Phytophthora: A Complex Problem

Daniel CooperDiseases

While the recent Florida Citrus Growers’ Institute presentations focused mainly on HLB, other topics were addressed, including phytophthora root rot. This disease has become more problematic in recent years. Megan Dewdney, University of Florida Institute of Food and Agricultural Sciences associate professor of plant pathology, spoke on the topic. She noted that phytophthora already is a complex problem and that has only been exacerbated by HLB. The two conditions interact and create even more problems in citrus trees.

Yellowing and thinning of the citrus canopy due to phytophthora infection.

“Research has shown that phytophthora plus HLB can cause 27% root loss while phytophthora alone causes about 10% loss,” she said. “So, it is eating away at the foundation of groves.”

Dewdney discussed two field trials located near Immokalee and another near Wauchula. The research showed that available fungicides, especially the higher-end products, can reduce phytophthora propagules in the soil. However, the trials showed no significant improvement in yields with any of the treatments.

Dewdney added there was a large amount of variability in the data collected in the trials, which complicated matters. This all leads to more questions, which Dewdney said requires more research.

“Should we be treating old and declining trees for phytophthora when they already are infected with HLB? That is a question we don’t have good answers for yet,” she said. “There is little effect on yields, which is significant. But there also is a concern that if we don’t treat in areas where it is a problem, the trees might collapse faster.”

Dewdney argued growers should protect young trees with healthy roots with measures like individual protective covers. Growers should continue to sample roots and take protective measures, especially where phytophthora is a problem.

There also has been some discussion that the threshold for treating phytophthora should be changed in the HLB era. This, too, would require new research to identify the correct propagule count.

Finally, Dewdney said more research will be needed to see what impact the introduction of oxytetracycline has on root systems and phytophthora.

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Frank Giles


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