Characterisation and pathogenicity of Phytophthora colocasiae on taro

Abstract

In 1993, taro leaf blight (TLB) disease decimated Samoa’s entire taro (Colocasia esculenta) crop within 6 months, halting all international export. TLB disease is caused by the fungus-like oomycete Phytophthora colocasiae. This oomycete is commonly spread in windy and wet conditions and causes necrotic lesions on taro leaves and causes corm rot post-harvest. Using 40 P. colocasiae isolates this project aimed to 1) establish the genetic differences between P. colocasiae isolates and their phylogenetic relationships, 2) determine the differences in pathogenicity between P. colocasiae isolates on taro leaves and corms, and 3) determine whether corms of tolerant taro varieties will become infected by P. colocasiae. P. colocasiae were characterised using elongation factor 1-alpha (EF1a) sequencing in the ambition to determine their intraspecific genetic variability; EF1a from only one isolate (Pc 50) was achieved and this showed remarkable variability indicative of either an impure isolate or a heterokaryon. An existing in vitro leaf disc assay was used to test the pathogenicity of the different isolates of P. colocasiae on three cultivars of taro whereby their growth rates were measured compared based on the diameter of the necrotic lesion. A range of pathogenicity was observed in the lesion diameter for the 27 P. colocasiae isolates. An inoculation assay was developed to infect corms in vitro with P. colocasiae isolates. This new assay was then used to test the pathogenicity of the P. colocasiae isolates on three taro cultivars (Fijian white, Fijian pink and Samoan pink). Infection of corms was assessed by destructive examination and re-isolation to determine the presence of infectious P. colocasiae and to prove Koch’s postulates. This novel assay demonstrated the infection variability from 16 P. colocasiae isolates on three taro cultivars. A weak correlation exists between the pathogenicity of P. colocasiae isolates on leaves and corms of Samoan cultivars. Further research that addressed a variety of taro cultivars may strengthened this correlation. The overarching aim, to address the relationship between genotype and pathogenicity of P. colocasiae isolates, was not resolved. Further investigation using pure isolates derived from hyphal tip or single spore isolation will provide more robust information about this relationship. In addition, the new in vitro corm assay may provide a basis for investigating tolerance of new varieties and the biosecurity risks of exported taro crops. Keywords: Phytophthora colocasiae, Colocasia esculenta, Taro Leaf Blight (TLB), elongation factor 1-alpha (EF1a)