A SPECIAL GROWERS’ REPORT OF AG ALERT ® C A L I F O R N I A Trees & Vines ® Prune growers protect trees from two forms of canker
ByKathyCoatney Prune cankers, both fungal and bacterial, are prob- lematic for farmers. Fungal cankers can be spotted in the spring and sum- mer with flagging branches. Hot temperatures push can- ker growth, and devastating impacts can be seen at the end of the season, according to LukeMilliron, University of California Cooperative Extension orchard systems advisor for Butte County. Of the fungal cankers, Milliron said, Cytospora is the biggest concern in prunes. Bacterial canker is associated more with cold, ear- ly-season conditions and are muchmore site specific— potentially with orchards that have ring nematodes or hardpan soils, he said. Farmers are generally aware if they have “bac canker ground” and will have a history of the disease, he said, whereas other growers will never see it. “Bacterial cankers are much more site specific,” Milliron said. He stressed that a major difference between fungal and bacterial cankers is that bacterial cankers will be worse in years with warmbloom conditions. “When it’s bad, it’s really bad, especially on those sites that can get bacterial canker. You can have devastating impacts; you can lose trees,” Milliron said. Cytospora (fungal canker) is a perennial canker, and it can grow year after year, which makes it a continual problem for growers, he said. Even if there aren’t new infections this year, farmerswill still have todeal with the old infections that are potentially spreading, he added. As a bark canker, Cytospora kills the bark. It is in a majority, if not nearly every mature prune orchard, Milliron said. Pruningwounds areapotential entrance forCytospora into the tree, so the disease could be coming in from the outside of the canopy and moving inward over time, he said, adding that there’s no curative—once it’s in the tree, there’s really no stopping it, other than pruning it out. Young trees are at extremely high risk, Milliron said, especially when making first training cuts around the trunk. Infection can be devastating, he said, because it can’t be pruned out because the tree is so small. For this reason, interplanting is a major concern. Although Milliron doesn’t see much interplanting in his area, he cautions farmers against the practice. “I see it more in Sutter/Yuba counties where there’s old prune blocks. Folks have realized, ‘Oh, these trees are just spaced toowidely. I know I need to capturemore sunlight,’” Milliron said, adding the more sunlight cap- tured with the canopy, the more fruit that is produced. “The problem there is that if you decide, ‘OK, I’m going to plant on the diamond or just in the middle of these two trees because they’re so widely spaced,’ all of the sudden you’re planting this susceptible tree where you have to make all these cuts to train these
Hedging of prune trees provides what a farm advisor calls “zillions of entry points” for spores that can cause prune canker. Immediate removal of brush and the use of fungicide can help protect the trees.
See PRUNES, Page 10
January 27, 2021 Ag Alert 9
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