Problematic Diseases Following Hurricanes

Hurricanes bring immediate tree damage from high winds and torrential rainfall, but storms also can lay the groundwork for citrus disease problems. Soon after Hurricane Milton passed through citrus-growing regions, the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) hosted a webinar that covered various topics including post-storm disease management.

CANKER

Megan Dewdney, UF/IFAS associate professor of plant pathology, presented observations about diseases that could be on the uptick after Milton. Key among those is citrus canker (Xanthomonas citri subsp. citr). Historically, active hurricane seasons have spread the disease around the state.

Citrus canker is moved easily by wind and rain. It only takes winds greater than 18 mph for canker bacteria cells to be blown through any protective coatings on citrus leaves. In addition, hurricane winds damage leaves, making them vulnerable to infection.

Canker is a particular concern for the fresh fruit segment. Dewdney said the good news is that most fruit types were far enough along developmentally that they should be immune to canker infection, with the possible exception of grapefruit depending on the grove and location.

“If you are worried about grapefruit, I would recommend scouting. And also note that copper protection will have been broken by the high winds forcing raindrops past the copper coating,” she said.

She cautioned growers with citrus under protective screen (CUPS) who might have had damage to screens that canker could enter through those openings or drip in from raindrops along holes in the roof.

Dewdney also addressed individual protective covers (IPCs). The bags normally help protect trees from canker by slowing winds and making rain droplets smaller, thus having less force behind them.

“However, after Hurricane Ian, we observed more canker in IPCs on the windward side of the trees. Also, where the trees grow and fill up the IPC, the foliage is right up next to the screen and doesn’t quite have the protection from the blocking effect of the net,” she said. “But we also didn’t find very much on the fruit in the IPCs; it was mostly on the foliage. We didn’t see much on the twigs, which was good news.”

She addressed what growers should do after Milton. Badly affected leaves are likely to drop in the spring. New flush will need to be supported. The greatest concern will be stem lesions on young trees.

“These stem lesions can ooze (canker) bacteria for up to four years and can provide inoculum for future seasons, which can be really devastating,” Dewdney said. “We have seen some situations where we had bad canker outbreaks the past two years, even in relatively dry periods. Part of this could be due to stem lesions on the trees.”

The lesions can lead to further fruit drop from canker. These lesions are very small and difficult to see. They can be pruned out if the trees are small, but that is labor intensive.

Plant immune system stimulators are an option to minimize lesions in young trees and can reduce the inoculum for future seasons. There are two products in this category — Actigard and Aura Citrus. Both should be applied in conjunction with copper. Dewdney recommended following the UF/IFAS Citrus Production Handbook for more detailed application guidance.

PHYTOPHTHORA

Standing water after hurricanes can support the development of phytophthora. Within less than 72 hours, root deterioration can become acute in standing water.

“Hopefully, with the rainfall patterns, growers didn’t have this problem this time (with Milton), but with Ian it was a real problem,” Dewdney said.

Toppled trees can be stood up, but care should be taken when the scion bark has touched soil or water because most all scions are extremely susceptible to the disease. It is not uncommon to see phytophthora lesions on trunks and limbs in this situation.

Groves that have had a history of root, foot and crown rot, flooding and saturated soils will likely have increased odds for problems. All these conditions are related to Phytophthora nicotianae and P. palmivora.

“We generally don’t recommend treating unless you have a propagule count above 10 to 20 per cubic centimeter of soil,” Dewdney said. “With the fact that HLB trees are more susceptible to phytophthora, and we have greater trouble treating for phytophthora effectively, I’d recommend treatment even if you have a threshold around that 10-propagule level.”

Dewdney said if growers are already on a phytophthora treatment program to stay on the current rotation of chemistries. Rotation is important to avoid resistance.

“Phosphite salts (only labeled as a fungicide) or Aliette have the same mode of action and are one option,” she said. “For foot, root and collar rot, mefenoxam products are an option. Also, fluopicolide and oxathiapiprolin are all good options for foot, root or collar rot.”

Dewdney noted that some of these applications require irrigation after treatments, so always read and follow the label instructions on each product.

For growers with fruit on trees that are concerned about brown rot, Dewdney noted it was late for phosphite salts and Aliette, but copper treatments can help with active infections and are a good choice for late-season applications.

Some newer products that she recommended for brown rot management are Revus and Orondis Ultra. Both can be applied in late-season situations post-hurricane.

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