The same bacterium used to make artificial snow, Pseudomonas syringae, is causing problems in blueberries right now.

The near freezing night time temperatures experienced widely in Western Oregon in April, combined with abundant rainfall and the near ubiquitous bacterium Pseudomonas syringae, are all adding up to severe tip and flower dieback in blueberry bushes.

A field of blueberries badly affected by P. syringae

A close-up of the P. syringae damage

The bacterium is common in the environment, and is likely already present in your backyard, garden, or production field. Many years it can live in association with plants without causing any trouble, but when environmental conditions are right, it can initiate formation of ice crystals when the temperatures are above freezing. This is due to a unique protein present in the outer bacterial membrane that acts as a nucleus for ice formation. Pure water will not form ice, even at temperatures several degrees below freezing, unless there is something that will act as a nucleus around which the ice will form.

pseudomonas syringae

Pseudomonas syringae shown using SEM. Source: Gordon Vrdoljak, U.C. Berkeley

Microscopic particles such as motes of minerals or other matter usually provide the nucleus for ice formation, in a way than can be thought of as similar to how a piece of grit aids in the formation of a pearl in an oyster.

The ice nucleating ability of P. syringae is a real advantage when the ski run needs more snow for an international competition, but isn’t very nice when you have blueberries in full bloom. Under the right conditions, as we have had in Western Oregon recently, the bacteria can initiate freezing when temperatures are actually above 32 °F, resulting in ice crystals that form and damage plant tissues (think of what happens when you put green leaves in the freezer).

We have had several samples of young blueberry twigs (see below), including those bearing flowers, damaged this way. And dead flowers mean no fruit.

Damaged blueberry flowers

A close-up of flower damage

Worse, the freeze damage to the plant does allow entry of the bacteria, which can then initiate disease.

Fortunately, this doesn’t happen every year, and the damage is not permanent. Judicious pruning can do a lot to prevent additional damage.

Hopefully conditions will be better next spring for a bountiful harvest.

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