Almond hull rot is a fungal disease that affects the hulls of nuts on almond trees. It can cause big losses in almond farming, but it may also impact the occasional backyard tree. Understanding basic hull rot information and identifying factors can help you manage this disease that can permanently destroy fruiting wood on your tree.
What is Hull Rot?
Nut crops with hull rot are often greatly diminished, and even worse, the disease will destroy the affected wood so that it dies. Hull rot can be caused by one of two fungal species: Rhizopus stolonifera causes black spores inside a split hull and Monilinia fructicola produces tan-colored spores inside and outside the hull after it has split. Before you can see the spores, though, you may see leaves on a small affected branch wither and then die.
Managing Hull Rot in Nuts
Ironically, it is the abundance of water and nutrients that you think are helping your almond tree grow well that invites hull rot. Agricultural researchers have found that putting almond trees in slight water stress—in other words, reducing watering slightly—a couple of weeks before harvest, around the time the hulls split, will prevent or significantly minimize hull rot.
This sounds pretty easy, but to actually make water stress work as a way to prevent rotting nut hulls you need to use a pressure bomb. This is a device that measures water stress by sampling leaves from the tree. Researchers say that simply reducing watering by an arbitrary amount won’t work; it has to be measured, slight water stress. This can be tricky if you have deep soil that holds water well. It could take a few weeks to achieve the stress necessary.
The effort and price of the pressure bomb may be worthwhile, though, as hull rot is a devastating disease when it takes over a tree. It destroys fruiting wood and can even ruin and kill the entire tree. The infected hulls also turn into great habitat for a pest called navel orangeworm.
In addition to creating water stress, avoid over-fertilizing. Too much nitrogen can lead to a fungal infection. Reducing water is the most effective way to manage or prevent hull rot in nuts, but you can also try fungicides and planting almond varieties that have some resistance. These include Monterey, Carmel, and Fritz.
Almond varieties most susceptible to hull rot are Nonpareil, Winters, and Butte.