To further investigate whether Xoo-induced necrosis in N. benthamiana is a form of HR, which is a hallmark of nonhost disease resistance, the number of bacterial cells in infiltrated leaves was analyzed. The results showed that the population dynamics of Xoo was correlated with the timing of necrosis (Figure 4). Bacterial numbers decreased slowly from 0–12 hpi before necrosis was observed; however, bacterial cell counts dropped dramatically when necrosis was readily visible, e.g. 2.8- and 6.4-fold reduction at 24 and 48 hpi, respectively (Figure 4). These results, together with the symptomology, cytology, and molecular data, demonstrate that N. benthamiana forms an HR and exhibits nonhost resistance in response to the non-adaptive pathogen Xoo YN-1.