crop reports

Long Irrigation Aids Phytophthora in Walnuts

Taylor HillmanPest Update, Specialty Crops

Phytophthora in Walnuts

Excessive saturation can aid the spread of phytophthora in walnuts. Growers need to manage irrigation carefully to avoid long sets and over-saturation.

At the annual Walnut Trade Show, Cooperative Extension Orchard Advisor Dani Lightle spoke to attendees about diagnosing and preventing crown and root diseases. Lightle said the first step is properly identifying why trees are experiencing reduced vigor. “You don’t want to be treating for a nematode problem, when actually it’s a nutrition deficiency,” she said. “It’s really important to know exactly what you are dealing with, and then you can plot the best course of action for managing the problem.”

There are several practices growers can do that will help prevent crown and root diseases in orchards. The most important is proper water management. Phytophthora, the pathogen that causes this type of rotting, thrives in wet conditions. Lightle said long periods of saturation will aid the spread of the pathogen. “The number-one takeaway I wanted everyone to have is that it takes about 24 hours of saturated conditions to set the stage for phytophthora infections,” she said. “Phytophthora needs water to infect. It needs water to reproduce and release spores, and so that 24 hours seems to be the threshold. If you saturate for over 24 hours, you are ripe for infection.”

This means growers need to know what their soils can handle. Lightle said good irrigation management can reduce a lot of rotting problems down the road. “Absolutely keep your irrigation sets timed for what your soil type can handle at that particular point in the summer,” she said. “Make sure you are trying to keep those sets at 18 to 24 hours maximum. So that could mean doing shorter sets more frequently rather than one long set that runs for 48 hours or more, which I have heard some growers say they do sometimes.”

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