The model was trained for 50 epochs with early stopping enabled. At the final epoch (50/50), the training accuracy reached 86.82% with a training loss of 0.3520. The validation accuracy at epoch 50 was 92.52%, with a validation loss of 0.2144. However, the highest validation accuracy of 92.83% was achieved at epoch 49. Since validation performance did not improve further, the model restored the weights from the best-performing epoch (epoch 49).
The training and validation curves show steady improvement in accuracy over epochs (Fig 3). Training accuracy increased gradually and stabilized near the final epochs. Validation accuracy followed a similar trend and remained consistently high, indicating good generalization. The validation accuracy being slightly higher than training accuracy suggests effective regularization, likely due to dropout and transfer learning.
This phenomenon can occur when regularization techniques such as dropout are applied during training but are disabled during validation and testing. Consequently, the model is evaluated under more stable conditions on the validation set, which may result in slightly higher validation accuracy than training accuracy. Furthermore, transfer learning from pretrained ImageNet weights provides robust feature representations that enhance generalization performance, particularly when the validation data distribution closely matches the training data. The relatively small difference between training and validation accuracy indicates good model generalization rather than overfitting.
The loss curves demonstrate a consistent decrease in both training and validation loss. Validation loss remained lower than training loss in later epochs, indicating stable convergence and minimal overfitting. The absence of sharp divergence between the curves confirms that early stopping successfully prevented performance degradation.
Fig 4 presents the confusion matrix of the proposed VGG16-based model evaluated on the independent test dataset for four-class classification. The diagonal elements show a strong concentration of correctly classified samples, indicating high predictive performance across all categories. For chocolate spot, 319 samples were correctly identified, with most misclassifications occurring as rust (65 samples), suggesting some visual similarity between lesion patterns. Gall disease achieved 312 correct predictions, with a moderate number of samples misclassified as chocolate spot (56) and rust (32), but none as healthy, indicating clear differentiation from non-diseased leaves. Healthy leaves were correctly classified in 326 cases, with limited confusion mainly toward chocolate spot (58) and rust (17). Rust demonstrated the highest correct classification rate, with 355 true positives and minimal confusion with other classes.
The classification report indicates strong overall performance of the proposed model across all four disease categories (Table 1). For chocolate spot, the model achieved a precision of 0.8939, recall of 0.8404 and an F1-score of 0.8663 based on 401 samples, indicating slightly lower recall compared to other classes. The gall class showed balanced and reliable performance, with precision of 0.9206, recall of 0.9275 and an F1-score of 0.9240 across 400 samples. The healthy class achieved the highest performance, with precision of 0.9470, recall of 0.9728 and an F1-score of 0.9597 from 404 samples, reflecting excellent model discrimination for healthy leaves. Similarly, rust classification demonstrated strong predictive capability, with precision of 0.9293, recall of 0.9525 and an F1-score of 0.9407 across 400 samples. Overall, the model achieved an accuracy of 0.9234 on 1,605 test images. The macro-averaged precision, recall and F1-score were all approximately 0.923, indicating consistent performance across classes, while the weighted averages further confirmed balanced classification effectiveness.
Fig 5 presents representative examples of model predictions for each of the four classes. In all cases, the predicted label matches the true label, indicating correct classification across different disease conditions. The model shows very high confidence for chocolate spot (99.79%) and gall (99.87%), demonstrating strong feature recognition for these disease patterns. The healthy leaf is also correctly identified with a confidence of 97.24%, reflecting reliable discrimination between diseased and non-diseased foliage. The rust sample is classified correctly with a confidence of 86.75%, which is slightly lower than the other classes but still indicates strong predictive capability.
Fig 6 presents the receiver operating characteristic (ROC) curves and precision-recall (PR) curves of the proposed VGG16 model for one-vs-rest multi-class classification of Faba bean leaf conditions. The ROC curves demonstrate the model’s ability to distinguish each class from the others across different threshold settings. All curves are positioned close to the top-left corner, indicating excellent discrimination performance. The Area Under the Curve (AUC) values are very high for all classes, ranging from 0.9791 for chocolate spot to 0.9978 for healthy leaves, confirming strong separability between classes. The Precision–Recall curves further evaluate model performance under class imbalance by measuring the trade-off between precision and recall. High Average Precision (AP) scores across all categories indicate that the model maintained high precision even at increasing recall levels. Healthy leaves achieved the highest AP (0.9949), followed by rust (0.9823), gall (0.9729) and chocolate spot (0.9399). Previous studies have widely applied VGG16-based deep learning models for plant disease detection across different crops, including grapevine, tomato, maize, mango, rice, cotton and mixed plant species (Table 2).
Alatawi et al., (2022) applied VGG16 to 15,915 PlantVillage images covering 19 plant disease classes and achieved 95.2% accuracy, showing the feasibility of large-scale automated diagnosis.
Mousavi and Farahani (2022) further enhanced VGG16 by integrating Faster R-CNN and drone-based image acquisition for grapevine disease detection, achieving 99.6% accuracy and outperforming several standard deep learning architectures.
Paul et al., (2024) implemented a pretrained VGG16 model within a mobile application to detect maize leaf diseases using 3,024 field and public images, achieving 93% testing accuracy and supporting real-time disease monitoring.
Similarly,
Sofiane et al., (2024) used VGG16 to classify tomato leaf diseases across ten categories using 16,012 PlantVillage images, achieving approximately 98.3% accuracy.
Kaur et al., (2024) applied VGG16 to mango leaf disease classification using 4,000 images across eight classes and reported 94% accuracy.
Suseno et al., (2023) combined a pretrained VGG16 model with image segmentation techniques, including thresholding and k-means clustering, to classify three rice leaf diseases, achieving 91.66% accuracy after data augmentation and hyperparameter tuning. In another study,
Kaur et al., (2024) fine-tuned VGG16 for cotton leaf disease classification using an augmented dataset of 5,000 images, achieving 95.5% accuracy and demonstrating improved performance with data expansion strategies.
Salau et al., (2023) highlighted the importance of image preprocessing in Faba bean disease detection by training an end-to-end CNN on both raw and enhanced images, achieving a substantial improvement in classification accuracy from 92.1% to 98.14% following preprocessing. Similarly,
Jeong and Na (2024) developed a deep convolutional neural network incorporating multiple convolutional, pooling and dropout layers and trained the model using a balanced 80:20 dataset split, demonstrating the effectiveness of deep feature extraction and regularization for disease classification. More recently,
Mostafa et al., (2025) and
Mohammad et al., (2026) employed a sequential CNN trained on expert-labeled and preprocessed Vicia faba leaf images and reported an accuracy of 98.92%, indicating that high-quality annotations and carefully curated datasets can significantly enhance classification performance.
The present study applied transfer learning with VGG16 to classify Faba bean leaf diseases. The proposed model achieved 92.34% accuracy with a macro-averaged F1-score of 0.9227, demonstrating reliable multi-class classification under natural field variability. Collectively, these studies confirm that VGG16-based architectures provide robust and adaptable solutions for automated plant disease detection across different crops, datasets and deployment scenarios.