Hull Rot

The first indication of hull rot usually comes several weeks before harvest, when leaves on a shoot wither and die. Closely examine fruit on this shoot for a brown area on the outside of the hull and either tan fungal growth in the brown area on the inside or outside of the hull (this indicates Monilinia) or black fungal growth on the inside of the hull (this indicates Rhizopus). Fungi invade hulls and produce a toxin that kills the spur and eventually the shoot attached to the fruit. Because the shoot is killed, nuts on affected shoots may be more difficult to remove during harvest. The disease causes dieback of shoots and fruiting wood (strikes) that reduces productivity in future years.

Almond hulls are susceptible to hull rot fungi from the beginning of hullsplit until the hulls dry—a period that can last from 10 days to 2 months depending on fertilization and irrigation.

Look for nuts or leaves stuck on trees well after harvest as an indication of hull rot infections. Managing tree vigor through proper irrigation and nitrogen fertilization is the most effective control.

• Regulated deficit irrigating (PDF) or reducing irrigation at the onset of hullsplit greatly reduces incidence of hull rot.
• Avoid standing water at hullsplit.
• Avoid excess nitrogen fertilizer and applications 45 to 60 days before hull split. Take leaf samples in July to be sure nitrogen levels, which should be below 2.6%, do not favor hull rot.

Almond varieties vary in their susceptibility. The most susceptible important varieties are Nonpareil, Monterey, Sonora, Fritz, and Winters. Hard-shelled varieties such as Mission, Davey, and Drake may exhibit rotted hulls but rarely shoot dieback.

Management of hull rot caused by Rhizopus stolonifer shows that demethylation (sterol) inhibitor (DMI; FRAC group 3) and quinone outside inhibitor (QoI; FRAC group 11) fungicides are highly effective against this pathogen. A single application at hullsplit, timed with the navel orangeworm insecticide treatment, may reduce hull rot incidence by 60 to 70%. Fungicide treatments should be integrated into deficit irrigation practices during hullsplit. Hull rot caused by Monilinia spp. is best managed with fungicide applications 3 to 4 weeks before hull split (early June).