A Little Stress Is Best
Less water at hull split reduces hull rot
There's always an exception to the rule. Amidst the expert advice to keep your trees fully irrigated and fertilized to achieve optimum tree growth and health - and maximum crop production - dwells the caveat that field tests indicate that water stress at hull split dramatically reduces hull rot strikes and substantially improves nut removal at harvest.
"Inducing stress early in hull split has worked gangbusters for us," says Beth Teviotdale, UC Extension plant pathologist at the Kearney Agricultural Center.
She and her team have been testing the stress remedy for a number of years, as has Wilbur Reil, farm advisor for Yolo and Solano counties. They've had similar results even though their tests have been conducted with different types of irrigation equipment and on different soils. Their results show that stressing almond trees at the beginning of hull split reduces hull rot by two-thirds or more.
If deprivation minimizes hull rot, then it isn't too great a stretch to conclude that the disease may be caused by just the opposite condition: plenty of water and fertilizer. You'd be right; that is exactly the case.
"I call hull rot the gout of almond trees," Teviotdale chuckles. "It comes from too much good food and drink."
The most severe cases are found in well-managed orchards with large, vigorous trees, according to IPM for Almonds. The UC publication explains that hull rot can be caused by either the common bread mold, Rhizopus stolonifer, or by the brown rot fungi, Monolinia fructicola and M. laxa. The disease appeared in almonds in the 1950s and now occurs in most almond-growing areas.
It strikes several weeks before harvest. The leaves on a shoot wither and die. The fruit on the shoot will have a brown area on the outside of the hull and either tan fungal growth in the brown area on the inside or outside of the hull (which indicates Monilinia at work) or black fungal growth on the inside of the hull (Rhizopus'signature). The fungi invade the hulls after they split in early July and apparently produce a toxin that causes the shoot and leaves to die. With the shoot dead, other green nuts on it won't mature. This is the most damaging aspect of hull rot: dieback of shoots and fruiting wood, and the resulting reduction in yield.
Almond hulls are susceptible to the disease from the beginning of hull split until the hulls dry, a period of ten days to two months, depending on conditions. Trees with dense canopies are most vulnerable, because of restricted air movement and sunlight.
Your choice of cultivar can influence your exposure to hull rot. According to the University, the most susceptible varieties are Nonpareil, Jordanolo and IXL. Also frequently affected, but suffering little damage, are Merced, Thompson, and Ne Plus Ultra. Hardshell varieties, such as Mission, Davey and Drake, may develop rotted hulls, but shoot dieback is rare.
Anything you can do to encourage uniformity of hull split and decrease drying time of the hulls without sacrificing yield or kernel quality will help minimize problems with hull rot. For example, prune your trees to increase the amount of sunlight that reaches the fruit. And follow Teviotdale's and Reil's suggestions for inducing water stress at the beginning of hull split. How you go about this depends on your own unique orchard conditions. There is no rule of thumb that applies across the board, except that water should be withheld early in hull split, then restored before entering the pre-harvest irrigation regime.
One grower, with whom Teviotdale worked, normally irrigated on an every-two-week cycle. During the trial, he counted back from the expected date of hull split, timed his irrigations to apply a full irrigation, wait two weeks and apply one-half of the usual amount, and then withhold irrigation at the beginning of hull split. Two weeks later he put on one-half of the usual amount of water and followed up two weeks later with a full irrigation. "For him, with his soils and his irrigation system, it worked very well," says Teviotdale. "But people have done it other ways."
Conditions vary greatly from one orchard to the next, Teviotdale observes, with some blocks needing water every few days while others need a drink only once every two to three weeks. "The people we worked with gauged how long until hull split and made their last irrigations at the appropriate interval to not irrigate during hull split," Teviotdale said. "You can't tell by looking at the trees."
Short of using a pressure bomb to accurately measure the water content of the leaves, a grower will have to rely on his knowledge and intuition when determining when and how much to hold back on the irrigation. "Most people have to guess at it," she says.
Guessing game or not, it is known that a little stress is a good thing when it comes to reducing hull rot. "It's been consistent for us and for Wilbur (Reil)," Teviotdale says. Maybe, with the University's results as encouragement, it's time to manage your orchards for less gout and more crop.