Retired University of California Extension Plant Pathologist Beth Teviotdale dubbed almond hull rot the “gout of almond diseases.”
“Too much food and drink is bad for almonds just like it is bad for us.” Teviotdale is often quoted when the subject of hull rot is on the agenda of field meetings like the one sponsored recently by The Almond Pest Management Alliance at Pik-A-Lok farms near Mendota, Calif.
Madera County UC Cooperative Extension Farm Advisor Brent Holtz calls hull rot the “good growers disease” since, ironically, the disease is often worse in well-maintained orchards.
With the continued high grower price of almonds and many new, high-density, well-irrigated orchards — growers are spending a lot of money to get trees into production quickly and keep them happy with plenty of water and nutrients. However, Holtz told growers and pest control advisers at the Mendota field day they are literally killing their trees with kindness.
Hull rot is far worse than a heavy mite infestation which may defoliate a tree and possibly reduce a subsequent year’s yields. Hull rot, by comparison, permanently destroying fruit wood.
Holtz called it “the single, greatest yield reducer of vigorous young almond orchards in the central San Joaquin Valley that are entering their prime production years.”
Fortunately, Holtz said work by Teviotdale, UC Irrigation Specialist David Goldhammer and former UCCE Kern County Farm Advisor Mario Viveros proved hull rot is preventable or at least can be significantly minimized by putting trees under slight, imperceptible water stress about two weeks before harvest.
Holtz said it should not be seat-of-the-pants eyeballing water stress, but water stress measured with a pressure bomb that measures the water stress level of plants using leaf samples.
Typically, the mid-day stem water potential for a well-supplied tree is minus 7 to minus 9 bars. However, to reduce hull rot, Holtz said pressure bomb readings of minus 14 to minus 16 bars significantly reduces hull rot.
Holtz said the induced stress as measured by the pressure bomb is not visibly evident in trees and does not over-stress the trees, making them susceptible to mite damage or defoliation.
It is not simple to reduce the soil moisture and water levels in the trees, admits Holtz. In some orchards, Holtz achieved the desired stress level in just a few days while in other orchards with deep, well-drained soils, it could take as long as 20 to 30 days. He suggested growers play around with irrigation sets to achieve the desired pressure bomb reading. “There is potential in achieving minus 14 bars if we think in terms of reducing irrigation sets from say 48 hours to 32 hours to get to that desired pressure bomb level.”
However, he said it is worth the effort. Otherwise, hull rot can literally kill the bottom portion of a tree or at least reduce fruiting wood and thereby reduce yields for the life of a tree. Once wood is infected and dies, it is lost forever.
A grower will know a tree is infected with hull rot when about mid hull split, clusters of dry leaves begin to appear. Individual spurs, small shoots and entire small branches may also collapse due to hull rot infections.
Nonpareil, the most popular variety in the state, is the most severely affected, since it is the first one to hull split when stem water potential is the highest compared to later-harvested varieties.
Many growers, Holtz said, confuse hull rot with shading out, since both result in lower canopy dead wood.
“I believe that hull rot actually enhances shading out as the tree abandons infected wood in the lower canopy for healthy wood receiving sunlight at the top of the canopy,” said Holtz. Hull rot is caused by two fungi. Ironically the initial infection does not affect yield that year. After hull split, one of the two fungi get inside the hull and infection is initiated. The fungi cannot penetrate a healthy, outer hull surface before hull split. However, when the fungi invade a split hull, the toxin can spread from the inside of the hull to outer areas of the hull. A toxin produced in the infected hull spreads to neighboring leaves and shoots, causing death of the fruiting wood. Infected fruit can become stick-tight during the first year of infection, remaining on the tree after it has been shaken. These infected hulls become habitat for Navel orangeworm. “Maybe it’s time we start putting our good orchards on a diet,” said the farm advisor.