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Ask Your PCA: How can almond farmers manage hull rot?

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Issue Date: June 9, 2021
Justin Nay, PCA, Integral Ag Services, Durham
Justin Nay

It has been dry and windy this year, with very little disease activity in almonds. But, as hull split approaches, hull rot becomes a concern as it provides an opening for the pathogen.

Hull rot is a hull infection. There are three pathogens—Rhizopus stolonifer, Monilinia spp. and, more rarely, Aspergillus niger—that will infect the hull. Ranches that are shaded and overgrown with a heavy crop are prone to developing hull rot.

Rhizopus stolonifer gets into the hull and produces a toxin that kills the spur and eventually the shoot, which will reduce the bearing surface of the trees. As a result, the trees become less productive.

Eventually, the toxin clears out the branch and hollows out the entire inner canopy of the tree. Over time, it pushes the crop up the tree and out of the inner canopy.

Limiting nitrogen and excess moisture are key. Adequate but not excessive nitrogen helps manage hull rot.

Water management seems to be a key factor, but orchards that use heavy deficit irrigation have seen more Aspergillus niger pathogen.

Aspergillus niger causes a black fungus inside the hull, rather than white or gray fungus that is seen with Rhizopus stolonifer and Monilinia. Though Aspergillus niger doesn't seem to kill as many spurs, it does cause problems with stick-tights. The nuts don't shake off at harvest, and this creates problems with winter sanitation later in the year. Mummies left in the orchard are feeding sites for navel orangeworm if they aren't removed.

There are fungicides for treating hull rot, but it's important to use management practices to prevent chemical resistance from building.

With another drought year in California and growers limiting irrigation, there may be more hull rot from the Aspergillus niger pathogen—so, hull rot in a different form than what growers are used to seeing.

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.




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