EcO157 population dynamics on romaine lettuce leaves (cultivar Green Towers) over time after inoculation and incubation of the leaves under conditions of warm temperature and 90-100% RH (A) or 65-75% RH (B). Population sizes of EcO157 were assessed on leaf discs of healthy, chlorotic, and necrotic tissue of B. lactucae-infected leaves immediately (white bars), at 24 h (gray bars), and 48 h (black bars) post-inoculation. Data points represent the mean of log-10 (cfu per disc) for six (high RH) and eight (low RH) replicate discs sampled from different leaves. Error bars indicate standard error of the mean. Different letters within panel indicate means that are significantly different at P < 0.05. Note that leaves at 65-75% RH were inoculated with greater concentrations of EcO157 than leaves maintained at 90-100% RH to allow for quantification of survival rate.