Late September to early October appears to be the best time to apply zinc sulfate to restore zinc levels in peach trees and avoid “little leaf” symptoms, says a University of California, Davis pomologist.
Scott Johnson, who is stationed at the Kearney Agricultural Center, Parlier, disclosed his research findings at a winter tree fruit meeting held there.
Small leaves, interveinal chlorosis, and wavy leaf margins are quite common signs of zinc shortage in San Joaquin Valley peaches, as well as nectarines and plums in the region.
“It also become common for most growers to put on a foliar application of zinc, generally in the fall or dormant season, in most years, no matter what,” he said.
But Johnson added that with better sampling methods, growers could identify orchards where zinc might not be needed and save costs of treatment.
The standard method of monitoring nutrients in peach and nectarine has been taking leaf samples in the summer. General practice has been to consider a zinc content of less than 15 parts per million as deficient, with more than 20 ppm desirable. However, recent research for various tree fruit crops suggests that many orchards, in fact, may not be deficient.
Johnson sampled for zinc in dormant shoots in the lower portion of the tree and found them a better indicator. “There's as much as fivefold more zinc in the lower part of the tree than the top.” And the shoots, he said, register variations in zinc content when leaves do not.
His sampling of shoots for zinc content in January revealed that fall applications of zinc sulfate provided plenty of zinc stored in the trees for growth the following spring.
Johnson said he plans to enlist the cooperation of growers to broaden his sampling in September of 2009. “We want to look at this in more orchards, but so far it is very promising.”
After screening several zinc-carrying materials, he concluded that zinc sulfate 36 percent worked as well as any and was more economical than most. In one experiment, he found that zinc sulfate in suspension painted on dormant wood showed 15 times more zinc taken up than zinc oxide. An application made in November was more efficient than one in January.
All zinc products, including zinc sulfate, he said, can cause toxicity when applied in the spring. Searching for the best dormant timing, he found that foliar application at lower rates made in September to October allows more time for zinc to be absorbed before defoliation.
Johnson also delved into soil applications of zinc sulfate. His preliminary trials last year with applications to the soil at planting proved to be highly toxic. “But the good news,” he added, “is we found a slow-release material that worked well.”
A difficulty with soil applications is the slow uptake of zinc by the ubiquitous Nemaguard rootstock, although Johnson said Controller 5 rootstock takes in the element faster.
The sulfur-coated, pelleted product, EnviroSul, is said to gradually release zinc in a soluble form. Johnson found that with it he could increase zinc content by sixfold in newly planted trees. He plans commercial-scale trials in 2009 and is seeking grower cooperators.
“The exciting thing about this is it may be able to supply enough zinc at planting to keep the trees going for at least three or four years,” he said.
The meeting also focused on inking, the skin discoloration of peaches and nectarines that has long been a cosmetic issue for the stone fruit industry.
Carlos Crisosto, UC Davis postharvest physiologist at Parlier, has worked on the problem since the early 1990s and said it is emerging again in recent seasons as a concern among growers, not only in California, but across the nation and abroad.
Inking is the brown to black spots that appear on the skin. Although the flesh is not harmed, the fruit is nevertheless unmarketable. The stains occur when skin cells collapse and release pigments, while the flesh cells beneath remain intact.
He said growers, packers, chemical manufacturers, and researchers will all have to cooperate to find solutions, and he offered his services.
Citing instances of inking in fruit after it had gone through the packinghouse in the 2008 season, he said apparently something new is going on with potential packaging contamination and that is being investigated.
Other factors contribute to inking, but even slight abrasion during harvest can be a significant cause. Gentle handling of fruit, avoiding long hauling distances, and maintaining harvest containers free of dirt are essentials for growers.
The more gentle the handling, the less inking occurs, but Crisosto conceded that extreme care in handling at some point starts to claim a toll on efficiency.
Important, too, is adherence to proper rates of post harvest fungicides, he said. “You want to fine-tune to make sure you are applying enough for control, but not too much, so you stay within the maximum residue limit.”
He said adding chlorine to cooling water will raise the pH and sodium content, but the additional chlorine is not the reason for inking.
He recommended adjusting cooling water pH to keep it between 6.5 and 7.2 and warned not to stop using chlorine because about five years ago he saw examples of white varieties developing brown spots from cooling water without chlorine.
Crisosto said wax applied to fruit can raise pH, but provided it is free of any contamination — not to an extent to cause inking.
Until more is known, he suggested that companies supplying chemicals to the stone fruit industry look into the composition of their products for contamination with iron, copper, or aluminum or other heavy metals, and remove any such materials.
Crisosto pointed out that analyses should be kept up-to-date, since chemical manufacturers maintain the same amount of active ingredient, but may change other components in their formulations from time to time.
He also recommended following preharvest intervals developed several years ago for foliar nutrients, fungicides, and other chemicals that may contain heavy metals.
Other tips he mentioned include keeping packinghouse equipment clean, avoiding dust contamination on fruit, and delaying packing for 48 hours to detect any fruit inking during grading.