Pathogen infection leads to dramatic changes in the carbohydrate metabolism of the infected plant tissue that was supported in our work by the alteration of C metabolism-related proteins (spots L14, L26, L33) in grafted watermelon seedlings infected with FON. Specifically, the abundance of malate dehydrogenase (MDH, spot L33) was up-accumulated extremely in RG under FON challenge. The gene discussed is MDH2; the disease is infection.