Downy mildew raises E. coli probability in lettuce
In a release, the Agricultural Research Service (ARS) highlighted downy mildew, caused by fungus-like water mold Bremia lactucae, was one of the biggest problems lettuce growers had to face.
In Albany, California, the ARS Produce Safety and Microbiology Research Unit's Maria Brandal has been invetigating why so many E. coli outbreaks can be traced back to lettuce fields when sources of the problem can be as diverse as undercooked beef, sprouts, raw dairy, shelled walnuts, fruits and vegetables.
Lettuce leaves are actually a harsh place for microbes to survive. But the epidemiological evidence is indisputable about how often lettuce is the source of E. coli O157:H7 contamination.
In earlier research, Brandl found E.coli preferred cut, injured and younger leaves to undamaged and older ones. Then, she collaborated with ARS geneticist and lettuce breeder Ivan Simko from the Crop Improvement and Protection Research Unit in Salinas, California.
They found that under warm temperature and on wet leaves, E. coli multiplied 1,000-fold more in downy mildew lesions than on healthy lettuce leaf tissue. Even on dry lettuce leaves, where most bacteria struggle to survive, E. coli persisted in greater numbers when downy mildew disease was present.
The researchers also found E. coli did not grow as well in downy mildew lesions on a lettuce line bred by Simko and a colleague, called RH08-0464, compared to a commercial variety that is highly susceptible to down mildew called Triple Threat.
The factors behind this finding still need to be explored, but if a genetic hurdle to E. coli colonization could be bred into commercial lettuce varieties, the ARS said it would add a new defensive line against disease and help farmers improve the microbial safety of their crops.