The common bean, Phaseolus vulgaris L., like many crop species is vulnerable to the destructive necrotrophic fungus Sclerotinia sclerotiorum (Lib.) de Bary (Ss), the causal agent of the white mold disease. To slow Ss spread, farmers rely on costly fungicides that problematically are most effective when applied early, during which the plant lacks visual signs of infection. Internally, an early indicator of Ss infection is the acidification or decrease in plant pH caused by the secretion of oxalic acid released by Ss. The objective of this study was to determine if this early drop in apoplastic pH post-Ss infection could be detected using an Arduino platform-based potentiometric pH sensor with a carbon reference electrode on the leaf surface of a common bean. Interestingly, plant pH did not decrease but was statistically unchanged in the cultivars resistant to Ss (WM-12, WM-1, and G122) or intermediate tolerant (Eldorado, ICA Bunsi, and Beryl), while increasing in the susceptible cultivar (Montrose). This hints at possible Ss resistance mechanisms not present in the susceptible cultivars. Importantly, in seven of the common bean cultivars tested, the direction and magnitude of pH change pre and post-Ss infection measured using the carbon sensor were indistinguishable from the labor-intensive manually extracted leaf apoplastic fluid. Therefore, in the future, these sensors could conceivably be used for high-throughput screening of large germplasm collections to identify novel sources of genetic resistance to Ss that could be introduced into elite common bean cultivars to counter the highly destructive white mold disease.