Editor's note: A special thanks to Dr. Monica Ozores-Hampton at the Southwest Florida Research and Education Center for coordinating The Immokalee Report, of which the article below is part.
The most important commercial orange cultivars grown in Florida are highly susceptible to canker. Current control methods have not been satisfactory, and the problem is exacerbated by tropical storms.
Nagami kumquat and Buddha Hand citron are resistant to canker.
Our research program has two objectives. The first is to determine why some citrus cultivars, such as kumquat and the citron Buddha Hand, are less susceptible to citrus canker than grapefruit and orange cultivars. The second objective is to improve the defense mechanism of susceptible cultivars using commercially available chemicals.
Kumquat and Buddha Hand rarely develop symptoms under field conditions or when artificially injected with canker in the greenhouse
We have observed that citrus cells near the inoculated area will rapidly die as part of a natural plant defense mechanism to isolate and kill the bacteria.
As part of this same research, we have observed how the canker bacterium, Xanthomonas citri subsp. citri, interferes with the disease resistance in citrus.
A key chemical that appears to increase in resistant cultivars is hydrogen peroxide, but this compound does not increase to the same degree in susceptible cultivars.
As a result, we use this same marker in plant defense to determine if commercial compounds will promote disease resistance in grapefruit and sweet orange.
We have attempted to restore the canker-defense mechanism in citrus by applying commercially available chemicals. To date, we have found that products containing salicylic acid restore part of the defense mechanism but not to a degree that provides sufficient grower-acceptable disease control.
Salicylic acid is not registered for use on citrus and therefore is illegal to apply.
Phosphite, the beneficial bacterium bacillus, humic and fulvic acid, as well as fertilizers we have tested have not reduced disease severity when applied alone. But we found that the products Fortress plus Safeguard II, which is a combination of fertilizer, phosphite and salicylic acid, provided some disease control.
Neonicotinoid insecticides are inducers of plant defense against pathogens and are registered for use on citrus. We are continuing to search for a cocktail of chemicals that growers can apply that would restore a full and complete plant defense mechanism response and give better disease control.
Robert C. Ebel, an associate professor and citrus physiologist at the Southwest Florida Research and Education Center in Immokalee, can be reached at email@example.com. Naveen Kumar is a post-doctoral researcher and citrus physiologist at the SWFREC. He can be reached at firstname.lastname@example.org.