Hull Rot

Hull rot isn’t a warning to sailors, but your almond tree may be at risk.

Roasted almonds (Kate Russell)

If you grow almonds, your tree goes through several stages before producing a crop. Being deciduous, winters are spent bare and quiet. In late winter or early spring, a riot of flower blossoms appears, followed by leafing out.

Almond tree in blossom (Kate Russell)

Almond tree in full leaf (Kate Russell)

Where each of those flowers had been, fuzzy green almonds begin to develop. It’s amazing how many almonds a small tree can produce each year! As those almonds ripen, they get fat. Ultimately, the hulls dry and split open. When that occurs, hull rot may invade the nuts, threatening the life of your tree.

Clusters of unripe almonds replace the blossoms (Kate Russell)

Almond spurs (Kate Russell)

Almond trees produce a lot of nuts! (Kate Russell)

Hull rot can be caused by one of three different fungi: Monilinia, Rhizopus, or Aspergillus niger. Aspergillus niger also causes sooty mold. If you notice leaves on shoots withering and dying, take a closer look. {Sadly, I was unable to find any photos I could use, so you’ll have to Google “almond hull rot” to see what it looks like.


Hull rot symptoms

You may see brown areas on the outside or inside of the hull (Monilinia). You may see black fungal growth on the inside of the hull, between the hull and the shell, or on either side of the edge that opens (Rhizopus stolonifer). Or, you may see black spores between the hull and the shell (Aspergillus niger). Each of these is a type of hull rot.


These fungi produce toxins that kill fruiting spurs and the twigs to which they are attached. Almonds on infected branches are harder to remove than healthy ripe nuts. Ultimately, shoots, strikes (fruiting wood), and lower limbs dieback.


Hull rot prevention and management

Healthy trees are less likely to become infected in the first place, so irrigate and fertilize trees properly. This means feeding your trees with aged compost or fertilizer at regular intervals in spring and summer, and providing 2-3” of water each week in summer. While an almond tree can survive and produce fruit with as little as 8” inches of water in a year, crop sizes and tree health are greatly improved when trees get a total of 54-58” of water each year.


As your almonds start to split, cut back on watering by 10-20%. This is called deficit irrigation and it should be maintained for 2 weeks before returning to regular watering. Deficit irrigation improves nut flavor and reduces the chance of hull rot.


Cut back on nitrogen fertilizers, too, for 40 to 60 days before harvest. Speaking of harvest, get those almonds off the tree as soon as they are ripe and toss them in the freezer for a couple of weeks to kill any worms that may be present.


Always remove and dispose of mummies, which can harbor hull rot and several other diseases. And avoid damaging your tree’s trunk. Bark provides good protection against pests and diseases. And if fungal diseases are common in your landscape, avoid overhead watering, which can bounce fungal spores up into healthy plants


If your tree is badly infected, you may need to resort to chemical fungicides. The products effective against hull rot are generally not available to the public, so you will need to contact a certified arborist for help.


Hopefully, your almond tree will never become infected with hull rot.