Location: Emerging Pests and Pathogens ResearchTitle: First report of rhizopus arrhizus (syn. R. oryzae) causing garlic bulb soft rot in the Hebei Province of China
|ZHANG, Y. N. - Chinese Academy Of Agricultural Sciences|
|WANG, J. Z. - Huaiyin Normal University|
|NIU, B. Y. - Chinese Academy Of Agricultural Sciences|
|XU, J. - Chinese Academy Of Agricultural Sciences|
|MA, X. - Cornell University - New York|
|GAO, M. - Chinese Academy Of Agricultural Sciences|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2022
Publication Date: N/A
Interpretive Summary: We identified Rhizopus arrhizus as causing disease on garlic bulbs in Hebei Province of China. This is the first time that this species has been observed to cause disease on garlic bulbs in China. The fungi were isolated from infected garlic bulbs using standard techniques. The isolated fungi were determined to be Rhizopus arrhizus using molecular methods. The isolated R. arrhizus fungi were determined to be responsible for the disease by demonstrating that they cause disease when inoculated onto healthy garlic and confirming that the R. arrhizus reisolated from these diseased garlic tissues were identical to these used in the test inoculation. This report expands the known range of plant species that R. arrhizus is able to infect and is the first to report the occurrence of this disease in China.
Technical Abstract: In November 2021, a disease outbreak in garlic bulbs suspected as Rhizopus soft rot occurred in Daming County, Handan City, Hebei Province of China (N 36°17', E 115° 13'). There were soft, water-soaked lesions on the surface of the diseased garlic bulbs, and the interiors were brown and soft. Infected garlic bulbs were sampled to isolate and determine the identity of the disease-causing organism. Symptomatic bulbs were surface sterilized and small pieces of the inner decayed tissue were removed and cultured. The colonies that formed were filamentous, started as white and then becoming brownish-gray to blackish-gray. The identity of the isolated fungi were determined by DNA sequence analysis of their internal transcribed spacer (ITS) region sequences. BLAST analysis of the ITS sequences indicated that all of the tested sequences were 100% identical to Rhizopus arrhizus (syn. Rhizopus oryzae). Phylogenetic trees were constructed using the ITS sequences provided additional evidence that these isolates were most likely Rhizopus arrhizus (syn. Rhizopus oryzae). We selected one isolated strain, DSF-0-2, to visualize its morphology and test its ability to cause garlic bulb soft rot. Under the microscope, nonseptate rhizoids, sporangia, and sporangiospores were observed. Sporangiospores were unequal, subglobose, numerous irregular, or oval, and 9.7 (6.2 to 12.5) × 6.5 (4.1 to 8.5) µm (n = 50) in diameter. The sporangia were globose, black, 121.5 (65 to 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 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 medium. 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. 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. To our knowledge, this is the first report of R. arrhizus (syn. Rhizopus oryzae) associated with soft rot on garlic in China.