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Cause of Dieback in Bingo Mandarin

At UF/IFAS, we are working on finding solutions for Florida’s citrus growers. This is a summary of one project made possible by state legislative funding for the UF/IFAS Citrus Initiative during the 2018-19 cycle. It documents how we are making progress and providing Florida growers with reasonable, pragmatic solutions to successfully grow citrus in the new age of citrus greening.

Researchers: Megan Dewdney, Plant Pathology; Christopher Vincent, Horticultural Sciences; Liliana Cano, Plant Pathology

IMPACT: The ‘Bingo’ mandarin is a promising citrus variety that many growers have planted, so understanding why the branches of many trees have died in the field is an urgent need. One or more species of Colletotrichum fungi were associated with twig dieback in ‘Bingo’ mandarin, but demonstrating infection requires more definitive research. 

In recent years, California growers have boosted demand for fresh citrus by promoting their seedless, easy-peel mandarins as healthy snacks. Florida citrus producers were interested and UF/IFAS citrus breeders responded with a series of seedless mandarin hybrids, beginning with an early-season variety known as ‘Bingo’, released in February 2015. Bingo produces a small fruit with good flavor, that ripens in October, providing Florida growers with a marketing window in which they do not compete directly with California mandarins, which ripen later in the season. To date, more than 150,000 ‘Bingo’ trees have been planted in Florida, but growers have reported an unexpected problem — excessive twig dieback, a condition that reduces the tree’s leaf canopy dramatically, diminishing
both photosynthesis and fruit production.

In hopes of pinpointing the cause and offering a solution, UF/IFAS plant pathologist Megan Dewdney and colleagues conducted studies to determine whether the dieback phenomenon involved fungi representing the genus Colletotrichum. This genus contains numerous plant pathogens, including two species blamed for causing citrus shoot dieback symptoms in California since 2012. Tissue samples collected from ‘Bingo’ trees in growers’ fields displaying twig dieback  generated many isolates of Colletotrichum fungi, 13 of which were purified, and represent three species. Next, the team attempted — unsuccessfully — to induce dieback in healthy, greenhousegrown citrus trees by inoculating them with a solution containing all 13 Colletotrichum isolates. The results were not
entirely conclusive in regard to wounding but the researchers believe that dieback could be the result of an opportunistic infection that develops after a tree experiences severe physical damage or a physiological response to wounding, which can happen during normal field operations. Further experiments will help resolve this question.