Survivability of Pantoea stewartii Subspecies indologenes in Crop Residue and Its Transmission Risk to Onions in Poaceae-Allium Cropping System.

Pantoea stewartii subspecies indologenes (Psi) isolates can cause disease in several Poaceae hosts, including millets and rice and were recently known to cause foliar and bulb symptoms characteristic of center rot in onions. Cover crops such as millet and cash crops like corn are commonly grown in the summer after onion harvest in Vidalia, Georgia, USA. However, the risk of pathogen transmission to onions in the cropping systems where summer crops precede onion planting is largely understudied. We evaluated the survivability of Psi in corn and pearl millet residues and assessed its ability to colonize onions transplanted into the infested soil. Our microplot study showed that millet and corn residues support the transient survival of Psi. The presence of the pathogen in the soil also overlapped with the presence of onion transplants. However, despite planting onion seedlings in Psi-infested soil, no bacterial colonization was observed in their rhizosphere and foliar surfaces. We further investigated genetic determinants for bacterial survival in millet residue and bare soil by creating deletion mutants of the genes responsible for exopolysaccharides, flagellar motility, quorum sensing and pathogenicity in a Psi pathovar cepacicola strain PNA 14-12. All mutant strains persisted for at least 24 days in millet residue at high population levels and colonies of all the strains remained detectable in bare soil until 44 days. Overall, our findings suggest that the risk of Psi transmission from millet or corn residue to onions appears is considerably low.