Bloom Disease Treatments and Honey Bee Safety

Emily Symmes, UC IPM Advisor, Sacramento Valley and Franz Niederholzer, UC Farm Advisor, Colusa, Sutter & Yuba Counties.

Disease management during prune bloom using fungicide sprays is critical to setting a successful crop, particularly in years with wet bloom conditions that drive pathogen development and spread. Of particular concern, and often treated during bloom, are brown rot and scab.

Just as essential as disease management to producing a large and profitable prune crop is strong honey bee activity in the orchard during bloom. Unlike the majority of almond varieties, prunes are self-fertile. However, they do require bees (or other insects) to move pollen within flowers to set a crop. It is vital to understand the impacts of disease sprays on honey bees and adopt best practices for mitigating those impacts.

Prune growers should do all they can to maintain strong, healthy bee activity in their orchards by providing a safe working environment for bees. The most important single thing growers can do is KEEP THE POLLEN CLEAN. In addition to the obvious impacts of contact insecticidal materials on active adult honey bees, a wide range of pesticide residues (insecticides, fungicides, herbicides, etc.) found on pollen are known or suspected to have adverse impacts on the developing honey bee brood and overall hive health. Pollen is collected by foraging adult bees and taken to the hive as a source of proteins, fats, vitamins, and other nutrients for the brood (young, growing bee larvae). Pollen that comes in contact with pesticides can damage hive health when fed to the growing brood.

The release of pollen occurs gradually over the course of bloom, with more anthers “popping” each day. Pollen is released by some of the anthers in each flower in the morning as relative humidity drops and the orchard warms. Strong bee activity typically strips all available pollen from the flowers in a given day by early- to mid-afternoon. For this reason, any pesticides (i.e., fungicides) that need to be sprayed during bloom when bees are active in the orchard should be applied in the late afternoon or evening in order to minimize contamination of pollen that will become available the following morning.

IPM for bloom diseases in prune, considering honey bee colony health.

  • Brown rot. The causal fungal agents survive in mummies, twig cankers, and remaining diseased flower parts and spurs, providing inoculum for spring infections. Spores are air-borne or rain-splashed. Optimal conditions for disease spread and development are rainy weather during bloom and temperatures in the mid-70s (°F), although temperatures above 58oF promote infection. All flower parts are susceptible to infection.
    • Good orchard sanitation can reduce the amount of inoculum available for spring infections. Pruning crews should remove fruit mummies and blighted shoots from the orchard. Complete removal is ideal, as mummies left on the orchard floor will contribute inoculum, although cultivating the orchard floor to bury mummies can reduce inoculum load. Dense ground cover maintained during bloom favors the production of inoculum by mummies left behind.
    • Depending on conditions, one to three bloom brown rot treatments may be needed to protect the crop. Typically, two treatments (green bud and full bloom) are applied; a third at popcorn stage may be warranted if bloom is prolonged and wet conditions favor disease development. Under dry (no rain) bloom conditions, a single fungicide spray at 30 to 50% bloom using FRAC 3, 9, and/or 11 fungicides – those showing some systemic activity – is recommended. Fungicides are most effective if applied before a rainfall event and allowed to dry.
  • Scab. Scab develops in years with excessive rain during and just after bloom, although research has shown that prunes with <75% scab do not contribute to off-grade. Although not caused by a microorganism, this physiological disorder is minimized by the fungicides chlorothalonil and captan. Full bloom is the suggested treatment timing for controlling scab in years with conditions conducive to symptom development. These applications cause particular concern for honey bee health, as both materials are known to be toxic to the bee brood. If possible, wait until the bee hives have been removed from the orchard before applying the full bloom fungicide containing captan or chlorothanonil. If bloom conditions have been good for set (60-80oF maximum daily temps with limited wind), removing bees by full bloom will not harm cropload.
  • For any fungicide treatment during bloom:
    • Consider weather conditions and treat only when needed to avoid economic crop damage.
    • Employ good sanitation for brown rot (remove and destroy mummy fruit) to reduce inoculum load and possibly limit the need to apply fungicides.
    • Minimize pollen contamination by applying fungicides in the late afternoon or evening after the day’s pollen has been stripped from the flowers. Avoid early morning sprays, as the spray will not have sufficient time to dry before new flowers open, anthers shed pollen, and bees begin foraging.
    • Never spray the hives or foraging adults directly with any pesticide – insecticide, fungicide, and/or herbicide. Even being contacted by water sprays can adversely impact adult bee flight activity.
    • Do not add adjuvants, insecticides and/or nutrients to the tank with fungicide applications during bloom or when bee activity is high in an orchard.
      • Do not add any insecticide to the tank except B.t. (Dipel®, etc.) during bloom or when bee activity is high in an orchard. Even “soft”, selective insecticides such as Dimilin®, Intrepid® and Altacor® applied when there is high bee activity in a block can be very harmful to hive (brood) health.
      • According to the authors of the annual Fungicides, Bactericides, And Biologicals for Deciduous Tree Fruit, Nut, Strawberry, And Vine Crops (Adaskavag, Gubler, and Michailides 2017), most fungicides are formulated with adjuvants including wetting agents, spreaders, and stickers. Unless a material specifically indicates on the product label that an adjuvant should be added, the fungicide product does not need additional adjuvants mixed into the sprayer tank to improve performance. With few exceptions, adjuvants do not statistically improve the efficacy of fungicides for managing diseases of fruit and nut commodities.
      • All University of California efficacy trial results (+++’s in the efficacy table) are based on this premise and materials are tested without addition of adjuvants unless expressly indicated on the product label.
      • Adjuvants may increase the potential toxicity of fungicides to honey bees. To save money and protect bees, only put what is absolutely necessary in the tank at bloom.
    • Ensure that adult bees have a clean water supply by covering water sources (buckets, etc.) provided for bees when spraying or replacing water immediately after.
    • Consider arranging with your beekeeper to remove hives just prior to a full bloom application of captan or chlorothanonil, if possible.
    • Know the impacts of any materials being applied during bloom to honey bees and their brood. UC IPM has a searchable database with hundreds of pesticides of different classes (insecticides, fungicides, bactericides, herbicides, etc.) that provides a rating scale of impact to honey bees, adverse effects on developing broods, and indicates known interactions among materials on honey bee health.

More information on honey bee best management practices during prune bloom is available here.

Although designed with almond growers in mind, the Almond Board of California’s Honey Bee Best Management Practices materials are a great resource of information for all agricultural producers whose crops depend on honey bees for pollination.


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