Brown Rot Is A Good Grower’s Disease
August 1, 2007 
Brent Holtz has seen the symptoms too many times. The University of California (UC) pomologist will be called by a grower to look at a block of well-watered, well-fertilized almond trees with thick, handsome canopies. The only problem is they lack bloom in the lower half, along with a lot of dead fruiting spurs. Growers often think the vigorous canopy is to blame, and it’s a simple case of shade-out. But more often than not, Holtz knows that it’s a far more serious problem: hull rot.
In fact, Holtz, a UC Cooperative Extension farm adviser in Madera County, believes that hull rot is the single greatest yield reducer of vigorous young almond orchards that are entering their prime production years. The reason is that with other diseases, such as brown rot, a grower can solve the problem simply by spraying. The next year the tree will be fine. But hull rot actually kills lower limbs. “And they don’t re-grow,” he says, “so you lose yield for the entire life of the orchard.”
What’s also insidious about hull rot is that growers who do spray will see positive effects, but they are only short-term. It’s very hard to get coverage between the hull and the shell, and a week or two later the fungus generally returns. The only effective treatment for hull rot, says Holtz, is to reduce irrigation. “A lot of people want to put a fungicide on because it’s easier,” he says. “But irrigation management is the way to go, especially in this environmentally sensitive world.”
Drop The Bomb
But those growers are right, irrigation management is tricky, concedes Holtz. You can’t just reduce water willy-nilly, because you could actually be drying out some of your trees too much. There is simply no substitute for using a pressure bomb to measure the trees’ mid-day stem water potential. The ideal stem water potential for trees up until hull split is between -7 and -9 bars. (The higher the negative number, the more water stress.) But then during hull split, growers should shoot for stem water potentials between -14 and -18 bars.
Growers using pressure bombs for the first time are likely in for some surprises, says Holtz. He recently visited a grower who had two trees not far apart, on the same irrigation system, and one measured -12 and the other -18. Variations in the soil likely accounted for the difference, says Holtz. “That’s why we use pressure bombs,” he says. “You can’t expect every orchard to respond the same way.”
Some varieties, such as Sonora, Butte, Monterey, and the popular Nonpareil, do appear more sensitive to hull rot, says Holtz. But that’s not due so much to something peculiar to the variety, as to the fact that they are the first varieties to go through hull split.
“That’s why Nonpareil can really get it, because it’s usually well watered when the heat wave hits,” he says, noting that last year’s record San Joaquin Valley heat made for a particularly bad year. “It’s hard to practice reduced irrigation when it’s 110°F.”
Subscribe Today For
