Elevated CO2 Effects on Ragweed Ecotypes: Growth and Fungal Associations.

Herbicide resistant weeds exit due to repeated herbicide application. Elevated CO₂ has positive effects on plant growth including weeds; however, effects on herbicide resistant weeds is unknown. Glyphosate resistant and susceptible ragweed (Ambrosia artemisiifolia L.) ecotypes were exposed to ambient or elevated (ambient + 200 ppm) CO₂ for 8 weeks at which time they exhibited disease symptoms. Elevated CO₂ resulted in greater top, root, and total plant biomass. Herbicide resistant plants also had greater top, root, and total biomass than susceptible plants. There were no significant CO₂ by ecotype interactions. Fungi from 13 genera were associated with ragweed; several of which can be either pathogens (i.e., Alternaria, Fusarium, Rhizoctonia) or saprophytes existing on dead plant tissues. Powdery mildew was higher on susceptible compared with resistant ragweed. Susceptible plants also had an increased frequency of Rhizoctonia on leaves and Alternaria on stems; however, Fuarium was more frequent on resistant ragweed leaves. Fungi were not affected by CO₂ level or its interaction with ecotype. This is the first study to report effects of herbicide resistance and elevated CO₂ on fungi associated with weeds.