It has been shown that plant pathogenic bacteria can occur on non-host plant surfaces without inciting disease symptomatology.
For example, the causal agent of bacterialleaf spot of lettuce, X. campestris pv. vitians, colonised plant surfaces of several weed species belonging to members of the Asteraceae family, as well as some in the Chenopodiaceae, Malvaceae, Polygonaceae and Portulacaceae families.
The number of bacteria on plant surfaces varied significantly among these different weed species and significantly more were recovered on lettuce than on plants in the Chenopodiaceae, Polygonaceae and Portulacaceae families.