Agriculture: Almond Pest Management Guidelines

Hull Rot

  • Rhizopus stolonifer and Monilinia spp.
  • 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 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.

    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. 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).

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. DIFENOCONAZOLE/CYPRODINIL
      (Inspire Super) 10–12 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 14
      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 four applications per season of QoIs or SDHIs to limit the development of resistance.
     
    C. METCONAZOLE
      (Quash) 3.5 oz 12 25
      MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3)
      COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance.
     
    D. DIFENOCONAZOLE
      (Inspire) 7 fl oz 12 14
      MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3)
      COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance.
     
    E. 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 apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not make more than four applications per season of QoIs or SDHIs to limit the potential for the development of resistance.
     
    F. AZOXYSTROBIN
      (Abound) 12.0–15.5 fl oz 4 28
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
      COMMENTS: Do not apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than four applications of QoI or SDHI fungicides per year or apply more than 92.3 fl oz/acre per season to limit the potential for the development of resistance.
     
    G. TRIFLOXYSTROBIN
      (Gem 500 SC) 3 fl oz 12 14
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
      COMMENTS: Do not make more than four applications per season and no more than two sequential sprays of QoI or SDHI fungicides to limit the potential for the development of resistance.
     
    H. PYRACLOSTROBIN/FLUXAPYROXAD
      (Merivon) 6.5 oz 12 14
      MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7)
      COMMENTS: Do not make more than three applications per season of QoIs (strobilurins) or SDHIs to limit the potential for the development of resistance. Note that resistant populations have developed in selected almond-producing areas within California.
     
    I. 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: Do not make more than four applications per season of QoI or SDHI fungicides and no more than two sequential applications before rotation to a different mode of action fungicide to limit the potential for the development of resistance.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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 action. 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 a fungicide with a different mode-of-action group number.
    QoI = quinone outside inhibitor or strobilurin
    DMI = demethylation (sterol) inhibitor
    SDHI = succinate dehydrogenase inhibitor
    Text Updated: 08/17
    Treatment Table Updated: 08/17