Walnut growers face a “sleeping giant” in their orchards that can be roused by rain, irrigation or simple agronomic practices such as pruning.

Botryosphaeria is a fungal disease that causes die-back in shoots and can kill fruiting buds, which leads to lost production and tree health issues, says Themis Michailides, plant pathologist with the University of California. Michailides spoke on the topic during the Tri-County Walnut Day in early February in Visalia, Calif.

Michailides says the disease is not new. A symposium on walnuts in 1915 discussed the disease, he said.

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The reason he calls this a “sleeping giant” is because its symptoms are not easily recognized and it has the ability to access trees through walnut scales and pruning. It’s one of those diseases that is more insidious than the others, and can benefit from the effects of walnut blight and the lesions caused by scales.

Additionally, the disease has sometimes been misdiagnosed as branch wilt and has possibly not been taken as seriously as it should. Four of the 10 species of Botryosphaeria are very aggressive, he said.

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While panicle and shoot blight, caused by a Fusicoccum sp. (e.g. Botryosphaeria dothidea), is the most serious disease of California pistachios, studies seem to indicate that walnuts are impacted somewhat differently by Botryosphaeria than pistachios.

For instance, green fruit and walnut leaves tend not to be as impacted by Botryosphaeria when compared to pistachios. This needs to be studied further, he said.

Still, the disease can cross back and forth from pistachios to walnuts to almonds, Michailides says. It’s just that it has its differences in how it gets into the various trees and the symptoms it displays. Making it more difficult is symptoms of the disease are difficult to spot in walnuts.