value per tree of that group was $37.57, vs. $23.93 for the control, a gain of 57 percent. The value increases were due to both greater dry weight per tree and a greater percentage of choice, extra choice, and fancy grade fruit.
Yield increases for the 1997, 1999, and 2000 seasons were 22, 38.6, and 21 percent, respectively. Ferguson said the 2000 season was the poorest for pounds per tree yield (20.7 girdled vs. 17 in the control) and she suspects it was due to alternate bearing, plus the early harvesting of the orchard to avoid rain.
Ferguson said although the study was confined to the Black Mission, girdling may have application for other fig varieties. She plans to continue the work this year to determine a biofix for girdling timing according to development of shoot or fruit size. It would be more accurate than calendar dates.
Doyle said the top promising cultivar, 6-38W, has the largest fruit and highest productivity of the group of four winnowed from the crosses he made and set out in 1991.
The poor returns for fig paste have directed the industry to concentrate on varieties for specialty packs of whole fruit, such as the Calimyrna.
“We are at the final stages of the breeding program, and we've selected from the original 3,500 crosses. We also have a 50-tree planting of 6-38W near Chowchilla. We need to establish enough of all four of the selections to field-harvest and process the fruit before a final decision.”
The Calimyrna-like selections do not depend on caprification. They also have a relatively small ostiole, or “eye,” to restrict insect entry into the fruit.
Using classical breeding techniques, he made the back-crosses with Calimyrna and capri-fig to create plants with increasing Calimyrna traits and selected individual seedlings. Over time, he arrived at the top four.
The other three, still to be set out in field trials, all produce smaller fruit than 6-38W, which has produced up to 60 pounds of dried fruit per year from the vigorous tree at KAC.
The 12-33E has medium to large, attractive fruit with few mold problems. The 12-40E is smaller yet but still adequate for packaged figs. The 23-41E has large to medium fruit size, but its tree vigor and productivity are still uncertain, Doyle said.
Following evaluations for fruit decay this season, the seedling block at KAC will be removed.