How to Manage Pests

UC Pest Management Guidelines

Almond

Hull Rot

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.

MANAGEMENT

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 (PDF) 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.

Common name Amount/Acre R.E.I.+ P.H.I.+
(example trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. DIFENOCONAZOLE/CYPRODINIL
  (Inspire Super) 10.0-12.0 fl oz 12 60
  MODE OF ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3) and anilinopyrimidine (9)
  COMMENTS: The demethylation (sterol) inhibitor component is the only ingredient in this product that is active against Rhizopus stolonifer.
 
B. FLUOPYRAM/TRIFLOXYSTROBIN
  (Luna Sensation) 5.0-7.6 fl oz 12 60
  MODE OF ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (7) and Quinone outside inhibitor (11)
  COMMENTS: The QoI component is the only ingredient in this product that is active against Rhizopus stolonifer. Do not make more than 4 applications per season of QoIs or SDHIs to limit the development of resistance.
 
C. PYRACLOSTROBIN/BOSCALID
  (Pristine) 10.5–14.5 oz 12 25
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7)
  COMMENTS: The QoI component is the only ingredient in this product that is active against Rhizopus stolonifer. Do not make more than 4 applications per season of QoIs or SDHIs to limit the development of resistance.
 
D. AZOXYSTROBIN/DIFENOCONAZOLE
  (Quadris Top) 14 oz 12 28
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than 4 applications per season of QoIs or SDHIs to limit the potential for the development of resistance.
 
E. AZOXYSTROBIN
  (Abound) 12.5 fl oz 4 28
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not make more than 4 applications per season of QoIs or SDHIs to limit the potential for the development of resistance.
 
F. TRIFLOXYSTROBIN
  (Gem) 3 fl oz 12 60
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not make more than 4 applications per season of QoIs or SDHIs to limit the potential for the development of resistance.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (For more information, see www.frac.info). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431

Diseases

J. E. Adaskaveg, Plant Pathology, UC Riverside
W. D. Gubler, Plant Pathology, UC Davis
Roger Duncan, UC Cooperative Extension, Stanislaus County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
B. A. Holtz, UC Cooperative Extension, San Joaquin County

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