L. Li et al. [5] found that research in the field of crop leaf disease detection has 3 major categories. First is using classic well-known deep learning architecture, second is using that architecture pre-trained on large datasets and third is using new/modified dl architectures.
1. Classic well-known deep learning architecture:
Because each disease location has distinct characteristics, Barbedo [6] and Lee et al. [7] investigated the utilisation of individual lesions and spots rather from taking into account the entire leaf. This method's benefits include the capacity to recognise the presence of many diseases on the same leaf and the ability to enhance the data by breaking down the leaf picture into various sub-images.
In a diverse field setting and in an experimental setting, publication [8] employed the GoogLeNet model to detect 79 illnesses in 14 plant species. Compared to the full picture (82%), isolated lesions and spots had a better overall accuracy of 94%.Through experiments, Lee et al. [7] showed that model training using common diseases was more universal, regardless of culture, and did not see new data or cultures obtained especially in different fields. This fresh approach to detection emphasised on the disease's common name rather than its crop-target category to identify illnesses.
Liang et al. [9] compiled a collection of 2906 positive samples and 2902 negative samples to identify rice blasts. The experiment's findings also demonstrated that, in terms of identification and effectiveness, senior features produced by CNN beat traditional manual techniques like the wavelet transform (Haar-WT) and local binary pattern histogram (LBPH).
Qiu et al. [10] employed Mask-RCNN, whose feature extraction network is either ResNet50 or ResNet101, to identify wheat disease-affected regions. The test data set's average accuracy was 92.01%.
Li et al. reported that the accuracy for the lab dataset was 98.44% and for the field dataset it was 92.19%. [11] used it to determine the various levels of ginkgo biloba. The accuracy of the Inception V3 model was 92.3% and 93.2% respectively.
2. Transfer Learning:
Ahmad et al. [12] used four different pretraining convolution neural networks, including VGG19, VGG16, ResNet, and Inception V3, and the models were trained by adjusting parameters. The experimental results showed that Inception V3 performed the best on the two datasets (the laboratory dataset and the field dataset). Additionally, the average performance on the laboratory dataset was 10–15% better than the field dataset.
Xu et al. [13] suggested a convolutional neural network model (VGG16) based on transfer learning to accomplish maize leaf disease picture identification (healthy, leaf blight, and rust) in complicated outdoor backdrops with constrained samples. After being trained on ImageNet, the VGG16 model's weight parameters were added to the model, and the average recognition accuracy was 95.33%.
The ResNet50 network, previously trained on ImageNet, was used to analyse four distinct apple leaf diseases in the Plant Pathology 2020 Challenge dataset. Overall test accuracy for the model was 97%. However, excluding the group of complicated illness patterns, the identification accuracy was just 51% [14]. (the mixture of numerous disease symptoms). AlexNet was used for two different types of training by Long et al. [15] to identify illness on camellia leaves. training from scratch while incorporating ImageNet knowledge (4 diseases and health). The findings shown that transfer learning may substantially increase the model's convergence rate and classification capability, with classification accuracy reaching 96.53%.
Improved Deep Learning Architecture:
To analyze high-resolution photos of corn disease, Dechant et al. [16] used many CNN classifiers. According to the experimental findings, the accuracy rate was 90.8% if only one CNN classifier was applied, 95.9% when two first-level classifiers were employed, and 97.8% when three first-level classifiers were used.
In order to extract the specifics of the symptoms of the wheat disease, Picon et al. [17] substituted the first 7 x 7 convolutional layer of the ResNet50 network with two 3 x 3 convolutions and used the sigmoid activation function in lieu of the softmax layer. and identified the first three wheat diseases with 96% accuracy on the balanced dataset using the improved ResNet50 network (septoria, tan spot, and rust).
Fan et al. [18] added a batch standardisation layer to the convolutional layer of the Faster R-CNN model to produce a mixed-cost function. To enhance the training model, they also used a stochastic gradient descent method.
They selected nine distinct varieties of maize leaf diseases with complex field histories as their research topic. Under identical testing settings, the upgraded technique outperformed the SSD approach with an average accuracy increase of 4.25% and a single picture detection time decrease of 0.018 s. The improved technique also improved accuracy on average by 8.86%.
Guo et al. [19] modified the fully connected layers of the AlexNet network, removed the local response normalisation layer, and set a multi-scale convolution kernel as the feature extraction mechanism in order to create a multi-receptive field recognition model based on AlexNet known as Multi-Scale AlexNet. The PlantVillage dataset and the self-collected dataset of seven different tomato-damaged leaf kinds serve as the study objectives. The model reduced the memory needs of the original AlexNet by 95.4% while increasing the average recognition accuracy of tomato leaf illnesses and each disease in its early, middle, and late phases to up to 92.7%.
Chen et al. [20] introduced an improved VGG model (INC-VGGN) based on the VGG model framework by adding two Inception modules, a pooling layer, and altering the activation function. Furthermore, an average of 92% of leaf diseases in maize plants could be correctly recognised..Additionally, 92% of corn plant leaf diseases could be accurately identified on average.
In Table 1, we have summarized all the above mentioned research works about the implementation of deep learning in plant disease detection. In this table, detailed analysis of the studies have been highlighted with the model types, dataset, sample size and evaluation metric.
Table 1
Summary of recent research works about the application of DL framework directly and improvement of DL in plant disease detection.
Reference
|
Object
|
DL frame
|
Dataset
|
Sample Size
|
Data Enhancement
|
Metric
|
[7], 2020
|
Plant
|
VGG16, Inception V3, GoogleNet
|
PlantVillage, IPM, Bing
|
54305
|
Random Cropping and Mirroring
|
Top-1
|
[12], 2020
|
Tomato
|
VGG-16, VGG-19, ResNet, Inception V3
|
Self-acquired in field and LAB
|
2681–15216
|
|
Average Accuracy
|
[9], 2019
|
Rice
|
CNN
|
Self-acquired in field and LAB
|
5808
|
None
|
Accuracy, ROC, and AUC
|
[10], 2019
|
Wheat
|
Mask RCNN
|
Self-acquired in field and LAB
|
20-2809
|
Chunking + traditional enhancement
|
Average Accuracy
|
[11], 2020
|
Ginkgo biloba
|
VGG, Inception V3
|
Self-acquired in field and LAB
|
3730–15670
|
Rotate, Flip
|
Average Accuracy
|
[8], 2020
|
Apple
|
ResNet152, inception V3, MobileNet
|
Self-acquired in field
|
334–2004
|
Random Rotation for cutting and grayscale
|
Average Accuracy, Processing time of each image
|
[13],2020
|
Corn
|
VGG-16
|
Self-acquired in field
|
600–5400
|
Rotate, Flip
|
Average Accuracy
|
[18], 2020
|
Corn
|
|
China Science Data Network, Digipathos Network and self-acquired in field
|
1150–1150*8
|
Flip, Brightness Adjustment, Saturation Adjustment, and add Gausian Noise
|
Average Accuracy, Average Recall, F1-score, Overall average accuracy
|
[19], 2019
|
Tomato
|
|
PlantVillage + Self-acquired in field
|
5766–8349
|
Random Cropping, Flip, Rotation, Color dithering, Add noise
|
Average Accuracy
|
[20], 2020
|
Rice, Corn
|
|
Self-acquired in field
|
966–4000
|
Rotate, Flip, Scale transformations
|
Accuracy, Sensitivity, Specificity
|