Resistance of Red Maple and Freeman Maple Cultivars to Phytopythium vexans, a Causal Agent of Root and Crown Rot Disease.

Phytopythium root rot caused by Phytopythium vexans is an emerging threat to red maple and Freeman maple production that seriously impacts plant growth, aesthetic, and economic values. This study reports on the resistance of red maple and Freeman maple cultivars against root rot disease caused by P. vexans. Rooted cuttings were received from a commercial nursery and planted in three-gallon containers. For each cultivar, six plants were inoculated by drenching 300 mL/plant P. vexans suspension, prepared by blending 2 plates of ten-days-old P. vexans culture/L sterile water. An equal number of plants remained non-inoculated and were drenched with 300 mL of sterile water. Two trials were conducted for four months in the greenhouse during the summer of 2023. Plants were evaluated for growth, physiology, Phytopythium root rot severity (0% to 100% roots affected), and pathogen reisolation frequency. Out of seven cultivars, Somerset had the lowest Phytopythium root rot severity and pathogen reisolation frequency. Cultivars Autumn Blaze, Brandywine, and October Glory were highly susceptible to P. vexans, whereas Sun Valley, Summer Red and Celebration were found to have a partial resistance to P. vexans. Cultivars Autumn Blaze, Brandywine, and October Glory had significantly lower chlorophyll content, net photosynthesis, and stomatal conductance compared with the other three cultivars under pathogen inoculation. Phytopythium root rot reduced plant height, width, total plant, and root fresh weight. The disease severity was negatively correlated with width, chlorophyll content, net photosynthesis, and stomatal conductance. These results can aid growers and landscapers in developing effective P. vexans management strategies.