Spikelet diseases pose severe threats to crop production and crop protection requires timely evaluation of disease severity (DS). However, most studies have only investigated the spikelet diseases within a short period of crop growth. Few have examined the consistency in DS monitoring accuracy across growth stages. This study aimed to investigate the differences in spectral responses among growth stages and to develop a spectral index (SI), rice spikelet rot index (RSRI), for multi-stage monitoring of the rice spikelet rot disease. Proximal hyperspectral images were collected over spikelets with various levels of DS at heading, anthesis, and grain filling stages. The reflectance was related to the DS extracted from concurrent high-resolution RGB images. The proposed RSRI was evaluated for the DS estimation and lesion mapping across growth stages in comparison with existing SIs. The results demonstrated that the spectral responses to DS in the green and near-infrared regions for filling were weaker than those for anthesis, and blue bands were necessary in DS quantification for early infection. The RSRI-based models exhibited the best validation accuracy for heading and the most consistent performance across growth stages as comparison to other SIs (Heading: R² = 0.65; anthesis: R² = 0.84; filling: R² = 0.78). Moreover, RSRI-based DS maps exhibited the best lesion identification for slightly, mildly, and severely infected spikelets. This study suggests that RSRI could be promising in breeding and crop protection as a novel index for DS estimation regardless of the spikelet ripening effect.

小穗病对作物生产构成严重威胁，作物保护需要及时评估疾病严重程度 (DS)。然而，大多数研究只调查了作物生长短期内的小穗病害。很少有人检查过不同生长阶段 DS 监测准确性的一致性。本研究旨在调查不同生长阶段的光谱响应差异，并开发光谱指数 (SI)、水稻小穗腐烂指数 (RSRI)，用于水稻小穗腐烂病的多阶段监测。在抽穗期、开花期和籽粒灌浆期收集具有不同 DS 水平的小穗的近端高光谱图像。反射率与从并发高分辨率 RGB 图像中提取的 DS 有关。与现有 SI 相比，对拟议的 RSRI 进行了跨生长阶段的 DS 估计和病变映射的评估。结果表明，灌浆期绿色和近红外区对 DS 的光谱响应弱于开花期，蓝色波段是早期感染 DS 量化所必需的。与其他 SI（航向：R²  = 0.65；开花期：R ²  = 0.84；填充：R ²  = 0.78）。此外，基于 RSRI 的 DS 图显示出对轻微、轻度和严重感染的小穗的最佳病灶识别。这项研究表明，无论小穗成熟效应如何，RSRI 作为 DS 估计的新指标在育种和作物保护方面可能很有前途。