Brown Rot or Bacterial Blast?

I have had a few calls regarding blossom dieback within almond trees. In many orchards, especially in Northern Merced, I have seen dried, crispy blossoms still stuck to the tree. In some cases, this may be brown rot, but in a year where we had a cold snap during bloom, this could also be bacterial blast. How do we tell the difference?

Figure 1: Brown rot blight of an almond blossom.
Note the grayish brown fuzz found at the base of the blossom.

Brown rot will kill blossoms and will often move into the spurs and branches on the tree. As temperatures warm up, cankers form on the wood which sometimes cause gumming. These cankers serve as an over-season resting place for the fungus. Even more obvious than cankers is the formation of light brown to gray fuzz on the jacket or at the base of the flower (Figure 1). This “fuzz” is the spores of the fungus.

Bacterial blast is a blossom blight that is caused by the bacteria Pseudomonas syringe. This is the same bacteria that causes bacterial canker and bud drop. Found naturally all over the tree’s surface, this bacteria is an opportunistic pathogen, meaning it will not affect a healthy tree. When a tree is stressed from nematodes (particularly ring nematode), poor soil conditions, lack of nutrients, or an extreme frost event, this bacteria makes it way into the tree’s tissues through cracks and natural openings killing affected tissues (Figure 2).

Figure 2: Bacterial Blast of almond blossoms.
Photo courtesy of M. Moran. 

To determine if the dead blossoms in your almond trees was from blast or brown rot, ask yourself these questions:
1). Did I spray for brown rot this year?
2). Am I seeing the dead blossoms across all my varieties?
3). Are the trees stressed in some way or located on sandy or extremely heavy soil?

If you answered “yes” to all of these questions, most likely the blossom death is due to bacterial blast. To confirm the diagnosis, place a few blossoms in a humidity chamber for a day or two (Note: A homemade humidity chamber can be made with an airtight plastic container with a wet paper towel). These conditions will trigger blossoms infected with brown rot to sporulate, which will be indicative by brown fuzz forming around the infected flowers. If no fuzz is found, most likely the culprit was bacterial blast.

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8 thoughts on “Brown Rot or Bacterial Blast?

  1. Why not look at displacing Bacterial blast with large communities of diverse beneficial aerobic biology, both on tree surface as well as soil drench?

  2. Sounds feasible, but this is easier said than done. Within every bacterial community, there is an intimate association between the host and the established bacteria. Not saying that the plant “wants” these bacteria to be here, but rather the bacteria have established a niche within the eco-system of the tree.

    Pseudomonas sp., being a large group of bacteria, tend to be pretty good at finding a niche somewhere. Even more of importance is their ability to rapidly colonize an area that provides resources – hence why bacterial blast occurs. Spraying trees with sugars or some other substrate probably will not reduce the population of these bacteria. In fact, it may actually increase populations.

    The same goes for applying an augmentive bacterial spray. The idea behind applying bacteria is competitive exclusion – meaning that the applied bacteria have to out compete the native bacteria. This isn’t easily done as the native bacterial community is pretty unique and resilient. In some cases these products are applying bacteria that will not even survive on the host. Furthermore, these epiphytic bacterial populations tend to be high, sometimes exceeding 10 to the power of 8 – even higher in moist, nutrient rich soils. Applying a couple of ounces of bacterial soup to the surface of these plants is similar to placing a drop of food coloring into Lake Tahoe. Sure, if enough is added, an effect will occur, but how much does it take, and is it affordable?

    Products have been developed that have successfully colonized plant flowers and provided protection against bacterial infection. A good case is the development of P. fluorescens that is used to out compete Erwinia amylovora, the Fireblight pathogen of Apple and Pear. This research, however, has taken years and research has shown that the bacteria helps reduce, not eliminate fireblight due to competition for resources within the apple blossom.

    So, in summary, there are many products that claim to change soil or plant microbial biology, but few that have scientifically been proven to provide benefit for disease control.

    If you are aware of any documented products that show promise within almonds, please let me know. I spent four years as a graduate student looking for such products in regards to soil-borne diseases of almonds. It would be to my relief to finally find a product.

    1. Copper can help reduce blast by reducing the bacterial counts on the surface of the plant, but the effectiveness varies. Improving tree health also helps – keeping trees hydrated as well as irrigating as the temperatures drop to help reduce frost damage.

  3. My orchard is 9th leaf, sandy ground, leaves are large and vigorous green color, new growth, flood irrigation, has blast, 80-85%light interception great potential! 3000 plus but only yielding 1800- 2000 lbs. average. # of blooms look fairly good but many don’t develop. No brown rot. starting to use Movento- foliar. Can you offer suggestions? Thank- you

    1. Randy – A few questions first…What varieties are planted and which rootstock? Also, have you done an nematode analysis – if so, what, if any, nematodes were present? What is your nutrition program (sounds like your N is in good range)? Did you performed a hull analysis for boron last year? Thanks, David

  4. Hi
    Wander if you can help we have bought a house in Turkey with an mature almond tree which was healthy but over the last two year has started to lose its leaves half way through the summer and now has lots of gum over the base and branches. We are not there all the year so a treatment we can dig in or paint on would be helpful.. Any help would be welcome.

    John Driscoll

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