Evaluation of Hydrangea Cultivars for Tolerance Against Root Rot Caused by Fusarium oxysporum

Abstract

Root rot caused by Fusarium oxysporum Schltdl. is a newly identified disease in oakleaf hydrangea. Some cultivars such as Pee Wee and Queen of Hearts grown in pot-in-pot container system showed root rot symptoms after late spring frost in May 2018 with 40% and 60% incidence in the infected nursery, respectively. This experiment was carried out to evaluate the tolerance among different hydrangea cultivars against root rot caused by F. oxysporum. Fifteen hydrangea cultivars from four different species were selected and rooted cuttings were prepared from new spring flushes. Twelve plants from each cultivar were transplanted in a one-gallon pot. Half of transplanted plants (6 - single plants) were inoculated by drenching 150 mL conidial suspension of F. oxysporum maintaining the concentration of 1×106 conidia/mL. Half of the plants remain non-inoculated (control) and were drenched with sterile water. After four months, root rot was assessed using a scale of 0-100% root area affected and recovery of F. oxysporum was recorded by plating 1 cm root in Fusarium selective medium. Fusaric acid (FA) and mannitol were extracted from the roots of inoculated and non-inoculated plants to see the effect and role on pathogenesis. Further, mannitol concentration was analyzed using absorption wavelength in spectrophotometer and FA was analyzed using high-performance liquid chromatography (HPLC). Results indicated that no cultivars were resistant to F. oxysporum. Cultivars from Hydrangea arborescens, H. macrophylla and H. paniculata were more tolerant to F. oxysporum compared to cultivars from H. quercifolia. Among H. quercifolia, cultivars Snowflake, John Wayne and Alice were more tolerant to F. oxysporum.