1. A plant health diagnostic method of diagnosing a health state of a plant in accordance with chlorophyll fluorescence emitted from chlorophyll of a plant body, the method comprising: obtaining a first minimum point after a maximum point having largest chlorophyll fluorescence intensity and a first maximum point after the minimum point on a time course curve of the chlorophyll fluorescence intensity; defining chlorophyll fluorescence intensity values at the minimum point and the first maximum point after the minimum point as S and M, respectively; obtaining a value M/S from the values S and M; and evaluating an initial state of photosynthetic functional disorder of the plant body with use of the value M/S and a value M/S of a healthy site of a plant body or of a healthy plant individual body as a reference value.