Published 2009-12-01
Abstract
Since the Asian citrus psyllid Diaphorina citri, vector of the Huanglongbing (HLB) disease, feeds primarily on young leaf flushes, it was hypothesized that applying pesticides on only new leaves could mitigate the impact of pesticide loading on citrus trees. An automated technique capable of recognizing new leaves and spraying the pesticide only on those targets would be of great interest to growers by making the spray applications economical and environmentally sustainable. A sensing system was developed that could differentiate new (immature) from old (mature) leaves on citrus tree canopies, based on spectral reflectance characteristic of the leaves. Based on preliminary results, a laboratory test stand was built consisting of a four-band active optic sensor, control box and spray system components. Using the spectral reflectance data, various vegetative indices were calculated and the utility of each index for differentiating young and old leaves was investigated. The control box and its algorithm, managed different tasks, e.g., communication with the sensor, computation of the vegetation indices and operation of the solenoid-controlled spray nozzles. Series of tests were carried out to evaluate the performance of the system under various static and dynamic conditions. Six of the eleven different vegetation indices used for the test were able to differentiate new from old leaves. Normalized Difference Vegetation Index (NDVI-870) gave the best results in both static and dynamic tests. The orientation of the sensor also provided a significant effect on target identification of the leaves but mounting it perpendicular to the tree detected a higher number of new leaves than mounting it at an angle. This paper reports on the development of the system and shows the performance of the system under various operating conditions.