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.  

At least one on-line weather forecasting site is predicting rain for Aug 20-22.  That is right about the time prune harvest may begin in the Sutter/Yuba area.   The forecast could be mistaken, but it might be a good idea to make sure your sprayer is ready and you have the best program (fungicide, sprayer set up and calibration) ready if needed.

Sprays to protect prune fruit from brown rot must be applied before rain.  Research by Dr. Jim Adaskaveg, University of California professor of plant pathology, has shown the following materials and application practices to provide the best possible protection:

Best Materials (++++) in the 2011 UC Fungicide Efficacy and Timing publication followed by fungicide class (in parenthesis) appear below.  Dr. Adaskaveg advises growers not to use the same fungicide chemistry more than 2 times in a single growing season.  Check the label, consult with your packer and PCA before applying a fungicide preharvest. 

• Bumper/Tilt (3)
• Indar (3)
• Quash (3)
• Pristine (7/11)
• Quilt Xcel (3/11)

Complete information at:   http://www.ipm.ucdavis.edu/PDF/PMG/fungicideefficacytiming.pdf

I strongly suggest growers talk with their PCA about tank mixing two chemistry groups if allowed by the label and a premixed material has not been selected.  For fruit brown rot control, Group 9 materials are rated +++ (good and reliable) by Dr. Adaskeveg compared to ++ (moderate and variable) for Group 11 materials.  Group 9 materials should provide better resistance management support of Group 3 materials for fruit brown rot control than Group 11 materials.   

Best results with research by Dr. Adaskaveg on prune fruit brown rot control include the following practices:

• Adding 1-2 gallons of 415 weight summer oil for each 100 gallons of spray water in the tank.  Use of  440 weight oil instead of 415 oil may provide similar results, although this has not been tested.  440 weight oils are labeled for summer use, but max rate is 1 gallon per 100 gallons.  Check the label of the oil product before spraying.   Make sure there is NO sulfur in your spray tank (or sprayer filters) before adding oil to the tank. 
• Using high spray volume ( for example 160-200 gallons/acre)

The most difficult spray job of the season is preharvest, when the canopy is most dense and weighed down by the weight of the crop.  Slow tractor speeds will deliver the best spray coverage under these conditions.  Slow down and do the best job possible.

Aerial application will provide less control than careful ground application. 

Finally, lower your expectations.  Fruit brown rot control preharvest is difficult.  This is especially true when conditions (warm and wet weather + clustered fruit) favor infection.  Complete control may not be possible.