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Detection and counting of flowers on apple trees for better chemical thinning decisions

Farjon, Guy, Krikeb, Omri, Hillel, Aharon Bar, Alchanatis, Victor
Precision agriculture 2020 v.21 no.3 pp. 503-521
Malus domestica, algorithms, apples, data collection, experts, flowers, fruit trees, humans, orchards, regression analysis
Accurate chemical thinning of apple trees requires estimation of their blooming intensity, and determination of the blooming peak date. Performing this task, as of today, requires human experts to be present in the orchards for the entire blossom period or extrapolate using a single observation. Since experts are rare and in high demand, there is a need to automate this process. The system presented in this paper is able to estimate the blooming intensity and the blooming peak date from a sequence of tree images, with close-to-human accuracy. For this purpose, a two years dataset was collected in 2014–2015, partially tagged for the flowers location and completely annotated for blooming intensity. Using this dataset, an algorithm was developed and trained with three stages: a visual flower detector based on a deep convolutional neural network, followed by a blooming level estimator, and a peak blooming day finding algorithm. Despite the challenging conditions, the trained detector was able to detect flowers on trees with an Average Precision (AP) score of 0.68, which is on a par with contemporary results of other objects in detection benchmarks. The blooming estimator was based on a linear regression component, which used the number of flowers detected and related statistics to estimate the blooming intensity. The Pearson correlation between the algorithm blooming estimation and human judgments of several experts indicated high agreement levels (0.78–0.93) which were similar to the correlations measured among the human experts. Moreover, the developed estimator was relatively stable across multiple years. The developed peak date finding algorithm identified correctly the orchard’s blooming peak date, which was used to determine the thinning date in the current practice (the entire orchard is thinned in the same day). Experiments testing the algorithm’s ability to find a blooming peak date for each tree independently showed encouraging results, which may lead upon refinement to a more precise practice for tree-specific thinning.