Interactions between an acute ozone dose, eastern cottonwood, and Marssonina leaf spot: implications for pathogen community dynamics

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Abstract
The effect of an acute ozone dose applied to eastern cottonwood (Populus deltoides Bartr.) on subsequent lesion production of the leaf spot fungus Marssonina brunnea (Ell. et Ev.) P. Magn. was investigated. Two cottonwood clones were exposed to 393 μg∙m−3 (0.20 ppm) ozone for 5 h and then inoculated with conidia of M. brunnea in leaf-disk and whole-plant assays on leaves of different ages (according to the leaf plastochron index (LPI)). There was no significant difference in lesion production on ozone-treated or control foliage, and significantly more lesions were produced on LPI 5 leaves compared with LPI 3, 7, and 9 leaves, irrespective of clone or treatment. These results show that infection of plants by facultative parasites will not necessarily increase when plants are exposed to air pollutants. In another study, urediospore production by an obligate parasite, the cottonwood leaf rust fungus (Melampsora medusae Thum.), was shown to decrease on cottonwood plants treated with an acute ozone dose in comparison with control plants. Thus, we hypothesize that ozone exposure of cottonwoods may have the potential to alter the relative destructive potential and abundance of members of the cottonwood pathogen community by differentially altering host susceptibility to different pathogens.

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