Bacterial canker: impact of seed-borne inoculum on plant infection in greenhouse tomatoes

Clavibacter michiganensis subsp. michiganensis (Cmm) is an important seed-borne pathogen that causes bacterial canker of tomato and significant crop losses worldwide. Few studies document the long-term survival of Cmm in infected seed and less is known about the relationship between inoculum levels and resulting plant infection. Greenhouse studies were undertaken to gain insight into Cmm viability on seed and transmission of disease to the subsequent crop. Seeds harvested from infected plants were assayed individually to enumerate the colony-forming units (cfu) of surviving Cmm. Data collected at regular intervals for five years revealed that Cmm was unevenly distributed in infected seed and the rate of population decline varied with the seed lot and some lots showed exceptionally high survival. Seed germination rates, seedling survival, transplant survival, and recovery counts of Cmm from fruit of infected greenhouse-grown plants were used to calculate contamination levels required to produce infected plants in the next generation. Two greenhouse trials were carried out, the first in Hawaii only and the second occurring concurrently in Hawaii and Mississippi. In the latter trial, not all infected plants produced infected fruit and infected fruit were unevenly distributed on infected plants. Transmission rates from the same infected seed lot varied considerably. Plant symptoms were generally more severe and occurred earlier in Hawaii and only 36.7% of the germinated seeds produced infected plants, whereas in Mississippi, the transmission rate was higher (51.8% of the germinated seeds produced infected plants) but a lower percentage of infected plants produced infected seed in next generation fruit. Based on cumulative frequency graphs extrapolated values for plant infection were greater than 525 and 115 cfu/seed in Hawaii and Mississippi, respectively. These studies give insight on specific variables to be considered in the effort to obtain threshold values for pathogen transmission.