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Too much water at almond hull split can be costly for a long time

Many Americans are damaging their long-term health by eating too many potato chips and too much ice cream and chocolate cake.

A lot of California almond producers are guilty of the same sin; gorging their orchards to death.

And like obese people, almond farmers will pay a heavy price in the future for indulging their almonds with far too much water near harvest, University of California (UC) Cooperative Extension farm adviser Brent Holtz of Madera County, Calif., told a group of about 50 growers and pest control advisers at a recent in-orchard field day north of Madera.

California almonds need to go on a diet, according to Holtz, who “went out on a limb” blaming hull rot for reduced one-season yields, and reduced lower limb vigor and possibly death to branches, robbing yields for the life of the orchard.

Retired UC plant pathologist Beth Teviotdale calls hull rot, caused primarily by too much water at hull split, the “gout” of almond trees.

Holtz calls hull rot the “most important disease almond growers are dealing with today.” And it will only get worse with the new, close-planted orchards irrigated with drip or sprinklers, unless growers change the way they irrigate late in the season just before harvest.

A bad case of almond hull rot in just one year is bad. However, the long-term fallout from hull rot — lost vigor low in the canopy — will become a much larger issue as almond prices are likely to be considerably lower in the future than they are now.

“We may not see $2 almonds any more. If prices get down to $1 per pound and you lose a third of an orchard’s production to hull rot, you will be losing your profit,” he said.

For example, a bad case of hull rot can take a 3,500-pound orchard down to an annual production of 2,500 or even 2,000 pounds per acre — as much as one-third of an orchard’s potential yield. With a production cost of $1,500 to $2,000 per acre, profit is in jeopardy at $1 per pound with this kind of lost production.

“And you may never get that lower canopy production back,” said Holtz.

Ironically, it is a disease which growers, pest control advisers and irrigation consultants are bringing on themselves. It can be prevented as easily as forgoing that piece of lemon meringue before bedtime.

All it takes is managing water content and using irrigation stress measured by a $2,000 to $3,000 investment in a pressure bomb to measure moisture content in leaves. “It is a lot better investment than a new John Deere tractor,” said Holtz.

A pressure bomb measures how much pressure it takes to extract water from leaves. It measures in bars. Typically, a well-watered orchard prior to hull split is at minus 7 bars. Taking that down to a range of minus 14 to minus 18 bars can make a huge difference in hull rot prevention.

However, Holtz said the deficit irrigation stress he recommends at hull split is not even visible in the trees.

In one Madera County orchard over two years of testing, Holtz reduced hull rot strikes (hull rot symptoms) in the first season from 44 strikes per tree in normally irrigated trees to less than 18 strikes on trees only slightly stressed. The second season there were 17 strikes in normally irrigated trees and only 2 strikes in trees stressed at hull split minus 14 to minus 18 bars measured with a pressure bomb.

Although Holtz did not mention it at his field day, UC researchers have found fertilizer management is also important in reducing hull rot. The UC recommendation is to take leaf samples in July to below 2.6 percent to reduce hull rot.

The worst case of hull rot Holtz has seen cost a Madera County grower 500 meat pounds per acre in one year. This was the amount of (from) stick tights shaken off trees after the regular harvest. They also contained 100 percent Navel Orangeworm (NOW).

This is not a problem. It is a disaster. At $2 per pound, which has been the average return for the past five years, this represents a $1,000 per acre loss from hull rot not just for one season, but a major crop loss potential every season from the toxin spawned from the hull rot fungus.

Holtz says growers are far too concerned about brown rot and spider mites, neither of which poses a 500-pound per acre loss threat.

Two fungi cause hull rot. One is the same fungus that causes brown rot in peaches. The other is the same fungus that causes mold in bread. These fungi emit a fumaric acid that attacks the shoot at the point where the nut is attached, killing the shoot and leaves to the tip of the spur. It can spread further throughout the limb; reducing vigor the next season and can eventually kill the entire limb.

The first sign of this fumaric acid damage is in the springtime after a bad hull rot season. It is evident by branches on the lower part of a tree with only a few leaves. It appears anemic as though it has been shaded out, but the cause is not a lack of sunlight, but the fumaric acid from hull rot, said Holtz.

He believes this acid-weakening wood could be a factor in the mysterious lower limb dieback problem. Researchers have identified several fungi in this lower limb dieback syndrome. Holtz believes it could start with the weakening of limbs from the fumaric acid from hull rot.

Hull rot attacks the first-harvested varieties, like Nonpareil, because they are the first to split hulls. “The first 20 percent of hull split is the most critical for hull rot, and the first variety to hull split in say a Nonpareil/Carmel orchard is Nonpareil,” said Holtz, who emphasized that hull rot is not variety-specific although Nonpareil seems to be the most susceptible.

“I have seen hull rot in Carmel that was flooded out. I have seen it in a Butte/Padre orchard where Butte is the first to hull split and harvest,” he explained.

“I have tried fungicides at hull split and they really do not work. You can knock the disease down, but it will come back with moisture,” he said. “The only way to manage hull rot is with irrigation.”

He cautioned against arbitrarily reducing post hull split by percentages of evapotranspiration (ET) or reducing irrigation sets. “Without a pressure bomb to tell you exactly what you are doing, you may reduce leaf water potential in the trees too much. You are just guessing without a pressure bomb,” said Holtz.

Extensive research by UC irrigation specialist Dave Goldhammer and Teviotdale led to the minus 14 to minus 18 bar recommendation, which Holtz offered to growers and pest control advisers at the field day.

Gradually reduced irrigation water through the season had no impact on hull rot at harvest in the Goldhammer/Teviotdale trials. Slight irrigation stress, however, in orchards at hull split made a dramatic difference in reducing the disease in the split-hulled almonds.

Although he is adamant about the pressure bomb reading, he was just as resolute about his recommendation that growers gain knowledge of their soil and leaf moisture content in orchards over time.

“If you have an orchard on good deep soils, it may take you three to four weeks to it dry down to minus 14 bars before hull split. On the other hand I have seen orchards in the San Joaquin soils series—the old hard pan—that would go from minus seven bars to minus 14 in two days,” he said.

Holtz’s pressure bomb recommendations are for readings taken from leaves tested between 1 p.m. and 3 p.m.

With the high price of almonds over the past few years, irrigation consultants and growers have been pushing orchards to the limit right through hull split. A grower at the field day commented that her irrigation consultant was adamant about not deficit irrigating at hull split because it would affect the subsequent year’s crop.

“I do not care what ET is at the beginning hull split. Irrigating on ET early in the year and through the season is fine. I want to see orchards at minus 14 to minus 18 bars to prevent hull split.

“ET recommendations were determined without considering hull rot. Besides the crop for next year is being set long before hull split,” said Holtz. “A lot of these ET recommendations are also causing a lot of Phytopthera root all over the place.”

At the minus 14 to minus 18 bar hull split deficit irrigation recommendation Holtz said spider mites and premature defoliation are not a factor. “You do not want to deficit irrigate to minus 22 bars at hull split or your will lose leaves and hurt a crop.”

Plus, Holtz said water deficits that may affect next year’s crop could be made up post harvest. In fact, farm advisers have determined that post harvest irrigation is a major factor on the following year’s crop.

Holtz noted that the hull rot problem is the most severe in the newer, more closely spaced orchards, which are constantly wet and spoon-fed fertilizer with drip or sprinklers.

“These orchards are much later to harvest. Some of the newer orchards I have seen on the West Side of the valley are not even to hull split until August and harvested in September,” he said. This is about a month later than normal and cause many late season problems, like a NOW infestation.

As icing on the cake, Holtz suggested that deficit irrigating at hull split will not only reduce hull rot and minimize lower limb decline, it may actually increase overall yields.

“They found in peaches that with deficit irrigation, trees still produced a normal crop. More importantly, though, the following year there was a bigger crop because the deficit irrigated trees put more energy into producing fruit buds than shot growth,” he explained.

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