Hull Rot

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

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

Almond tree in blossom (Kate Russell)

Almond tree in full leaf (Kate Russell)

Those flowers will fall, and fuzzy green almonds will start to grow. As they 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 is a fungal disease initiated by Monilinia, Rhizopus, and Aspergillus niger. Aspergillus niger also causes sooty mold. If you notice leaves on shoots withering and dying, look more closely.

Hull rot symptoms

You may see brown areas on or in the hull (Monilinia). You may see black fungal growth inside 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 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 more difficult to remove than healthy nuts. Ultimately, shoots, strikes (fruiting wood), and lower limbs die back.

Hull rot prevention and management

Healthy trees are less likely to become infected in the first place, so care for almond trees properly with these tips:

• Feed trees with aged compost or fertilizer regularly in spring and summer.
• Provide 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 improved when trees get a total of 54-58” of water each year.
• Employ deficit irrigation for two weeks once almond hulls start to split. Cutting back on watering by 10-20% improves nut flavor and reduces the chance of hull rot.
• Cut back on nitrogen fertilizers for 40 to 60 days before harvest. 
• Harvest almonds 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.
• Remove and dispose of mummies, which can harbor hull rot and many other diseases. 
• Avoid damaging tree trunks. Bark provides good protection against pests and diseases. 
• Avoid overhead watering, which can bounce fungal spores and spread infection.

Trees infected with hull rot need help from a professional arborist. The fungicides effective against hull rot are generally unavailable to the public.

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