Skip to main content

Plan Ahead to Avoid Damage from Bacterial Blast

When frost and chilly temperatures occur during bloom, bacterial blast can easily become a menacing threat to almond orchards. Learn how one application can help to prevent blast on your trees and help prevent crop losses.


Bacterial Blast Image.jpg

We are in the middle of a California winter, a time when cold temperatures and, ideally, rain provide replenishment and critical chilling hours for almond trees. However, fast forward a month, when those same trees have burst into bloom, and extremely cold temperatures are no longer a blessing. Not only do frost and chilly conditions impact bud survival as well as honey bee activity in the orchard, they also create an opportunity for bacterial blast to devastate the crop.

The bacteria that causes the blight (Pseudomonas syringae) is ubiquitous in almost every orchard, but it only becomes an issue if cold storms occur when buds and blossoms are forming – such was the case three of the past four years. In addition to outbreaks in the San Joaquin Valley as far south as Fresno County, some growers with outbreaks in the Sacramento Valley reportedly experienced crop losses up to 40%.

Blast occurs when bacterial cells move into plant tissues damaged by ice formation, resulting in broken cells and cell leakage. When this takes place, bacterial blast can display a variety of symptoms: Elliptical lesions can develop on trunks and branches, developing leaves will show signs of small necrotic flecks, shoot tips will experience dieback and gumming, blossoms may suddenly wither and turn brown, and dormant buds will be covered with gum and die by springtime. Often, there is a profuse amount of gum on the surface of the canker and pale, reddish-brown streaks under the bark near the area. Growers can also sometimes identify dead tissue by its sour smell.

Blast is often worst in the lower portion of the canopy and in sections of the orchard with the lowest elevation (generally areas where frost conditions are most severe). The disease can also be more prevalent in sandy or acidic soils. Tree wounds caused by shaking or pruning don’t automatically create an opportunity for bacterial blast to occur, but the wounds’ presence can increase a tree’s vulnerability.

UC Riverside professor and plant pathologist Jim Adaskaveg is recognized as an expert on bacterial blast, as he has conducted research that has helped identify an effective blast treatment for almond growers. Known as kasugamycin (Kasumin), this application should be sprayed in orchards in February and early March before freezing temperatures are expected.

“If you see the symptoms of bacterial blast, it’s too late,” Adaskaveg warned during an Almond Stage presentation at The Almond Conference 2020.

When applied, kasugamycin helps growers prevent bacterial blast by providing a protective layer on emerging green tissue and blossoms.
The active ingredient in this application is absorbed by the plant’s green tissue and provides even broader control of targeted bacterial pathogens.

As kasugamycin has not yet received full registration (via Section 3), and since there are no registered and effective alternatives for controlling blast, Adaskaveg
worked with the Almond Alliance of California using data from research supported by the Almond Board of California to submit an emergency exemption request (Section 18) to the California Department of Pesticide Regulations (CDPR) and the Environmental Protection Agency. Those exemption requests have been granted the past two years and are being pursued again for 2021. The joint effort to obtain an emergency exemption registration, which permits the use for a limited time, helps to ensure growers have this tool available should they require it for the 2021 bloom season.

Last year, the authorization allowed up to two treatments at least a week apart during bloom – no spraying could occur after petal fall. Treatments are recommended at 64 ounces per acre.

Growers who are concerned about bacterial blast occurring in their orchards this year are highly encouraged to start the process of ordering kasugamycin now as it can take quite some time to receive the product. Those interested in applying should contact their
local County Agricultural Commissioner’s office. Uses under a Section 18 emergency exemption must be requested from the county ag commissioner’s office and be renewed annually, so growers who received permission to use the product last year must apply again this year.

Adaskaveg summed up the potential risk of bacterial blast this way: “It's one of those diseases where, when you have an orchard that’s prone to these conditions, you can [experience] extensive losses. And then next year, it might not be there, but you’re left with the die-back, the dropped leaves and the impacts on your orchard.”

For more information on the process of applying for a Section 18 exemption as well as more information about kasugamycin and its benefits to growers, please view Adaskaveg’s full presentation
from The Almond Conference 2020.

1. To note: Since the disease is caused by a bacterium, the fungicides commonly applied during bloom to control fungal pathogens are not effective in controlling bacterial blast.