Abstract
Huanglongbing (HLB) and Phytophthora foot and root rot are diseases that affect citrus production and profitability. The symptoms and physiological changes associated with these diseases are diagnosed through expensive and time-consuming field measurements. Unmanned aerial vehicles (UAVs) using red/green/blue (RGB, true color) imaging, may be an economic alternative to diagnose diseases. A methodology using a UAV with a RGB camera was developed to assess citrus health. The UAV was flown in April 2018 on a grapefruit field infected with HLB and foot rot. Ten trees were selected for each of the following disease classifications: (HLB-, foot rot–), (HLB+, foot rot–), (HLB-, foot rot+) (HLB+, foot rot+). Triangular greenness index (TGI) images were correlated with field measurements such as tree nutritional status, leaf area, SPAD (leaf greenness), foot rot disease severity and HLB. It was found that 61% of the TGI differences could be explained by Na, Fe, foot rot, Ca, and K. This study shows that diseased citrus trees can be monitored using UAVs equipped with RGB cameras, and that TGI can be used to explain subtle differences in tree health caused by multiple diseases.
Highlights
Citrus is grown around the world and is produced in more than 80 countries, creating a worldwide interest in its development [1]
These results are significant because previous studies have found that triangular greenness index (TGI) was highly correlated with N, our study indicates that Na, K, Ca, and Fe as well as disease status affect TGI while N does not
Using Unmanned aerial vehicles (UAVs) to calculate TGI can simplify this process by distinguishing between the diseases over a large area in a short amount of time
Summary
Citrus is grown around the world and is produced in more than 80 countries, creating a worldwide interest in its development [1]. Citrus production worldwide is threatened by many abiotic or biotic factors. The most significant abiotic factors posing a threat to citrus production are flooding, salinity, heat, drought, and nutrient deficits [3]. Diseases like huanglongbing (HLB or citrus greening disease) and Phytophthora foot rot and gummosis are among the most devastating to the citrus industry in the US [4,5]. HLB, caused by the bacterium Candidatus liberibacter asiaticus (CLas), has no known cure and has caused a severe decline in citrus production in Florida, the most affected state. As the disease has spread, other citrus producing states like Texas and California are trying to prevent large scale tree decline. Tracking the spread of disease for effective management has been difficult due to asymptomatic trees and interaction with diseases such as Phytophthora foot rot
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