Alternaria season starts in the Sacramento Valley

In a Colusa County almond orchard, this past week saw the beginning of summer conditions that can produce alternaria leaf spot.  Based on hourly tracking of leaf wetness and temperature, the treatment threshold may not have been reached, yet, but the conditions that can build up to disease infection have begun.  Note:  For more information on alternaria leaf spot biology and control from the UC IPM program, click HERE.

Alternaria leaf spot infections are promoted when leaf wetness occurs when average temperatures are between 60-82^oF.  It often takes three weeks for symptoms to occur, so timely treatment based on weather conditions is key to cost effective control.  How can you tell when alternaria leaf spot infection risk increases?  Many growers have developed their own programs based on local experience and economics.  However, a relatively new tool that should help growers match current orchard conditions to disease risk is the DSV model for alternaria in almond, developed by Dr. Jim Adaskaveg, a professor in the Plant Pathology Department at UC Riverside.  Click HERE to read Dr. Adaskaveg’s research report (2010-11) to the Almond Board of CA.  The Almond alternaria leaf spot DSV model is a modification of a disease prediction model used for decades in row crops (TomCast).  It has yet to see wide spread commercial use in California and further experience may result in adjustments based on location conditions and varieties, but it was developed with years of sound field research and has value.

To use the DSV model, you need hourly leaf wetness and temperature data plus the table that appears below.  (Click HERE to see the model in degrees C.)  Some companies have weather stations that calculate DSV values from remote weather stations, but only one company I know has adopted Dr. Adaskaveg’s model – most use Tom-Cast.  Another option is to do it yourself — calculate daily and weekly DSV values from leaf wetness and temperature data downloaded from the field.  Here’s how to do that.

Each day at 11:00 AM, review the leaf wetness and temperature data for the previous 23 hours and calculate the DSV value for the previous 23 hours (shouldn’t be wet between 11 AM and noon in late spring or summer).  [If leaves are dry for at least 2 hours between wetness events, consider those to be separate events.]  Add the daily DSV value to those of the past 6 days to get a 7 day DSV accumulation.  On day 8, add the daily DSV to the total and drop the day 1 DSV value, and so on through the season.  When 6-12 DSV units accumulate in a week, then a treatment is recommended.  In an area of high disease history, the spray threshold may be 6-8 DSV units.  If the alternaria incidence history in an orchard is low, then the threshold may be 10-12 DSV units.  Once you spray for alternaria, restart from 0 for DSV accumulation.  The following is an example of 7 day DSV accumulation:

The Alternaria Disease Severity Value model developed by Adaskaveg from the TomCast model for alternaria risk in tomatoes.

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