As I write this, early bloom is just beginning to show in some prune blocks in the Yuba City area.  Even though it has been a relatively dry winter, there is rain predicted starting this weekend (Mar 10-11) or early next week, so growers should prepare to protect their crop from disease at bloom -- especially brown rot.

When crop stage and weather indicate there is a need to protect a crop, make sure to rotate between different pesticide classes to reduce the risk of fungicide resistance developing in a disease population in your orchard.  Using different FRAC (Fungicide Resistance Action Committee) group pesticides -- pesticides that use different modes of actio on disease organisims -- is the best way to manage fungicide resistance in your orchard.  See the tables below for details on timing, relative resistance risk, and efficacy of many pesticides registered for prunes.  The FRAC group or groups in a particular fungicide product are listed on the first page of the label. 

In wet bloom weather, UC recommends a two spray program for blossom brown rot control -- once at green bud and once at full bloom.  The full bloom spray is the most important, although the green bud spray should be included as well.  At full bloom, include a captan or chlorothanil (Bravo, etc.) material for russet scab control.  As stated above, alternate between FRAC classes with each application.  For example, for blossom brown rot control, you could use Vangard (FRAC Group 9) at green bud and Tilt/Bumper (FRAC Group 3) at full bloom.  In a wet year at full bloom, combining two fungicides in the tank -- one for brown rot and one for scab management (for example, Tilt + captan) -- is the best approach.  Captan and chorothanil are effective for scab control, but only mediocre brown rot materials.

In a dry bloom season, research by Dr. Jim Adaskaveg, UC Professor of Plant Pathology at UC Riverside, has shown that effective brown rot control can be acheived with a single bloom spray at 50-70% bloom.  This year is not shaping up to be a dry bloom.  Hopefully that will help with fruit set!
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Table 1. Examples of general effective timing, resistance risk, relative efficacy, and target diseases of different FRAC group fungicides (alone or as mixtures) available to prune growers in California at the beginning of bloom, 2012.  Low, medium, and high resistance risk groups are listed under “Low RR”, “Med RR”, and “High RR”, respectively.  Stars after each mixture indicate relative efficacy of the product(s) –   limited and erratic (*),moderate and variable (**), good and reliable (***), and excellent and consistent (****).   See Table 2 for product names to match with FRAC numbers listed here.  Consult with your PCA for specific fungicide application rates and timing(s) for disease control in your orchard.   The information in this table is drawn from Efficacy and Timing of Fungicides, Bactericides and Biologicals for Deciduous Tree Fruit, Nut, Strawberry and Vine Crops, 2011.

 Table 2.  Examples of FRAC groups (2, 2, 9, etc.) and FRAC group combinations (3/9, 3/11, etc.) available to prune growers in California at bloom, 2012.  Always read the label before applying pesticides.   Consult with your PCA regarding spray timing, rates, etc.  Not all generic products available in the market are listed in this table.

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The best single source for information on disease management tool efficacy and timing in California deciduous tree and vine crops is Efficacy and Timing of Fungicides, Bactericides and Biologicals for Deciduous Tree Fruit, Nut, Strawberry and Vine Crops, 2011 written by UC researchers/advisors J. Adaskaveg, D. Gubler, T. Michailides, and B. Holtz.  Please follow the hyperlink in the title above to view this excellent, free document.