How to Manage Pests
UC Pest Management Guidelines
Pathogens: Rhizopus stolonifer and Monilinia spp.
(Reviewed 3/09, updated 11/12)
In this Guideline:
SYMPTOMS AND SIGNS
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 shoot attached to the fruit. Because the shoot is killed, other green fruit on the shoot don't mature and they remain on the tree after harvest. The disease causes dieback of shoots and fruiting wood that reduces productivity in future years.
COMMENTS ON THE DISEASE
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. No fungicides are suggested for hull rot. Irrigation management is the most important cultural control. Regulated deficit irrigating or reducing irrigation at the onset of hullsplit greatly reduces incidence of hull rot at hullsplit. Fertilizer management is also important; avoid excess nitrogen fertilizer. Take leaf samples in July to be sure nitrogen levels, which should be below 2.6%, don't favor hull rot.
Almond varieties vary in their susceptibility. The most susceptible varieties are Nonpareil, Kapareil, Kochi, Sonora, Jordanolo, and IXL. Although frequently affected by hull rot, damage tends to be minimal on Merced, Thompson, and NePlus Ultra. Hard-shelled varieties such as Mission, Davey, and Drake may exhibit rotted hulls but rarely shoot dieback.
Research on management of hull rot caused by Rhizopus stolonifer shows that demethylation (sterol) inhibitor and Quinone outside inhibitor 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 60-70%. Fungicide treatments should be integrated into deficit irrigation practices during hullsplit. Fungicide treatments for hull rot caused by Monilinia spp. have not been successful when applied at hullsplit or at later times.
UC IPM Pest Management Guidelines:
J. E. Adaskaveg, Plant Pathology, UC Riverside
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