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IPF 2014: Detecting stealth crop diseases

Physicists help Brazilian orange groves to thrive.

Jarbas Caiado de Castro Neto, an entrepreneur and professor at the Instituto de Física de São Carlos, told participants at the Industrial Physics Forum how researchers in his small startup company are using optics to improve agriculture, specifically by saving orange trees.

Brazil is the seventh largest country in the world, but the third largest exporter of food. It is the number one exporter of sugar and coffee. Brazil also has large areas of unused arable land that represent still-untapped agricultural potential.

Brazil produces 31% of all oranges in the world, with production centered in the state of São Paulo.

Orange trees in Brazil and around the world suffer from a disease called citrus greening, or huanglongbing (HBL). The disease is caused when bacteria hitch a ride from tree to tree on a small plant-eating fly. The fruits from affected trees are small and irregularly shaped, with a thick, pale peel that stays green at the end. Diseased oranges taste bitter and can’t be sold. The disease also turns orange tree leaves yellow and blotchy. There is no cure, and when farmers discover a diseased tree, they must kill it in an effort to prevent the disease’s spread.

GreeningSymptoms of citrus greening include underdeveloped fruit and yellow, blotchy leaves. CREDIT: Fundecitrus

Symptoms of citrus greening include underdeveloped fruit and yellow, blotchy leaves. CREDIT: Fundecitrus

Since 2005, citrus greening has caused $4.5 billion in economic damage in Florida, and has necessitated the eradication of 27 million trees in Brazil, accounting for approximately 5–15% of production costs.

Orange farmers traditionally detect citrus greening by visually inspecting their tree leaves. In Brazil a tractor carrying four leaf inspectors rolls up and down the rows of trees. Visual inspection accurately identifies the disease 60% in the best cases and 30% on average. Unfortunately, diseased trees typically do not exhibit visible symptoms for more than a year, during which time they can spread the bacteria to other trees in a grove.

Castro and his research team spent one and a half years developing an inexpensive and portable way to detect citrus greening before the trees exhibit visible symptoms. The end product, which is about the size of a credit card scanning machine, contains a laser, a spectrometer, and a processor to analyze the results. It contains a battery that can power the equipment for a full day of leaf checking, and can report results in the field.

The portable spectroscopic system tests for citrus greening in an orange grove. CREDIT: Jarbas Caiado de Castro Neto

The portable spectroscopic system tests for citrus greening in an orange grove. CREDIT: Jarbas Caiado de Castro Neto

Diseased and healthy orange tree leaves emit different fluorescence spectra. Leaves affected by citrus greening emit more light in the middle of the visible spectrum, but have a lower peak—around 750 nm—associated with the degradation of chlorophyll in the leaves.

The spectrographic signature of citrus greening disease also appears almost immediately after a tree is infected, compared to the year or more it takes for visible symptoms to develop.

Debora Milori, a researcher from Embrapa (the Brazilian Agricultural Research Corporation), led a project to develop a benchtop fluorescence spectroscopy technique to diagnosis citrus greening disease, but farmers needed a more portable device to test leaves in the field. Embrapa transferred the technical know-how behind the spectroscopic technique to the company Castro and his colleagues started, called Agricultural Optronics Systems (AgriOS). AgriOS then developed a portable spectroscopic system for use in the orange groves.

When Castro and his team tested their portable spectroscopic system in August 2014, it detected citrus greening with greater than 95% accuracy. The fact that the device can identify diseased trees in the year before they exhibit visible symptoms is a huge advantage, Castro said, because it allows farmers to eradicate trees before they spread the disease to others. The researchers estimate that this early identification could save Brazilian orange farmers billions of dollars by 2020 if widely adopted today.

AgriOS has an agreement to test their portable spectroscopic system with the largest producer of oranges in Brazil. Castro noted that one of the challenges the company faces may be convincing farmers to eradicate trees that have no visible signs of disease. He said this challenge might be overcome by marking trees that the spectroscopic analysis identifies as diseased, and then closely monitoring them. If farmers see the trees succumb to citrus greening, they may be more likely to trust the technology.

AgriOS plans to extend their use of optical techniques to detect diseases in other crops, and eventually to integrate the devices into comprehensive farm management systems.

Catherine Meyers is a writer in the Media Services division of the American Institute of Physics.

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