LAUSR

Geranium (Pelargonium hortorum) is a widely cultivated ornamental plant in the world. In August 2021, geranium plants exhibited stem and root rot symptoms in a garden in Tianjin, China (117°20'E, 39°13'N) with an approximate incidence of 10%. Symptoms were rotted stem and root, chlorotic leaves, wilting, discolored vascular tissues and eventually plant death. White mycelia were observed growing on the surface of the infected stems and roots in a high humidity environment. Symptomatic plants were collected from three nurseries (20 plants per nursery) in the Tianjin garden, out of which 10 plants were arbitrarily selected for pathogen isolation. The diseased stem and root were rinsed under tap water, blotted dry, and excised into 3 mm2 pieces (three pieces per plant) at the boundary of the symptomatic area. The pieces were surface disinfected with 75% ethanol for 30 s, washed with sterile distilled water for three times, and then placed on potato dextrose agar (PDA) containing 50 mg/L of streptomycin sulfate and incubated at 26℃ in the dark. After 6 days, 24 of 30 isolates generated white to pale pink aerial hyphae on PDA. Macroconidia were slightly curved at apex, hyaline and falcate, 3 to 5 septate, 19.2 to 41.08 μm in length and 2.95 to 5.62 μm in width (n = 30). Microconidia were single-celled, hyaline, non-septate and ovoid, and measured 4.98 to 16.48 μm in length and 1.58 to 4.69 μm in width (n = 30). These characteristics matched the description of Fusarium oxysporum (Leslie and Summerell 2006). The ITS gene, TEF1-α gene, mtSSU gene and RPB2 gene of representative isolates (T7, T8 and T9) were amplified and sequenced for species identification (Carbone and Kohn 1999; Li et al. 1994; Miller and Huhndorf 2005). BLASTn analyses of the ITS amplicon (OK326801, OK326789 and OK326823), TEF1-α amplicon (OM974708, OM974710 and OM974711), mtSSU amplicon (OK330481, OM948931 and OM948932) and RPB2 amplicon (OM974709, OM974712 and OP331332) obtained with cognate sequences available in GenBank showed 99.31% to 100% identity to sequences of F. oxysporum in the NCBI database and the Fusarium MLST database. The multilocus phylogenetic analysis from the Fusarium MLST database performed by UPGMA showed that the representative isolates were assigned to the group of F. oxysporum. Ten two-month-old healthy plants of geranium (cv. Horizon Scarle) were inoculated by dipping the roots into a conidial suspension of 1×107 conidia/mL for 10 min (Monther et al. 2021). Another ten plants dipped in sterile distilled water served as controls. After inoculated or noinoculated treatments, plants were planted in sterilized nutrient soil in plastic pots (one plant per pot). Each treatment was repeated three times in a greenhouse at 28℃ and 75% relative humidity. Two weeks later, inoculated plants exhibited similar symptoms to those of diseased plants in the garden. Whereas the control plants remained healthy. To fulfill the Koch's postulates, F. oxysporum was reisolated from each inoculated, symptomatic plant, but was not recovered from any of the control plants. The experiments were repeated twice with similar results. F. oxysporum is reported to cause disease on Pelargonium graveolens in Kenya (Nattrass 1961) and on Pelargonium sp. in California (French 1989), to our knowledge, this is the first report of F. oxysporum causing stem and root rot on Pelargonium hortorum in China. Identification of disease caused by F. oxysporum on geranium will guide disease management.