First Report of Rhizopus arrhizus (syn. R. oryzae) Causing Garlic Bulb Soft Rot in Hebei Province, China.

Abstract

Rhizopus soft rot occurs on the succulent tissues of vegetables, fruits, and ornamental plants throughout the world (Cui et al. 2019). When the garlic is in the seedling stage in the fields (Fig. S1) in November 2021, a disease outbreak on garlic bulbs suspected as Rhizopus soft rot occurred in Daming County, Handan City, Hebei Province of China (N 36°17', E 115° 13'). This disease symptom was first found in the garlic seedling stage in China. Disease incidence was 10% to 30% in cultivated garlic bulbs. There were soft water-soaked lesions on the surface of diseased garlic bulbs and the interiors were brown and soft. In the disease severe field, white to gray mycelia were observed on the diseased garlic bulbs. Infected garlic bulbs were sampled to isolate and determine the identity of the disease-causing organism. Symptomatic bulbs were surface sterilized with 1% NaClO for 2 min, dipped in 75% ethanol for 3 min and rinsed three times with autoclaved distilled water. Small pieces of the inner decayed tissue were removed and cultured on potato dextrose agar (PDA) at 28°C for 2 to 3 days. Five white colonies grew on PDA and then they became brownish gray to blackish-gray mycelium. The fungal strains were purified by hyphal-tip isolation method. To determine the identity of the five isolated fungi, we analyzed their internal transcribed spacer (ITS) region sequences (Jung et al. 2012). BLAST analysis of the ITS sequences from DSF-0-2 (accession no. ON706022), DSF-0-3 (accession no. ON706021), DSF-0-4 (accession no. ON706020), DSF-0-5 (accession no. ON706019) and DSF-0-6 (accession no. ON706018) were all 100% identical with Rhizopus arrhizus (syn. Rhizopus oryzae). Phylogenetic trees were constructed using the neighbor-joining method of MEGA11 based on the sequences of ITS rRNA gene (Walther et al. 2013). Phylogenetic trees indicated that isolates were most likely Rhizopus arrhizus (syn. Rhizopus oryzae) (Fig. S2). We selected one isolated strain, DSF-0-2, for characterize the morphology and test its ability to cause garlic bulb soft rot. Under the microscope, nonseptate rhizoids, sporangia, and sporangiospores were observed (Fig. S1). Sporangiospores were unequal, subglobose, numerous irregular, or oval, and 9.7 (6.2 - 12.5) × 6.5 (4.1 - 8.5) μm (n = 50) in diameter. The sporangia were globose, black, 121.5 (65 - 198) μm (n = 50) in diameter. Based on the rDNA-ITS sequencing and the morphological characteristics, the DSF-0-2 isolate was identified as Rhizopus arrhizus (syn. Rhizopus oryzae) (Zheng et al. 2007; Abeywickrama et al. 2020). To complete Koch's postulates, surface-sterilized healthy garlic bulbs were inoculated with R. arrhizus isolate DSF-0-2. A 1.0-ml sterile syringe was used to inject 50 μl of a 106 conidia/ml suspension into each of five healthy bulbs. As a control, garlic bulbs were treated with sterile distilled water. The inoculated and control bulbs were incubated at 28°C for 7 days. The bulbs inoculated with R. arrhizus DSF-0-2 showed symptoms of water soaking, and the tissues were brown and soft throughout the bulb at 7 days (Fig. S1). Results of the three trials were the same. No symptoms were observed in the control group. R. arrhizus was reisolated from the symptomatic garlic bulb and confirmed as such based-on colony and sporangia morphology and ITS sequence. There were some reports that R. arrhizus infects cassava tubers and potato tubers (Amadioha and Markson 2007; Cui et al. 2019). To our knowledge, this is the first report of R. arrhizus (syn. Rhizopus oryzae) associated with soft rot on garlic bulb in the seedling stage in China. This disease has posed a potential threat during garlic seedling stage in the field. Management measures should be considered before this disease outbreaks widely. Garlic bulbs died in the seedling stage, which caused production reduction, serious economic loss and soil pollution. This finding may help to take effective control measures for this disease.