It’s a wrenching sight, walking into your pomegranate orchard and seeing crippled or dead trees.
To varying degrees, that happened this year to at least three growers who were among those who gathered for a pomegranate field day in Parlier and Reedley where researchers solicited anecdotal information on the mystery of just what is harming the trees.
The three growers — David Lometti, who farms in the San Joaquin area; Gary Davis, who manages a farm in the Firebaugh area; and Mike Brenner, who welcomed workshop participants to his orchard in Reedley — blame cold temperatures in part.
They have agreement from Themis Michailides, a University of California plant pathologist, who said many questions remain, but “low temperatures have a lot to do with this problem.”
Michailides cited a research paper from Iran that showed similar cold snap damage in 1998.
He said still more research needs to be done on what is being called “tree decline,” a problem that dates back at least four years in the Central Valley, where acreage has grown to 33,500 acres.
None of the three growers is about to walk away from the problem. Davis, for example, said the loss of 10 percent of the trees in one orchard has not dampened enthusiasm for new plantings. With 220 acres in production, the farm he manages, J&J Farms, planted 53 more acres this year and may soon add more than 100 more.
But the loss of trees is an obvious source of frustration and puzzlement.
Lometti wondered if nematodes could be part of the problem. Researchers say nematodes, like the cold temperatures, could perhaps open the door to decline.
Brenner’s losses totaled 30 percent to 40 percent “on average,” he said.
He and others say younger trees — his damaged Wonderful trees are about four years old — are more vulnerable than older plantings. Brenner said that suckers that emerged around trees did not fare well, becoming brittle.
Davis had more luck, he said, because his is a trellised orchard of high density, and he was able to tie new growth to the trellis. He said an Israeli variety was unscathed.
Michailides said one of the mysteries concerning the dieback is the fact that some trees remain healthy while others die, and they may be side-by-side.
Black heart and cankers
Apart from dieback, with research paid for by Paramount Farms, Michailides has studied diseases in pomegranates that include “black heart,” an infection from Altenaria fungi that turns the inside of fruit black while leaving the outside appearing normal.
A single damaged fruit can contaminate others juiced with it, and if the fruit is sold fresh it’s obviously not received well even if it’s limited to a few pomegranates.
Michailides also identified cankers appearing on some diseased trees caused by species that include Neofusicoccum mediterraneum, but it is not known if they are primary pathogens or need stress points such as freezing injury or wounds to cause damage.
Michailides and Richard Molinar, a UC farm adviser in Fresno County, said damage from dieback this year was more common in sandier soils. There was some speculation that heavier soils held beneficial moisture.
Among the unknowns is whether some areas in the Valley may be more susceptible to damage. “Over time we will learn where the sweet spots are for pomegranates,” one grower remarked.
Symptoms of dieback damage include yellowing of leaves, dead branches and loss of sap.
Maxwell Norton, a UC farm adviser in Merced County, summarized observations by those at the workshop, saying variables in damage appeared to include the age of the tree, the soil type, the moisture level of the soil, presence or absence of nematodes, cold spots and whether frosts were early or late.
He urged growers to keep good records on any variability in irrigation or fertilizer applications to see if they made a difference. “Keep good notes,” he said.
Claude J. Phene, a soil scientist and former director of the U.S. Department of Agriculture’s Water Management Research Laboratory in Fresno, said use of potassium nitrate in citrus has been effective in cutting freeze damage and might be useful in pomegranates.
But Norton said using nitrogen in the late summer or early fall could force growth that would be damaged in winter freezes.
Phene also discussed irrigation and fertigation at Kearney Research and Extension Center in Parlier during a visit to a 3.5-acre plot of pomegranates that are just over two years old.
The plot shows use of surface and sub-surface drip. Phene, a pioneer in championing sub-surface drip for processing tomatoes, is an advocate of the latter. He said it results in no weeds, releases no nitrous oxide and uses less water.
He hopes to show that using even higher levels of nitrogen through the two irrigation methods do not result in high levels of leaching of nitrates. Why? Material is released near the trees roots with a fully automated system that delivers product multiple times daily in small doses.