Abstract: Methods are provided for determining plant stress, including: using a computer-based camera system having thermal imaging and visual imaging to capture foliage at close proximity of at least one plant to provide high resolution images/video thereof; analyzing both thermal and visual images/video therefrom to form a composite image; determining the thermal activity of the composite image/video and photosynthesis state of the at least one plant; and deriving the plant stress from the determination. A handheld computer-based camera system is also provided and includes a smartphone and thermal imaging device.