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How to Minimize Hull Rot

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Early irrigation cut-off and harvest can help

Almond Facts, May/June 1996

Hull split is just around the corner, which means hull rot may soon visit your orchard, eating into this years crop and reducing your prospects for next year. Ironically, the more vigorous your orchard is, the more likely it is to be struck by hull rot. And there is no spray recommendation for hull rot control.

But you don't need to be a victim. Recent research indicates that by taking a few simple precautions_prune for open canopy, cut back on irrigation before harvest, and timely harvest_you can minimize damage from this fungal pest.

Kills Fruiting Wood

Hull rot was first noticed in California in the 1950s, and has since spread throughout the almond-growing district. Today it is considered to be a somewhat sporadic but potentially serious disease of almonds. All almond cultivars are susceptible but Nonpareil is the most vulnerable.

Hull rot is generally caused by the common bread mold, Rhizopus stolonifer or by the brown rot fungi, Monolinia fructicola. The fungi set up shop in almond hulls as soon as the hulls begin to split. As they colonize, they produce a toxin that attacks the shoot at the point where the nut is attached, killing the shoot and leaves all the way to the tip of the spur. Sometimes the toxin spreads in the other direction, causing even more damage up the branch.

Hull rot doesn't damage the nuts inside the hulls that it invades. It diminishes crops by killing the shoots, which stops other green nuts on those shoots from maturing. The immature nuts remain on the trees after harvest, providing overwintering sites for navel orangeworm, unless removed, which can be an expensive proposition.

By killing fruiting wood, the toxin cuts into the next year's crop. According to University of California publication 3308, Integrated Pest Management for Almonds, "when more than 30 percent of the fruiting wood is killed, a significant reduction in yield results. If less than 20 percent of the wood is killed, economic losses are minimized by the fact that most trees can offset the loss with new growth."

Symptoms

Dr. Beth Teviotdale, University of California plant pathologist at the Kearney Agricultural Center, describes the first symptom of hull rot as "a grayish lesion that soon turns tan to brown on the hull of the maturing almond fruit." Soon, nearby leaves and part or all of the spur or shoot bearing the fruit die. The dead leaves remain attached to the twigs, giving the infected area a "scorched appearance," she explains.

Inside the infected hull, spores of the Rhizopus fungi are easily seen between the hull and shell. Spores of M. fructicola are found on inner and outer hull surfaces.

The Critical Period

Hull rot infections occur at hull split. Up until that time, the hull exterior protects against the fungi. But as soon as the hull opens along the suture, wind- and insect-borne pathogens find their way into the inner hull and begin to grow in the moist environment. Almond hulls are susceptible to infection from the beginning of hull split until the hulls dry _a period of 10 days to two months. Once the hulls are dry, the fungi can no longer get a foothold.

A number of factors influence the length of time that the hulls are susceptible, including the amounts of irrigation and fertilizer the orchard receives, the space between the trees, and density of the canopy. Rainfall and sprinkler irrigation between hull split and harvest will also increase drying time and susceptibility.

Controlling Hull Rot

Dr. Teviotdale points out that hull rot is "most prevalent in certain almond cultivars and in high yielding, densely canopied orchards which receive ample nitrogen and water." She has observed that "sudden outbreaks of hull rot often follow late preharvest irrigations."

These contributing factors are all easy enough to manage. Consequently, with a few minor changes in orchard practices, you can minimize the likelihood of a serious outbreak of hull rot in your orchard. Here are some recommendations gleaned from U.C. Davis publications and personnel.

Plant resistant varieties_When possible, select resistant cultivars for new plantings. According to IPM for Almonds, the most susceptible varieties are Nonpareil, Jordanolo, and IXL. Merced, Thompson, and Ne Plus Ultra are affected, but the damage is usually minimal. Hulls may rot on hardshell varieties, such as Mission, but shoot dieback is rare. Even though Nonpareil is susceptible to hull rot, Blue Diamond® recommends planting it, because of Nonpareil's marketability and importance to the overall sales effort.

Prune for uniformity_IPM for Almonds recommends pruning trees to increase the amount of sun exposure that the almonds receive, but not to the point that yield is sacrificed. Proper pruning will increase the uniformity of hull split and decrease hull drying time.

Fine tune the nitrogen_Excessive levels of nitrogen may slow the maturation of the crop, says Dr. Teviotdale. This would lengthen the amount of time it takes for the hulls and nuts to separate and dry, giving the fungi more time to do their damage.

Lonnie Hendricks, Merced County farm advisor, tells his growers to "use only enough nitrogen to maintain a 2.2 percent to 2.5 percent leaf N level." He says nitrogen test applications showed a sharp increase inhull rot from the 125 pound-per-acre N application to the 250 pound- per-acre rate. He reminds growers that "composts, manures, legume covercrops, and even your irrigation water can all contribute to the total N application, so consider those sources as well."

Stress for less moisture_Recent experiments in Kern and Yolo counties support mildly stressing almond orchards at the beginning of hull split to reduce hull rot. It has been shown that a sharp cutback in water just prior to harvest can dramatically reduce the incidence of hull rot. Wilbur Reil, farm advisor for Solano and Yolo counties, and David Goldhamer, UC irrigation specialist at the Kearney Ag Center in Parlier, have conducted separate studies of the effects on hull rot of different irrigation regimes. Both performed their research on Nonpareil.

Reil found that irrigating at 50 percent evapotranspiration at early hull split drastically reduced hull rot while greatly improving nut removal. Trees in his trials were on either surface or subsurface drip.

Goldhamer's work in a Kern County orchard watered with microsprinklers revealed that irrigation within two weeks of harvest resulted in a 10-fold increase in hull rot compared to earlier cutoff dates.

Dr. Teviotdale has been following the tests. She says that best results appear to be achieved by inducing a two-week period of stress starting with the beginning of hull split. Cut water delivery to the trees by 50 percent, then after two weeks, resume normal irrigation, she says. It is important to cut the water supply all at once. A slow reduction is not as effective. The objective is to reduce hull moisture, making it more difficult for hull rot fungi to colonize. Do this, and you can expect hull rot damage to drop by half to two-thirds.

No need to worry about damaging the trees or affecting yield, says Dr. Teviotdale. The stress is inconsequential, and when irrigation returns to normal, the trees quickly recover. Teviotdale points out that best results are obtained on light soils where a 50 percent reduction in water gets a quick response from the trees. Trees on heavier soils may not notice the change in irrigation, because they will draw on residual moisture.

Reil cautions that tree age is an important consideration. Young trees, he observes, do not respond as well. This is because trees of different ages need different amounts of ET reduction.

Proper Harvest Timing _"The longer nuts hang on the trees, the more opportunity there will be for infection and toxin production," says Hendricks. IPM for Almonds recommends starting harvest as soon as nuts at eye level show 95 to 100 percent hull split. Early harvest will lower the incidence of twig dieback.

Remove mummies_After harvest, remove infected nuts that remain on the trees. This can help reduce the level of inoculum in the orchard and will also eliminate overwintering sites for navel orangeworm.

Try these steps and perhaps you can cut your losses to hull rot this year. Remember: No spray program has proved useful, but a few changes in your routine may save the day.