In the wet and cold spring of 2019, the disease bacterial blast blasted almond orchards from Butte to Fresno county, destroying shoots and dormant buds and exposing trees to canker infections. There was no treatment or preventative product allowed for this infection at the time.

By the spring of 2020, a Section 18 emergency registration use in almonds was secured for a bactericide that proved effective against bacterial blast, but warmer and drier conditions limited the opportunities for the bacteria to attack trees. A renewal of the Section 18 registration for UPL’s Kasumin has been submitted to DPR and EPA by the Almond Alliance of California. Kasumin is already being used in walnuts for walnut blight control after receiving federal and state registrations in 2018.

Almond growers who previously lost production to bacterial blast and those with vulnerable orchards will be carefully watching weather forecasts this year so they can apply the preventive bactericide if cold and wet weather is forecasted just before or during bloom.

Disease Conditions and Effects

Almonds are the tree nut most susceptible to bacterial blast as the trees bloom when frost conditions are most likely to occur. Almond shoots exposed to –4 degrees C for two hours have been shown to be more susceptible to blast. Ice-nucleating bacteria and freezing temperatures are major factors in this disease. There is no treatment for this disease after the infection occurs. UC IPM Guidelines report bacterial blast symptoms are shriveled up ‘fried’ looking blossoms often on twigs that have died back. Additionally, buds can die, and dieback can occur on larger branches as a result of severe infections.

UC Riverside Plant Pathologist Jim Adaskaveg, who has worked on bacterial blast in almonds and assisted the Almond Alliance of California in preparing the Section 18 renewal for Kasumin use in almonds, said outbreaks of bacterial blast are caused by the bacteria Pseudomonas syringae. He said the disease was found in 2019 to be most severe in some self-pollinating varieties of almond as well as other cultivars that bloomed during frosty weather conditions. Losses in production in affected trees can reach 30% to 40%, he said.

Bacterial blast infections affect almond varieties depending on their stage of bloom when cold, wet weather hits. In early blooming Independence, Aldridge and Monterey, the disease affects blossoms and leaves which wilt and die. The bacteria, with the right environment and susceptible host, colonize plant tissue damaged by cold weather. Non-bearing trees are also affected, and the subsequent dieback of fruiting wood can delay production. Established orchards can also lose fruiting wood due to the disease, which will affect production for more than one season, Adaskaveg said.

Blast infections of dormant buds result in bud death. Blossoms may wither suddenly and turn dark brown. When there are multiple blasted flowers, the shoot tip may become necrotic and exude gum. Other signs of infection include necrotic flecks on leaves that may have a chlorotic halo around the lesion. Necrotic tissue may fall out to produce a shot hole–like symptom.

Blast infections are most severe in the lower part of the tree canopy and in low places in an orchard where colder air has settled. Growers who experienced previous bouts with this disease now know which varieties of almonds are most susceptible and orchard locations where trees are most vulnerable due to environmental conditions.

Adaskaveg said almond trees that are stressed due to disease are also susceptible to injury from frost and infection with the blast bacteria leading to bacterial canker infections.

Prevention is Key

Some protection from bacterial blast can come from frost protection practices, including use of sprinklers or wind machines to prevent dew or frost from forming. Copper applications can be phytotoxic to blossoms and green tissue, and previous research completed by Adaskaveg has shown that the bacteria that causes bacterial blast is usually resistant to copper. No biological controls are registered for bacterial blast in almonds.

Kasumin’s active ingredient, kasugamycin, has a unique site of activity and mode of action, acting locally as a systemic bactericide. Adaskaveg said it is being tested for effectiveness against other bacterial diseases and may be a new tool for growers.

UPL’s Joe Vassios said Kasumin is only effective in preventing bacterial blast if it is applied prior to cold and wet weather.

“Strategy is key going into bloom. If the forecast is for cold and wet weather and bloom is imminent, Kasumin should be applied in the orchard. Think of it as a preventative tool, protecting almond flowers from disease,” Vassios said.

The bactericide acts as a protective layer on emerging green tissues and blossoms and is locally systemic. Adaskaveg said an application of Kasumin days prior to a rain/frost weather event would protect the blossoms. The product will remain effective in protecting the plant for seven to 10 days following application, and a second application may be warranted if trees are blooming or close to bloom and another rain or frost event is forecasted.

If the Section 18 renewal is approved, Kasumin can be applied up to two times from bud break until petal fall. It is not allowed after petal fall.

Applications of Kasumin are made at the rate of 64 ounces of product per acre. Adaskaveg said applications with an air blast sprayer were at 100 gallons per acre. Good coverage is a must, he added.

Adaskaveg said it is not as important to hit the tops of the canopies as it is to make sure there is good coverage within the first 15 feet from the ground, which is typically the frost line. Non-producing almond trees should also be treated because infection can lead to canker and lower the production potential of a tree over its productive life.

Vassios said that as the product registrant, UPL has played a supportive role in the Section 18 process and that Adaskaveg and the Almond Alliance have conducted a large amount of work to pursue this Emergency Exemption on behalf of California almond growers.

Adaskaveg said EPA approved the Section 18 renewal for Kasumin for Bacterial Blast in Almonds in January 2020, and a request to renew with the same use rate was submitted in October. At press time, the renewal application was currently under review at EPA, and Adaskaveg indicated that he was hopeful that it will be approved prior to the 2021 almond bloom.