SEED TRANSMISSION OF ACIDOVORAX CITRULLI AND ITS DETECTION IN CUCURBIT SEED LOTS

The bacterial fruit blotch is an important disease of cucurbits, caused by the Gram negative rod Acidovorax citrulli. The disease has a great potential to cause significant losses, especially on watermelon. There are only two reports of the disease in Italy, but both the seed industry and farmers are particularly concerned, since the pathogen has been already intercepted in imported watermelon seed lots. In order to evaluate the risk of seed transmission and the relationship between seed contamination, disease outbreak and seed quality, an extensive field trial has been put in place in 2013. The highly sensitive watermelon cultivar Charleston Gray was used for the experiments. Three seed contamination levels were considered (1%, 5% and 15%) and for each contamination level a plot with 100 seedlings was transplanted. Outbreaks were monitored during the growing season until harvesting and disease was quantified by means of AUDPC. The harvested watermelons were used to produce seed with a natural contamination and such seed was used to implement a detection protocol for A. citrulli. The results obtained showed a significant correlation between initial contamination rate and disease severity in the field, with an AUDPC score of 1.312, 7.315 and 32.970 for the three contamination levels considered. The disease was observed on leaves and vines, but not on fruits. Nonetheless, seed obtained was contaminated by the pathogen, indicating that A. citrulli efficiently progressed along the vascular system, into the fruits and seeds. Pathogen detection in seed lots has been implemented by using samples of 5,000 seeds, extraction of bacterial DNA and its amplification with a RealTime PCR protocol.