A DEVICE FOR PRECISE AND NONDISRUPTIVE STOMATAL INOCULATION OF LEAF TISSUE WITH BACTERIAL PATHOGENS

A stomatal inoculation apparatus (SIA) was developed to produce water congestion of leaf tissues and provide a reproducible noninjurious means of introducing two Xanthomonas campestris pathovars of citrus into leaf tissues without wounding. The SIA consisted of a small inoculation chamber attached to an intact leaf. Water and inoculum were metered into an airstream and focused to impact on a 1-mm-diameter area of the leaf surface. Leaf tissues on the abaxial surface of Duncan grapefruit leaves expanded 50-75% were more susceptible to infection than were other growth stages. Inoculum concentrations of 10(6) cfu/ ml consistently induced infection and resulted in discrete individual lesions. Airstream impact pressures of 6.28-8.04 kPa against the leaf surface consistently produced tissue congestion and infection without wounding. These pressures were the minimum threshold for increasing water volume in the leaf. From calculations of volume versus concentration of inoculum that enters a leaf via SIA, it was determined that as few as 2 cfu were required to cause a single lesion.