Evaluating the Potential of Hot Water Treatment as Means to Reduce Bacterial Canker

Introduction - Bacterial canker is an important disease of stone fruit in the Northeastern United States. Although it is most serious on sweet cherry and apricot, it also affects tart cherry, peaches, plums, and prunes. Two species of bacteria are associated with bacterial canker: Pseudomonas syringae pv. syringae (herein referred to as P.s.s.) and Pseudomonas syringae pv. morsprunorum (herein referred to as P.s.m.).

Bacterial canker poses a serious threat to newly-established and young orchards. It has always been presumed that epidemics get their start as a result of infection from outside inoculum sources, e.g., infected wild Prunus spp. or epiphytic populations surviving on weed hosts. Although these sources of inoculum may indeed be significant, particularly the role wild Prunus spp., there have been too many instances where severe epidemics have occurred but no obvious source of inoculum exists. In a preliminary survey of 1-yr-old dormant vegetative buds (i.e., the buds used in propagation) we found 5 to 40% of apparently healthy vegetative buds were infected with P. syringae. These infections, we contend, have a direct impact in nursery production as well as in orchard production.

Several factors are to be considered. First, it is likely that heavily infected buds won't "take" or they will develop die-back symptoms quickly after growth begins. Both symptoms have been observed in local nurseries. It is likely that the inherent susceptibility of a variety plays a role on how quickly infection develops. Less susceptible varieties are likely to be more problematic for the grower because significant disease development will not occur until the tree is planted; infected trees will then serve as a source of inoculum in that orchard. When epidemics develop in nurseries, copper bactericides are used more intensively. Heavy use of copper bactericide puts selective pressure for the development of resistance. These strains eventually make their way in to grower's orchards. Given the implications, greater emphasis should be placed on reducing or eliminating nursery-born infections.

The objective of this research is to evaluate the potential of using heat treatment to eliminate systemic infection of budwood.

Accomplishments: Hot-water or heat treatment has been used to reduce or eliminate systemic bacterial infections in propagation material for a variety of crops. There are two important factors that must be considered when developing a heat treatment protocol. First, the sensitivity of Pseudomonas strains to heat treatment must be determined in vitro. To accomplish this, suspensions of actively growing cultures of P.s.s. and P.s.m. were placed in eppendorf tubes and immersed in hot water held at various temperatures. Samples of the heated bacterial cultures were collected at time intervals and dilution plated on KB to determine the proportion of surviving bacteria relative to a non heat-treated control. Results showed that P.s.s. was less sensitive to heat treatment than P.s.m. and we concluded that a hot water immersion at 52 C for 6 min was sufficient to eliminate all detectable P. syringae.

Second, for heat treatment to be readily adopted the viability of the propagation material must be preserved. That is, an effective heat treatment must eliminate the pathogen while not harming the embryonic tissues. Early researchers effectively eliminated necrotic rusty mottle virus from cherry budsticks with hot water treatments of 50 C for 10 or 13 minutes and 52 C for 5 minutes. Combining this information and the results of the in vitro study as starting point, 'Royalton' bud sticks grouped into experimental units of 5 sticks of 5 buds each were immersed in water held at 52 C for various durations (consistent infections rates were obtained by grouping 5 buds from five bud sticks for a total of 25 buds). Once treated, the buds were removed from the bud sticks and serial dilutions were made to detect bacterial populations.

Results indicated that the 6 minute immersion period was indeed sufficient to eliminate all detectable P. syringae in budwood collected in 2001. Bud survival was determined by placing treated bud sticks in beakers of water at room temperature and recording the number of buds that broke. Unfortunately, buds became more sensitive to the hot water treatment the longer they were held in storage at 4 C. Additional experiments were conducted this winter to determine the optimal timing for applying hot water treatments to propagation materials without damaging the tissue. It is too early to present results.