Canada thistle (Cirsium arvense) plants that are infected with the bacterium Pseudomonas syringae pv. tagetis (PST) develop "bleached tips." The bacterium produces a chemical called tagetitoxin that is a RNA polymerase III inhibitor that blocks the production of chloroplasts. Symptoms could be mistaken for exposure to a member of the photosynthesis inhibiting class of herbicide such as the triazines (e.g. atrazine) and nitriles (e.g. bromoxynil). Of course, the herbicides would tend to affect the entire plant whereas PST only affects the upper portions of infected plants and is described in the literature as "apical chlorosis."
PST infections will not only produce chlorotic stems and foliage, they will also reduce seedhead production and can occasionally cause plant mortality. The bacterium received a great deal of research attention in the early 2000s as a possible biocontrol agent for Canada thistle. Unfortunately, PST has defied being cultured in a laboratory; all testing thus far has been done using extracts from infected plants. Also, even though flower head production was reduced by as much as 87% in research trials, the thistle is such a prolific seed producer researchers concluded that PST would not be able to overcome re-seeding by surviving plants.