Strain identification and distribution of citrus Huanglongbing bacteria in Taiwan

摘要

The 48 representative Huanglongbing bacteria (HLBB) isolates were selected from 457 disease samples collected from Huanglongbing (HLB)-diseased citrus trees grown in seven main citrus-producing areas, including tropical and subtropical regions of Taiwan. After indexing and eliminating the citrus viruses, the selected HLBB isolates, free from the viruses, were used for identification of HLBB strains based on a pathogenicity and virulence test with the following differential citrus cultivars: Ponkan mandarin {Citrus reticulata Blanco), Liueheng sweet orange (C. sinensis Osb.), Wentan pummelo (C. grandis f. buntan Hay.), and Eureka lemon (C. Union Burm.). Four strains of HLBB were identified. Strain I showed pathogenicity on mandarin and sweet orange by inducing typical HLB symptoms. Strain II showed high virulence on all differential cultivars and multiplied fast in all cultivars. Strain III caused intermediate symptoms on mandarin and sweet orange and mild symptoms on pummelo, but did not infect Eureka lemon. Mild strain IV infected mandarin and sweet orange without causing symptoms and was rarely isolated. Strain II, which attacked all citrus cultivars grown in Taiwan, was found to dominate over the other strains in the field. Strains III and I were second in quantity. One-third (32.6%) of the diseased samples showing HLB-like symptoms in the field survey were found to be infected by HLB pathogen in PCR detection. Only 1.9% of healthy-looking citrus trees surveyed, including Wentan pummelo (5%) and Eureka lemon (5.7%), were infected by HLBB. The HLBB-isolates collected from mandarin, tangor, sweet orange, and kumquat (Fortunella margarita [Lour.] Swingle) were commonly co-infected with Citrus tristeza closterovirus (CTV) and/or Citrus tatter leaf capillovirus (CTLV). About 32% of the HLBB-infected trees examined were infected with HLBB only. Most HLB-affected mandarin (66.7%) and tangor (69%) trees were also infected by CTV while CTV was rarely detected in HLBB-infected pummelo (5%) or lemon (0%).