Florida growers, Extension agents and others have been baffled by unusual trunk symptoms during what Evan Johnson termed “the year of the weird trunk disorder.” Growers spotted the disorders, which were similar to phytophthora, in groves over the past one and half to two years. Johnson addressed three of the disorders, which have been found in multiple counties. Johnson, a University of Florida Institute of Food and Agricultural Sciences researcher, discussed the trunk disorders at this year’s virtual Citrus Expo.
Bark splitting causes vertical splits in the bark and is usually restricted to the rootstock. It appears primarily in 4- to 5-year-old trees. Oozing is rarely observed. Bark splitting creates a problem by opening the trunk to opportunistic pathogens.
Trees with the next disorder, bark dieback and adventitious roots, may have some root sloughing. With this disorder, the bark is alive and healthy below the soil line, and necrosis stops at the graft union. “We’ve got a lot of questions and no clear answers” regarding the cause, Johnson said. He speculated that herbicide damage or tree wraps that were too tight could be a culprit, but added, “It’s also possible it’s a completely unknown cause.”
The pathogen Kretzschmaria deusta was determined at fault when growers “see a canopy that looks perfectly healthy collapse within days to a week,” Johnson said. Symptoms of this disease include fungal stroma on the trunk and black staining lines in the trunk’s wood. There is also soft, spongy wood in the tree’s roots and crown.
Kretzschmaria deusta is common in landscape trees but uncommon in citrus. In citrus, it’s a sporadic disease that can infect weakened or stressed trees. Because oaks can harbor the disease, Johnson said oak trees near groves could be the source.
The Kretzschmaria deusta pathogen can survive in the roots, so Johnson recommended removing roots from trees that collapse. There are no recommended chemical treatments. Because trees that collapse have probably been infected a year or more, and the disease has been linked to weather events, Johnson speculated that Hurricane Irma could be a culprit. Irma hit Florida citrus hard in September 2017.
Johnson discussed some possible reasons all three of the weird trunk disorders emerged in the past couple years. The possibilities include growers pushing trees hard with fertilizer and irrigation, susceptibility of new rootstocks, past weather events, stress caused by HLB or a combination of these factors.
See Johnson’s full Citrus Expo presentation here. The presentation, and the continuing education units available to those who watch it, will remain online through the end of 2020.
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