Montana Field Guide

Canada Thistle Rust (Puccinia punctiformis) is a fungus, specifically a type of rust in the Family Pucciniaceae.

Canada Thistle Rust infects the root system of Canada Thistle (Cirsium arvense), a noxious and non-native plant, and kills its leaves (Montana Biological Weed Control Coordination Project). The rust produces four types of spores (Winston et al. 2012). The urediospores are visible in spring, and can be found on the thistle's stems and upper surfaces of leaves. The urediospores are dense golden-orange pustules. In the fall, brown rust colored teliospores are produced in preparation for winter.

HOST PLANT
Canada Thistle Rust attacks Canada Thistle plants (Winston et al. 2012).

DAMAGE TO PLANTS [Adapted from Winston et al. 2012; Field Guide for Biological Control of Weeds in Montana]
Spores of Canada Thistle Rust germinate on leaves and the fungus travels through the stem to infect the root system. Infected roots cannot effectively draw nutrients from the soil, which results in weakened plants. The rust can stunt the growth of young Canada Thistle plants, but it does not provide significant population control.

PLANT-ANIMAL INTERACTIONS
Some thistle biological control insects can disperse Canada Thistle Rust (Winston et al. 2012).

LIFE CYCLE [Adapted from Winston et al. 2012]
Canada Thistle Rust produces many generations per year. The rust produces four types of spores: teliospores, basidiospores, aeciospores, and urediospores. When the host plant dies in autumn, the fungus produces dark brown teliospores on the understide of leaves, which overwinter. In the spring, volatile chemicals in the roots of Canada Thistle stimulate the teliospores to germinate and produce basidiospores. Next, urediospores (spermagonia) develop postules on the underside of leaves which produce a scented, sweet, sticky nectar that attracts flies. The flies disperse spores of opposite mating types. These create the aeciospores which become chalky, rust brown pustules. The wind and movements by animals blows the spores to uninfected plants. The cycle repeats itself.

BIOLOGICAL CONTROL [Adapted from Winston et al. 2012; NRCS 2019)
A variety of biological control species (insects, fungi, etc.) have been brought into North America to control non-native thistles. A fair amount of preliminary work and an array of factors must be considered when developing a biocontrol plan for controlling non-native thistles. Readers are encouraged to consult the "Useful Links" and cited literature in this profile.

Canada Thistle Rust was accidentally introduced into the USA and Canada. It is still being researched as a potential future Canada thistle biological control agent and is not fully approved for re-distribution within the USA and Canada. Since 2017 it has been intentionally moved to uninfected Canada Thistle infestations and is being monitored.

Useful Links:
Montana Biological Weed Control Coordination Project
Field Guide for Biological Control of Weeds in Montana