Reconsidering Leaf Wetness Duration Determination for Plant Disease Management

ABSTRACT Hydathodes of tomato leaves served as extremely efficient infection courts for the bacterial canker pathogen, Clavibacter michiganensis subsp. michiganensis. Chlorotic lesions developed at the tips of leaflet lobes about 2 weeks after inoculation of guttation droplets. Lesions expanded along the leaflet margins and became necrotic. Movement of C. michiganensis subsp. michiganensis from the inoculated leaflet into the rachis was slow and erratic. Histological observations revealed that pathogen populations first developed within large intercellular spaces lying beneath the stomata, which serve as water pores in tomato hydathodes. Bacteria were first observed within vessels of the large marginal fimbriate veins 7 days after inoculation. By 14 days after inoculation, large populations could be seen within the vessels; and by 21 days after inoculation, tissue collapse was widespread and masses of bacteria could be seen in the intercellular spaces and within necrotic cells.