The high variation of Fv/Fm in leaf discs of controls and infected
susceptible lettuce plants as shown in Fig. 3 can be explained by
the accidentally contamination of disc by secondary microbes
other than B. lactuca (in particular Alternaria spec., Penicillium spec.
and Cladosporium spec.). In some cases, mean Fv/Fm of leaf discs of
controls decreased by more than 0.2 units. To prevent artificial
misclassifications due to this contamination, all leaf discs were
additionally examined and separated into the four categories
shown in Table 2.