Shining the Light on Tree Health

At a University of Florida Institute of Food and Agricultural Sciences Extension citrus field day held earlier this month, plant pathologist Amit Levy talked with growers about ways to evaluate tree health.

“A good evaluation should be correlated to the yield that the trees will produce,” explained Levy. “The evaluation should be concentrated on the tree health, and not on the number of bacteria in the trees. For example, when you are sick, you use the body temperature to evaluate the disease, not the amount of pathogen. Therefore, measuring CLas Ct values will only tell you if the tree is infected, but higher or lower Ct values should not be used to determine what will be the yield.” (See a Citrus Industry article that shows no correlation between Ct values and yield).

According to Levy, the most common current way to measure tree health is by determining a tree index rating, which divides a tree into eight segments. Each segment gets a score from 0 to 5 that evaluates the disease symptoms. “This method is good, since it evaluates the tree health and not the pathogen levels, but there is an inherent limitation with this method because it’s subjective,” said Levy. “Different people will rate symptoms differently, and even the same person can have different evaluations.”

Instead, Levy recommends growers use a light sensor to evaluate the canopy density of the tree. The sensor measures the light that can go through the canopy. “The idea is that if the canopy is denser, less light can go through,” Levy said. “We use a light sensor to first measure the light that reaches the canopy from the sun, and then the light under the canopy (that’s the light that went through). We then calculate the percentage of light that was intercepted, or absorbed, by the canopy. More absorption means less light went through, which means the canopy is denser and the tree is healthier.”

Levy and fellow researchers consistently found that trees with higher light interception produce higher yields.

“At the field day, we compared trees that were treated with controlled-release fertilizers to those that were supplemented with soil-applied sulfur. The supplemented trees, which produced higher yields during the 5-year experiment, had a light interception of 94% compared to the ones without supplement (87%),” reported Levy.

He said that measuring light interception is relatively easy and inexpensive. “A very cheap light meter can be used, although a more expensive line sensor will produce better results,” said Levy. “The main limitation is that the measurements should ideally be obtained when the sky is not cloudy.”