Minnesota Bacteria, Viruses,
and Other Pathogens

of Native Plants and Wild Animals


Pathogens: Unconventional grouping

This is an unconventional grouping of organisms that cause disease in native plants or wild animals. It includes bacteria, viruses, viroids, virus-like organisms, oomycetes, phytoplasmas, and and some parasites. It does not include protozoa, which cannot be identified in the field; nematodes, which cannot be identified without uprooting and thus killing the plant; or parasitic plants, insects, and fungi, which are covered elsewhere.




  apical chlorosis of Canada thistle (Pst)  


Recent Additions

Horsehair worms (Order Gordioidea)


Gordioidea is an order of parasitic horsehair worms. Larvae are parasites of insects, mostly grasshoppers, crickets, and katydids. They feed on and absorb nutrients from the gut of their host. It is thought that they influence the behavior of their host, bringing them near water when the adult is ready to emerge. Adults are free-living. They are found usually in freshwater habitats, sometimes in semi-aquatic habitats, or inside terrestrial hosts usually near water. They do not feed, but may absorb nutrients through their body walls.

  horsehair worm (Order Gordioidea)

Photo by Greg Watson

Adults are very long, hair-like worms. They are usually 12 to 16 long but some can grow up to 47 in length. The body color is purplish-brown to black in most species, tan in some species. There is a blunt head and a swollen tail, but there are otherwise no distinguishing features that can be seen in the field without magnification.

Apical chlorosis of Canada thistle (Pst)


Visitors to Minnesota’s natural places will occasionally come across a stand of Canada thistle with a few plants that are whitened at the top, appearing bleached. The discoloration is caused by the bacterium Pseudomonas syringae pv. tagetis. It has been called “White‐colour Disease of Canadian Thistle,” “apical chlorosis of Canada thistle,” and “Bacterial Speck”, but it has no widely-accepted common name. It is often referred to in scientific literature as Pst.

Researchers at the University of Minnesota conducted a study in 2002 to assess the viability of using Pst as a biological control agent for Canada thistle.

  apical chlorosis of Canada thistle (Pst)

Outside of a laboratory, a bacterium is recognized only by the symptoms it produces in its host. Pst produces the substance tagetitoxin, which blocks the production of chloroplasts, preventing photosynthesis. This results in whitened plant growth (chlorosis) on only the upper portion of the plant, stunted growth, fewer shoots, and inhibition of flowering. Pst infects plants in the Aster family, including Canada thistle, common dandelion, common sunflower, common ragweed, giant ragweed, Jerusalem artichoke, and some other plants not found in Minnesota.

Other Recent Additions







This list includes only pathogens that have been recorded in Minnesota, but not all of the pathogens found in Minnesota.


Profile Photo Video      
Profile Photo Video  

apical chlorosis of Canada thistle [Pst] (Pseudomonas syringae pv. tagetis)


horsehair worm (Order Gordioidea)


aster yellows [AYP] (Candidatus Phytoplasma asteris)


cottontail rabbit papillomavirus [CRPV] (Kappapapillomavirus 2)


fire blight (Erwinia amylovora)

Profile Photo Video  

horsehair worms (Order Gordioidea)


shot hole disease (Wilsonomyces carpophilus)

Candidatus Phytoplasma asteris (aster yellows [AYP])

Erwinia amylovora (fire blight)

Gordioidea (horsehair worms)

Kappapapillomavirus 2 (cottontail rabbit papillomavirus [CRPV])

Pseudomonas syringae pv. tagetis (apical chlorosis of Canada thistle [Pst])

Wilsonomyces carpophilus (shot hole disease)

No Species Page Yet?

If you do not see a linked page for an insect in the list at left, or the insect does not appear in the list, you can still upload a photo or video or report a sighting for that insect. Click on one of the buttons below and type in the common name and/or scientific name of the insect in your photo, video, or sighting. A new page will be created for that insect featuring your contribution.

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