Bacterial Spot on Almonds

Bacterial spot is a relatively new disease to California, but is common in the Southeastern U.S., Europe, Australia, and the Middle East.

When bacteria infect a plant, the plant’s first line of defense is often to surround the infected area with a strong barrier. While not always as effective as our immune systems, this often prevents further spread of the disease. These damaged areas usually appear as spots on leaves. Bacterial spot is just one of many different bacterial diseases that attack your plants.

First seen in 2006, this particular bacterial spot attacks almonds and other stone fruits, such as peach and nectarine. Bacterial spot (Xanthomonas arboricola pruni) first appears as an amber colored gum oozing from immature nuts.

​Bacterial spot symptoms

Symptoms first appear mid-April to early May. Damage looks similar to that caused by leaf-footed bugs and anthracnose. Use these notes to differentiate between the three conditions:

• anthracnose - sunken lesions, amber gum, pink or orange spores in the gum
• leaf-footed bug feeding - feeding punctures, clear gum
• bacterial spot - spotted leaves, amber gum, twig cankers 

Leaves may become spotted, develop angular or circular red lesions, and drop early. If you cut open an affected nut, you will find a pencil eraser-sized lesion. These lesions get bigger, transforming your delicious almond into an orange slime. (Ew!!!) 

Almond varieties most commonly affected are ‘Fritz’, ‘Monterey’, ‘Nonpareil’, ‘Mission’, ’Neplus Ultra’, and ‘Padre’.

How to control bacterial spot

Commercial growers use zinc sulfate in the fall to make trees drop all of their leaves, which are then gathered and destroyed. Leaf and mummy removal is followed by spraying with dormant oil and copper treatments, combined with the antibiotic oxytetracycline. Obviously, you are not going to hit your backyard fruit and nut trees with all these chemicals. These tips can help minimize the damage caused by bacterial spot in your garden:

• rake up and dispose of fallen leaves and mummies promptly and regularly
• do not add infected materials to the compost pile
• brush off shoes and clothing when moving away from infected trees
• harvest before autumn rains begin

To prevent infection, trees can be treated with oil and copper mixtures before winter. This is yet another reason why it is a good idea to quarantine new plants before installing them.