How To Prevent Armillaria Root Rot

Armillaria Root Rot

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Armillaria root rot, or oak root rot, can be a devastating disease for peach growers in the southeastern U.S. In fact, Clemson University plant pathologist Guido Schnabel says just about every commercial peach grower in the region has likely lost some trees to it. What’s worse, there are very few management options that have shown much success.

However, Schnabel is looking to change that, or at least help growers minimize armillaria damage. Some growers already have Schnabel to thank for helping them ward off brown rot resistance to fungicides, and are hoping his latest trials continue to prove successful.

Schnabel is evaluating a system called above ground-root collar excavation (AG-RCE), which is partially based on a technique used by some citrus and grape growers. It involves excavating the lower trunk of the tree enough to make the primary roots visible. By planting on “berms” in this manner, growers can avoid the difficulty of continuously digging out below-ground roots that would likely become retrenched in soil anyway. In essence, it places the root crown out of reach of armillaria.

Slowing The Decline

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Schnabel’s trials, which are taking place at two South Carolina commercial orchards, evaluated peach trees planted three different ways: one as the grower control standard, one in Smart Pots, and one planted in Smart Pots, then removed about eight months after planting and replanted with the lower trunk and upper primary roots located above ground. Each tree was planted in an area previously exposed to armillaria. Each of the planting areas were eventually analyzed for tree mortality.

Within the first five years of planting, the trees planted in Smart Pots, then replanted on berms, showed significantly less tree decline due to armillaria. “With the infection effectively stopped, the fungus is unable to move to the crown of the tree or to other roots via the crown,” says Schnabel. In addition, “the new cultural method of planting the trees higher than normal allowed easier access to the above-ground root collar and eliminated the threat of soil covering roots back up.”

The one drawback to the AG-RCE approach was an increased risk of drought stress. However, Schnabel says waiting another year or two before excavation could help improve root mass and tree stability.

More Work Ahead

The trials taking place in South Carolina will continue for a few more years. Schnabel notes that his team did not investigate yield or fruit quality differences between treatments; however, the canopy of the trees did not show any noticeable differences. Fruit size may be a bit smaller because the trees may be weaker, says Schnabel, but adding another year to the time frame before excavation could offset this.

Schnabel also notes that when planting, growers will need a suitable plow, preferably one that throws soil from both sides in a berm. In addition, the trees must be planted at the right depth, with root initiation 1 to 2 inches below the soil line.

More information on Schnabel’s research can be found on the American Phytopathological Society website at http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-06-11-0493.

Resistance Monitoring Kit Update

The efforts of Clemson University plant pathologist Guido Schnabel and his team on the development of a brown rot monitoring and resistance management kit have been well-documented on this website. Now, Schnabel says the use of the kit has been expanded to include gray mold in strawberries. “The service has received lots of positive feedback and has been used to serve growers in many states,” says Schnabel. The kits can help growers identify gray mold resistance to several important disease control tools.

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