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Tips for control of hull rot, NOW at hull split

This year’s unusual weather has delayed almond hull split, giving growers time to control any hull rot by cutting back on irrigation and temporarily curtailing nitrogen applications. Mildly stressing the trees this way can reduce the incidence of hull rot significantly, says David Doll, University of California Cooperative Extension Farm Advisor for Merced County. He expects hull split to occur mid-July this year, about a week or so later than normal.

Hull rot is caused by either of two fungi — black bread mold (Rhizopus stolonifer) or Monilinia. They enter the hull from the beginning of hull split until the hulls dry.

The problem isn’t the fungi; instead, it’s the toxin they carry with them. The toxin is translocated back to the branches, where it causes shoots and fruiting wood to die back, reducing productivity in future years.

Much of the nitrogen applied to almond trees is for nut development. In the case of Nonpareil, the kernel typically is filled and the jelly is completely solid by the second week of June. After kernel fill, excess nitrogen and water available to the tree are used to develop the hull.

“Once the hull splits, that fruit is full of nutrients and moisture, which can feed any fungi that get inside the hull, allowing them to grow quickly.”

That’s why withholding nitrogen and reducing the amount of water to trees can limit fungus development.

“It’s a good idea to make the last nitrogen application around the first week of June,” Doll says. “But, if you haven’t done so, stopping nitrogen can still help. Ideally, you shouldn’t be applying any nitrogen by the time of hull split. Nitrogen applications should resume during the post-harvest period to supply the tree with essential nutrients for fruit bud development.

“Once you start seeing the hulls split, apply about 30 percent to 50 percent less water to the trees,” Doll advises. “That will cause them to dry down a bit and pull some moisture from the hull, which the tree can use for other physiological purposes.

“UC research shows that lowering irrigation rates to induce a mild tree stress can reduce hull rot by as much as 80 percent. This irrigation reduction should occur about one week before hull split and continue until one week after, for a total of about two weeks. After this period, normal irrigation should resume.”

Hull split also provides an opening for the second flight of adult navel orangeworm (NOW) to lay eggs on the developing nuts. Extensive research statewide has shown that growers who practice very clean post-harvest sanitation, including aggressive removal of mummies from December through February, followed by disking or shredding of any nuts not buried in wet weeds or grass, can often escape NOW damage with only minimal insecticide application, Doll notes.

“Control can be very hit-or-miss. One year, a treatment program may provide a great reduction of damage at crack-out, and the next year the same treatment won’t. That’s why it’s important for growers to be aware of the insect pressure in their orchards and properly time sprays.”

In the area of his county north of State Highway 140 and up through the lower Sacramento Valley, one insecticide spray when the hulls are starting to open (2 percent to 5 percent hull split) will usually do the job in Nonpareil and other early varieties, Doll says.

“Some growers who practice good sanitation and scouting and know their orchards, can sometimes get away without making an application.”

In warmer areas of Merced County and the San Joaquin Valley south of Highway 140, more frequent spraying is usually needed to control NOW. Some growers make two applications.

“They make their first spray just as, or slightly before, the hulls are opening up and follow with a second spray once the hulls are completely open,” Doll says. “That kills that moths and larvae in the hull and nut tissues that weren’t exposed when the first spray was made.”

In the case of late varieties, like Fritz or Monterey, a third application may be appropriate if NOW pressures warrant, he notes. That’s especially true in areas of pistachio and pomegranate fields, where high overwintering populations of the insect can move into almond orchards mid- to early summer once the moths emerge.

In Merced County, some growers may not spray their Nonpareils, but will spray Fritz or Monterey trees because they are harvested later, Doll says.

Regardless of the number of applications made in a season, early treatment is critical. “You want to hit the insect as the eggs are laid or as they hatch.”

The ideal ground speed when spraying is 2 mph; any faster can reduce coverage in the upper canopy of the more mature trees.

The latest insecticide products are also helping to keep NOW in check.

“The new chemistries that are now available offer longer persistence,” Doll says. “You can spray them earlier — slightly before hull split — and get as much as 14 days of insect control.”

TAGS: Tree Nuts
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