Bacterial Spot Management

Almond growers and pest control advisors who attended our July bacterial spot field day in Manteca saw how devastating this new disease can be.  A map passed around at that field day showed two hot spots for the disease in our area.  The most severely affected area was between Highway 120 and the Stanislaus River in the Manteca/Ripon/Escalon area.  The second area was south of Turlock in the Delhi / Ballico area. Although we will be implementing some field trials this year, all of the information we currently have on control is from Australia where this disease has eliminated the Fritz variety in that country.  The strategy for control is a two part program; reduce the amount of inoculum from the previous year and then protect the nuts and leaves from infection in the spring.  The following is our suggested program if you had a bacterial spot problem in 2013.

• Spray 15-20 pounds per acre of zinc sulfate now to burn off leaves in the fall (Late October).  This may help reduce inoculum for next year.
• Apply copper after leaf fall and prior to significant storms.
• Remove as many mummies as possible during the winter.  Infected mummies are likely a major source of bacterial spot inoculum (and NOW too).
• Apply a second copper application in a delayed dormant spray (early February) (see note).
• Apply Manzate after bloom to protect new leaf and nut growth. (See note),

We aren’t sure if it is necessary to do all of these things every year.  For instance, I am not sure how important it will be to burn the leaves off with zinc because we don’t know what role leaf infections play in California.  Leaf infections appear to be important in Australia where it rains periodically during the season.  Our limited experience here suggests that leaf infections may play a minor role in California.  Also, copper sulfate will do a pretty good job burning leaves off by itself so maybe zinc is not necessary.  If you have Fritz but have not had bacterial spot, maybe you can get by with only a delayed dormant copper application or a single manzate spray in the spring.   In my opinion, if you had a bacterial spot problem this year, it may be best to start with a “Cadillac” program now and then we can determine later where we can cut back.  Bacterial spot is primarily a disease of Fritz and Ne Plus, so I don’t know if it is important to spray the other varieties in the same orchard.  We have seen the problem to a lesser extent on Padre and experience in Australia suggests that Price is moderately susceptible.  Maybe a less intensive program would be adequate with these varieties.  Nonpareil, Carmel and many other pollinizer varieties seem to be pretty resistant to the bacteria and shouldn’t require treatment for bacterial spot.