Long used to protect other fruit and vegetable crops, kaolin can also conceal citrus trees from hungry psyllids by confusing their visual sensory system, said Michael Rogers, director of the Citrus Research and Education Center in Lake Alfred, part of the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS). Consequently, kaolin has emerged as a viable weapon in the battle against the bacterial disease huanglongbing, also known as HLB or citrus greening disease.
“We are encouraging growers to experiment with kaolin, because there’s a lot of potential upside to this material and not much risk,” said Rogers, also a professor with the UF entomology and nematology department. “Kaolin is inert, non-toxic and widely available. We think it’s worth a look, and UF/IFAS is conducting studies now to determine how we can optimize its usefulness for citrus production.”
In its commercial form, kaolin is a white powder consisting of tiny particles that can be mixed with water to make an opaque, white liquid suitable for application with standard spray equipment, said citrus tree ecophysiologist Christopher Vincent, an assistant professor at the Lake Alfred center and principal investigator of the current study, which was supported by the Citrus Research and Development Foundation.
“As the water evaporates, a thin coating of kaolin remains behind on the leaves and other tree surfaces that were sprayed,” he said. “It temporarily gives the leaf canopy a whitish color that’s very noticeable, and it provides several benefits to the treated groves.”
Chief among those benefits is the fact that kaolin can “hide” citrus trees from the ACP, the insect that transmits the pathogen responsible for causing HLB, Vincent said. The psyllid feeds on citrus foliage and new growth, and finds host trees by using its visual system to search for the wavelengths of light that people perceive as green or yellow. But a coating of white kaolin can mask the leaf canopy so much that psyllids don’t recognize it as citrus foliage, he said.
“Our findings thus far indicate that white kaolin won’t make your groves 100 percent psyllid-proof, but there’s no question it helps,” he said.
Vincent and his team are almost two years into a three-year study that uses young citrus trees planted in outdoor blocks, comparing the growth and health of kaolin-sprayed trees against that of trees sprayed with a foliar insecticide as well as untreated control trees. The study began in December 2016; field experiments began the following April.
The research team has counted psyllids on all trees under study every week to date, Vincent said. Untreated trees have had about five times as many psyllids present, compared with trees sprayed with a kaolin coating.
“Our findings are highly consistent with those reported in previous studies from the U.S. and Brazil,” Vincent said.
Aside from confusing psyllids, white kaolin has been shown to prevent sun damage and increase photosynthetic activity by diffusing light more effectively throughout the trees’ canopy.
“You know how lots of Floridians drive white cars because white paint reflects more sunlight than darker colors, and keeps the passenger compartment cooler?” Vincent asked. “White kaolin has a similar effect. When the light waves strike it, some of them are reflected into other parts of the leaf canopy. Potentially, the reflected light will reach leaf surfaces that don’t normally receive much direct sunlight, and so the photosynthetic activity in those leaves will increase.”
Currently available kaolin products are colored white. But as part of the overall project, Vincent’s team is evaluating a red-colored kaolin developed at UF by an interdisciplinary team that included Michael Rogers. Though not yet sold commercially, the red kaolin is available for licensing from UF. When mixed with water and applied to trees, it temporarily imparts a pink color to treated areas.
Preliminary findings indicate that red kaolin performs much the same as the white variety, except that red kaolin seems to be slightly more effective at discouraging psyllids and slightly less effective at promoting photosynthesis, Vincent said.
“We’re confident in saying that growers should consider using kaolin, but from there it’s a question of whether it makes financial sense for your operation,” Vincent said. “Frankly, one of the biggest barriers that causes growers to hesitate is the expense — kaolin typically costs around $40 to $50 per acre, per application, and it needs to be reapplied after you’ve had 2 or 3 inches of rain.”
Consequently, he said, growers who’ve begun using white kaolin generally apply it only during the spring and fall dry seasons, when a single application will last longer.
“We have been evaluating adjuvants, trying to find one that will substantially improve the rain-fastness of the kaolin particles and enable them to remain on the tree for longer periods of time,” Vincent said. “So far, we haven’t found anything ideal, but we will keep searching. If there were a low-cost method of assuring that a single application of kaolin would remain effective through multiple hard rains, I think we’d see more grower interest.”
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