What is in this article?:
- Pomegranate decline baffles growers
- Black heart and cankers
- Pomegranate tree decline is a problem that dates back at least four years in California's Central Valley, where acreage has grown to 33,500 acres.
- 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.
Pomegranates at a research plot at the Kearney Agricultural Center in Parlier, Calif.
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.