Phytophthora Increasing; HLB Complicates Control

Phytophthora incidences are increasing in Florida citrus groves, plant pathologist Ozgur Batuman reported during a May 21 presentation. Phytophthora diseases include foot rot of trunk and limbs, root rot and brown rot. Batuman, a University of Florida Institute of Food and Agricultural Sciences associate professor, said these diseases led to yield reductions of 3% to 6% per year even prior to HLB being in Florida.

HLB reduces the efficacy of phytophthora management, Batuman said. Possible reasons he cited include:

• Trees’ response to therapies has changed. Tree response to all manner of stress events and therapies, including response to fertilizer, are affected.
• The pressure of phytophthora has been magnified, and the average root mass of trees is reduced.
• The cycles of root production and disease are disrupted, atypical and fluctuate widely. Consequently, timing of fungicide applications becomes more complicated.

CHEMICAL MANAGEMENT

Batuman’s suggested chemical management of phytophthora includes:

• When the phytophthora count is greater than 10 to 20 propagules/cm³, recommend rotation of fungicides: fosetyl-Al/phosphite after spring shoot flush.
• For a severe root problem, use mefenoxam after spring/early summer rains begin.
• Use fosetyl Al/phosphite after fall shoot flushes.
• Remember, root flushes follow shoot flushes.

For phytophthora control in groves with HLB, Batuman said growers should decide which root flushes to protect. Start with root flushes that give the best return, he added. 

HLB RESEARCH

Batuman said he is working with other researchers on developing an automated delivery system for HLB therapies. Many growers in Florida are currently injecting trees with oxytetracycline in an effort to combat HLB. But Batuman, who works at the Southwest Florida Research and Education Center in Immokalee, noted that the current injection process is labor intensive and expensive.

VIRAL DISEASES

During the same presentation, Batuman discussed the viral diseases citrus leprosis and citrus yellow vein clearing virus, neither of which are currently in Florida. Learn about his discussion of those diseases here.

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