The influence of six commercially available fungicides incorporated into a water-retaining polymer and applied to the root system of horsechestnut (Aesculus hippocastanum L.) as a dip at the time of planting was conducted. Potential increases in resistance against the foliar pathogen Guignardia leaf blotch (Guignardia aesculi) was then monitored over two growing seasons. Trials were conducted in 2007 and duplicated in 2008. A comparative evaluation of the fungicide penconazole commercially used for Guignardia leaf blotch control was studied by spraying trees at the manufacturer’s recommended rate of four times during the first growing season but none in the second. None of the treated or control trees died as a result of Guignardia leaf blotch attack during the course of the study and none of the fungicide and water-retaining polymer combinations evaluated was phytotoxic to the test trees. Efficacy as Guignardia leaf blotch protectant compounds over the first growing season was demonstrated when fungicides were incorporated into a water-retaining polymer. Reductions in Guignardia leaf blotch severity were mirrored by increases in leaf chlorophyll fluorescence as a measure of leaf photosynthetic activity and leaf chlorophyll content SPAD values. There were little differences in the magnitude of control efficacy between the fungicides evaluated. Limited efficacy of any of the fungicide and water-retaining polymer combinations as Guignardia leaf blotch protectant compounds was, however, demonstrated the following year after application indicating a fungicide and water-retaining polymer root dip provided one growing season protection only. Application of a water-retaining polymer alone had no effect on reducing Guignardia leaf blotch severity. Based on visual Guignardia leaf blotch severity ratings, greatest protection in both the 2007 and 2008 trial was provided by the synthetic fungicide penconazole applied as a foliar spray four times during the growing season. No efficacy of penconazole foliar sprays as leaf blotch protectant compounds was demonstrated the following year, indicating annual sprays against Guignardia leaf blotch are required for control.