Holey Bleached-Out Canada Thistle

Published on

Canada thistle (Cirsium arvense) in southwest Ohio is starting to show symptoms of two “problems” … a relative term!  Infections by the bacterium Pseudomonas syringae pv. tagetis are producing the most obvious symptoms causing plants to look like they were dipped in bleach.  The second problem may require an up-close inspection with the annual holey-handiwork of the Thistle Tortoise Beetle (Cassida rubiginosa) beginning to produce see-through leaves.



A Beetle Blast

The beetles are pale green or yellowish-green, which allows them to blend with their host's leaves.  Like other tortoise beetles, the adults have a body shaped like a flattened pith helmet.  The head and legs of the adults are typically hidden under the flares of their helmet-like body.  The antennae can be hidden or extended out from underneath the front of the beetle.


Thistle Tortoise Beetles


The oval-shaped larvae are grayish-green and have a ring of spines arranged crown-like around the edge of their bodies.  The larvae sport a pair of spike-like appendages (cerci) at the tip of their abdomen that they use to spear and carry an odious collection of feces and shed exoskeletons in an umbrella-like fashion over their bodies; presumably for protection against predators.


Thistle Tortoise Beetle Larva


Both the adults and larvae consume the upper or lower leaf epidermis and leaf mesophyll leaving behind the epidermis on the opposite side of their feeding site.  Feeding damage first appears as irregularly shaped depressions with one leaf epidermis still intact producing what looks like a windowpane.  Eventually, the intact epidermis dries and drops from the leaf to produce holes.


Thistle Tortoise Beetle Damage


Thistle Tortoise Beetle Damage


This weed-whacking beetle was intentionally imported from Europe and northern Asia into North America as a biological control for thistle.  The beetle is also known as the "thistle defoliating beetle" and it feeds on other non-native thistle-nasties including musk (Carduus nutans) and plumeless (C. acanthoides) thistles.  This is a very good non-native beetle!



Bacterial Bleached Tips

Canada thistle plants infected with the bacterium Pseudomonas syringae pv. tagetis (PST) develop "bleached tips."  The bacterium produces a chemical called tagetitoxin, which 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."


Canada thistle infected with a bacterium


Canada thistle infected with a bacterium


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, Canada thistle is such a prolific seed producer researchers concluded that PST would not be able to overcome re-seeding by surviving plants.