Q. I've begun my almond harvest and have found black substance on the inside of some of them. It is almost like soot. Do you know what it is and is it toxic? Should I wash it off or throw them out?
A. You didn’t say if it was inside the nut (kernel) or just on the husk. I am assuming it is only on the husk because this is much more common. If this mold is on the nut, discard these infected nuts and do not contaminate other nuts. This reduces the potential of a mold “toxicity” to humans due to aflatoxins.
Yours is most likely one of the “bread molds” infesting the husk or hull and probably developed due to recent rains and high humidity. The nut is safe to eat if not infected. Normally hull rot, as it is called, is not as common here as it is in more humid climates where it can actually attack fruiting portions of the tree and cause a decline in production.
Hull rot is accentuated if you apply too much water and nitrogen fertilizer to the tree. Let them get a little “hungry” for fertilizer and start applying less water to them 4-6 weeks before harvest (skip an irrigation day). If this problem will probably not continue in future years unless it rains again during nut hardening.
Harvest the nuts, dry them out, remove the husk and inspect the nut when you open it. If the nut is infected, discard these nuts and wash your hands.