Paper Reviewed
Gilardi, G., Gisi, U., Garibaldi, A. and Gullino, M.L. 2017. Effect of elevated atmospheric CO2 and temperature on the chemical and biological control of powdery mildew of zucchini and the Phoma leaf spot of leaf beet. European Journal of Plant Pathology 148: 229-236.

Agricultural producers suffer significant losses every year due to foliar diseases that infect their crops. To combat such losses they apply fungicides and biological control agents to limit the damage. In the future, however, it is not known how changes in climate and/or atmospheric CO2 concentrations will impact the incidence and severity of plant diseases; they could be exacerbated, diminished or remain about the same.

The latest scientific paper to provide some insight in to this topic comes from Gilardi et al. (2017), a four-member Italian research team that investigated the interactive effects of CO2 and temperature on foliar diseases of two important food crops, zucchini (cv. Genovese) and leaf beet (Beta vulgaris subsp. Vulgaris L.). To achieve their objective the scientists inoculated zucchini and leaf beet plants with the powdery mildew (Podosphaera xanthii) and phoma leaf spot (Phoma betae) pathogen, respectively, in controlled environment chambers under six different combinations of temperature and CO2: (1) 400-450 ppm CO2, 18-22°C, (2) 800-850 ppm CO2, 18-22°C, (3) 400-450 ppm CO2, 22-26°C, (4) 800-850 ppm CO2, 22-26°C, (5) 400-450 ppm CO2, 26-30°C, (6) 800-850 ppm CO2, 26-30°C. After inoculation the scientists applied two chemicals (penconazole and sulphur) and one biocontrol agent (Ampelomyces quisqualis) to the zucchini plants and two other fungicides (Mancozeb and azoxystrobin) to the leaf beet plants. Then, they measured the incidence and severity of disease on the two plant species over a period of days and weeks to evaluate the single and combined effects of CO2 and temperature on those two parameters.

And what did their measurements reveal?

With respect to zucchini, Gilardi et al. report that elevated CO2 "significantly improved the powdery mildew control provided by A. quisqualis, by 8.6% and 16.1% for disease incidence and disease severity" (meaning it reduced pathogen damage); but it had little impact on the efficacy of penconazole and sulphur chemical treatments. Temperature alone was found not to be a significant factor of influence on either disease incidence or severity; but when combined with elevated CO2, pathogen incidence and severity declined by 23.3 and 22.8 percent, respectively, when temperatures were increased from the 22-26°C to the 26-30°C treatment. Similar results were reported for leaf beet, with the authors reporting that the effectiveness of both fungicides was significantly improved at 22-26°C for higher CO2 levels (a 15.3% decline in disease incidence and a 20.2% decline in disease severity for mancozeb and a 20.6% decline in disease incidence and a 9.6% decline in disease severity for azoxystrobin).

In commenting on their findings, Gilardi et al. say their work suggests that "the activity of A. quisqualis against zucchini powdery mildew is generally improved in a climate change scenario [of elevated CO2 and elevated temperature]," as is the effectiveness of the two fungicides against Phoma leaf spot disease, all of which findings bode well for the future of zucchini and leaf beet in battling the negative impacts of these two pathogen-induced diseases.