California Almonds: Bloom Diseases – A Quick Overview

Almond bloom is quickly approaching. Diseases require specific conditions for infection – susceptible tissue, moisture, conducive temperatures – and these are often all present during bloom. Appropriate material choice, application timings and coverage are critical to minimize losses. Below are susceptibility and treatment information for the primary spring diseases in almond production.

Brown Rot (Monolinia laxa or M. fructicola)

  • Most susceptible cultivars: Butte, Carmel, Winters, Wood Colony.
  • Infection timing: Pink bud through petal fall; highly susceptible when flowers are open.
  • Favorable conditions for infection: Rainy weather (or high humidity/dew) and warm temperatures (over 58oF).
  • Treatment timing: If it’s a dry (no rain) spring, one application at 20-40% bloom. If infections were severe last year, or it is a wet spring, treat at pink bud (5% bloom) and full (80%) bloom.

Anthracnose (Colletotrichum acutatum)

  • Most susceptible cultivars: Butte, Fritz, Monterey, Peerless, Price, Winters. Nonpareil is less susceptible than others. All cultivars are susceptible.
  • Infection timing: Pink bud through May if rain continues.
  • Favorable conditions for infection: Warm, rainy weather. Wet tissue and temperatures over 59oF carry the greatest risk for anthracnose infection.
  • Treatment timing: Apply beginning at pink bud (5% bloom) and repeat every 10-14 days if rain continues (possibly into May, if late spring rains occur).
  • Material considerations: Rotate fungicide FRAC groups to delay resistance development. See the fungicide efficacy table (separate insert) for effective materials.

Green Fruit Rot (Botrytis cinerea, Sclerotinia sclerotiorum, and Monilinia laxa)

  • Infection timing: Latter part of full bloom to petal fall when senescing petals and anthers are present
  • Favorable conditions for infection: Cool, rainy weather, and nut clusters that trap senescing flower parts.
  • Treatment timing: Full bloom, when bloom is extended and weather is cool and wet.
  • Material considerations: DMI fungicides (FRAC 3) are ineffective against Botrytis.

Bacterial Blast (Pseudomonas syringae)

  • Most susceptible cultivars: Ne Plus Ultra, Peerless. Plum rooted trees are more susceptible than those on peach rootstock.
  • Infection timing: Swollen bud through the small nut stage.
  • Favorable conditions for infection: Cold, wet, and/or frosty weather.
  • Treatment timing: None recommended at this time. A dormant copper treatment is sometimes suggested, but efficacy studies have not demonstrated control with a copper program. Where copper is used regularly Pseudomonas has been confirmed to be copper resistant.

Shot hole (Wilsonomyces carpophilus)

  • Most susceptible cultivars: Butte, Carmel, Fritz, Price, Sonora
  • Infection timing: Bloom through spring. Prolonged, wet spring weather results in the greatest risk of damaging levels of this disease, which can drop nuts and defoliate trees.
  • Favorable conditions for infection: High levels of inoculum and wet weather. Monitor for fruiting bodies (sporadochia) in leaf lesions in spring.
  • Treatment timing: If fruiting bodies were found on leaves the previous fall, spray at petal fall or when new leaves emerge. If no fruiting bodies seen the previous fall, wait to spray until fruiting bodies are found on current season leaves. Once present, apply protective fungicides prior to additional rain.

Bacterial spot (Xanthomonas arboricola pv. pruni)

  • Most susceptible cultivars: Fritz (highly susceptible), Butte, Carmel, Nonpareil, Price
  • Infection timing: Infections occur in the spring but a delayed dormant and a bloom spray can deliver good to excellent control of this spring disease.
  • Favorable conditions for infection: wet and warm (>68oF) conditions
  • Treatment timing: Dormant or delayed dormant (copper + mancozeb) reduced infections by more than 75% in UC research. Single applications of copper or copper + mancozeb at full bloom or petal fall provided excellent disease control in the same studies. Reminders for best control:
  • Spray every row after pink bud. Every-other-row spraying = every-other-row control.

Also, follow the rules for resistance management:

  • Rotate fungicide chemistries.
  • Use labeled rates.
  • Limit the use of a single site fungicide to once or twice a year.
  • Educate yourself on fungicides and their modes of action.
  • Start strong: begin your fungicide program with a multi-site fungicide or mixture of different single site materials to reduce the pathogen populations and resistance risk to any single site fungicide used later in the season.

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