Dipartimento di Protezione delle Piante, UniversitÓ degli Studi della Tuscia, Via S. Camillo De Lellis, 01100 Viterbo, Italy

Background and objectives
Bacterial speck of tomato (Lycopersicon esculentum) caused by Pseudomonas syringae pv. tomato is one of the main bacterial diseases of this horticultural crop. The bacterium is ubiquitous and is able to survive on the host phylloplane during extended periods of hot and dry conditions. Among the environmental factors that assist spread of this pathogen, rain appears to play a fundamental role as a natural force involved in the release of bacteria from the soil and/or vegetation. The aim of this study was to verify the influence of rain on the horizontal and vertical distribution of P. s. pv. tomato on tomato leaves.

Materials and methods
The role of rain in epiphytic bacterial distribution was studied by a new rain simulator developed in the greenhouse. Two different kinds of rain (drops 1.6 and 2.7 mm in diameter) were simulated on infected tomato plants. Around the inoculum source (tomato plants infected by a bacterial suspension of 108 c.f.u./ml), the research area was divided into three sub-areas (A1, A2, A3). To follow horizontal distribution after rainfall simulation, samples were taken after 1, 3, 5, 7, 10 and 15 days. Randomly collected leaves were washed in an orbital shaker. Serial dilutions of washing water were plated on KB. To study the vertical distribution of P. s. pv. tomato, in addition to washing leaves, the replica print technique was applied at the 1st and 3rd node levels. The environmental factors (temperature, relative humidity, light) were automatically controlled and registered. Wind was deliberately not involved in the experiment. Plants of tomato cv. S. Marzano were grown for 6 weeks in the greenhouse before use. The study was repeated three times for each kind of rain.

Results and conclusions
The horizontal distribution of P. s. pv. tomato cells was influenced by drop size: the smallest (1.6 mm) determined long-distance distribution; whereas the largest drops (2.7 mm) seemed to cause short-range bacterial distribution (A1), even if, at the end of the experiments, the pathogen was also detected in the distant areas (A2 and A3).

Comparing the two different kinds of rainfall, interesting results were recorded for vertical P. s. pv. tomato distribution. Quantitatively, cells of P. s. pv. tomato were present mainly on the 3rd node leaf level within 3 days; later, bacteria were more easily detected on the 1st node leaf level within the 15th day, especially on the upper leaf surface. Qualitatively, the distribution of P. s. pv. tomato cells on the upper leaf surface (1st and 3rd node leaf level) was limited on the leaf edge within the first 3 days, and afterwards bacterial cells appeared uniformly distributed. On the lower leaf surface, bacterial distribution was on the leaf edge in A2 and A3 areas with both kinds of rain, while in A1, the P. s. pv. tomato cells were uniformly detected even on the lower leaf surface when the largest drops were used.

It appears that the smallest drops are more suitable for long-distance transport and distribution of P. s. pv. tomato cells. The largest drops showed a remarkable influence for a short bacterial distribution (A1), near the inoculum source. The uniform distribution of the pathogen, even on the lower leaf surface in A1, could be caused by more raindrops reaching the 1st node leaf level, directly or indirectly (falling from the superior node leaf level), and subsequently from the aerosol which is generated. Qualitatively, P. s. pv. tomato distribution (especially vertical) could be explained considering the morphological characteristics of tomato leaves, the gravitational forces involved and the different temperatures between the middle and edge of the tomato leaves. The influence of rain on bacterial distribution must be considered in relation to the phases of immigration, emigration, death, and growth of P. s. pv. tomato populations in the host phylloplane.