Manage Hull Rot with Deficit Irrigation, Balanced Fertility

Ongoing research shows certain fungicides can reduce hull rot caused by the bread mold Rhizopus stolonifer (top).Past and current ABC-funded research illustrates that effective hull rot management takes an integrated approach that includes regulated deficit irrigation at hullsplit, and a balanced fertility program that does not apply nitrogen in excess of tree and crop demands.

Hull rot can be reduced 60% to 90% by inducing mild stress at hullsplit initiation and maintaining that stress for two weeks afterward. This is best executed by tracking tree water status with a pressure chamber; the goal is to reach mild stress with readings of –14 to –18 bars. Pressure chamber readings should be done in concert with soil moisture monitoring to ensure deep moisture is not depleted, imposing severe stress that is difficult for trees to recover from.Hull rot caused by Monilinia spp. (bottom) does not respond to fungicide applications. This disease can grow on the outside of the hull. Photos: Jim Adaskaveg, UC Riverside

Depending on the soil type, reaching this stress level can take just a few days or much longer, so timing is a challenge. One new tool to help predict the onset of hullsplit is the UC Almond Hullsplit Prediction Model.

Under high-pressure situations, recent research shows that some registered fungicides applied at around 20% hullsplit can reduce hull rot caused by the bread mold Rhizopus stolonifer from 30% to as much as 60% to 80%. Fungicides applied at this timing are not effective against hull rot caused by Monilinia spp. Research is ongoing to determine if fungicide treatments can be an effective tool for hull rot caused by Monilinia spp.

More information on hull rot management and ABC-funded research findings can be found on the Almond Board website at