From my first link, which cites many peer-reviewed studies, not the bilge produced by Singer and Idso:
“…It has also been found that excess CO2 can make certain agricultural plants less nutritious for human and animal consumption…”
“…Increased CO2 has been shown to lead to lower production of certain chemical defense mechanisms in soybeans, making them more vulnerable to pest attack and diseases (Zavala et al. 2008 and Eastburn et al. 2010)…”
“…There is also some evidence suggesting that invasive species and many “weeds” may show relatively higher responses to elevated CO2 (Ziska and George 2004), and become more resistant to conventional herbicides (Ziska et al. 2004, Ziska and Teasdale 2000)…”