Bare, dormant stems begin to swell in spring, transforming from green to red tips, from which tight clusters of pink blossom buds emerge. Those buds will bloom, drop their petals, and generate fruit, assuming they have been pollinated. That is, of course, unless blossom brown rot has taken hold.
Blossom brown rot (Monilinia laxa), also known as brown rot blossom blight, is a fungal disease of almonds, apricots, cherries, and other stone fruits.
Similar to brown rot (Monilinia fructicola), blossom brown rot can affect flowers from pink bud stage through petal fall. All parts of the flower are susceptible.
Symptoms of blossom brown rot
The first sign of blossom brown rot is the death of young blossoms. What should be a colorful, flower-laden tree, buzzing with pollinators, looks more like clusters of brown, dried up tissue paper. [That would be an extreme case.] More often, infected flowers are intermittent (at first).
Gum may ooze from the base of infected flowers and cankers may form on twigs. Those cankers will have tan centers and dark edges. Blossom spurs and their leaves may collapse. Under humid conditions, you may be able to see tan to grey spore masses.
Blossom brown rot lifecycle
Fungal spores overwinter in twig cankers, on mummified fruit, and on any diseased flowers that remain attached to the tree. As temperatures rise in spring, fungal spores begin populating nearby twigs and other blossoms, causing twig and branch dieback, along with blossom losses. Spores are airborne, and spread by irrigation and rain drop splash, and by insects.
How to control blossom brown rot
This fungi thrives in rainy weather with temperatures in the 70s. High humidity can also encourage spore development. In fact, this fungi’s growth is almost directly related to humidity and temperature, both of which are difficult to control in the home garden.
Proponents of compost tea recommend foliar sprays as a treatment for blossom brown rot, but research has shown that compost tea either has no effect, or that it worsens the condition.
Unless you want to apply chemical fungicides, you are best off selecting varieties that are resistant to this disease in the first place. In the world of almond trees, the following species are most susceptible to blossom brown rot: Butte, Carmel, Drake, Ne Plus Ultra, Winters, and Wood Colony.
You can also reduce the likelihood of blossom brown rot by removing all mummies, as soon as they are seen, and disposing of them in the garbage. Pruning and training for better air flow can also reduce the amount of time blossoms take to dry.
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