In November 2009, many farmers in Abia State were alarmed by complete destruction of their taro (Colocasia esculenta (L.) Schott.) crop. Symptoms, suggestive of leaf blight caused by Phytophthora colocasiae Raciborski (2), began as small, brown, water-soaked lesions that rapidly enlarged to form large, dark brown, coalescing lesions, sometimes with orange host exudations. White sporulation was evident on the lesion surface under wet conditions. The pathogen caused rapid defoliation and killed plants. The epidemic was widespread in 2010 during the rainy season (April to November) in all taro-growing areas of Nigeria. Diseased leaves were collected from taro in Iwo Village near Ibadan, cut into 4-cm2 pieces, washed in several changes of sterile water, and incubated in petri dishes lined with wet filter paper at 22°C. Newly produced sporangia were collected from the incubated leaves and plated on a selective medium (1). Sporangia were hyaline, papillate, and measured 25 to 55 × 15 to 30 μm. Zoospores encysted within 30 min after release; cysts were 9.7 to 19.5 μm in diameter. Sporangia and zoospore formation were induced in water and by chilling, respectively (1). Two leaves each of three 1-month-old taro and three Xanthosoma plants (both unknown clones) and six detached leaves of taro were inoculated with a 1 × 105/ml zoospore suspension of isolates PC01 and PC02. Detached leaves were incubated in moist chambers at 22°C. Plants were covered with polyethylene bags for 12 h after inoculation and maintained in a screenhouse. Water-soaked lesions appeared on detached leaves within 24 h postinoculation and the leaves were completely rotted 48 h later. All inoculated attached leaves of taro, but not Xanthosoma, showed typical leaf blight symptoms including abundant sporangial production. Noninoculated control detached leaves and plants were disease free. Sporangia from detached and attached inoculated leaves, when plated on selective medium, produced typical P. colocasiae colonies. The internal transcribed spacer (ITS) region of rDNA was amplified using the ITS1 and ITS4 primers (3). Amplicons (786 bp) were sequenced in both directions and submitted to GenBank (Accession Nos. HQ602756, HQ602757, HQ602758, and HQ602759). A BLASTn search revealed 99% similarity to a P. colocasiae strain of the Pacific Region (Accession No. GU111604), but only 94% similarity to a P. colocasiae strain from India (Accession No. GQ202149). The sequence analysis, morphological characteristics, and pathogenicity test confirmed the taro leaf blight pathogen as P. colocasiae. There are previous reports of occurrence of taro blight-like disease attributed to P. colocasiae in Ethiopia, Equatorial Guinea (1), and more recently in Cameroon, but comprehensive details on pathogen or disease are not available. To our knowledge, this is the first confirmed record in Nigeria of P. colocasiae causing taro blight. This disease poses a serious threat to the production and biodiversity of this important food crop. Urgent interventions are necessary to halt this emerging epidemic in West and Central Africa.
References: (1) Phytophthora colocasiae, In: CABI-Crop Protection Compendium. CAB International, Wallingford, UK, 2005. (2) P. S. Tsao. Page 219 in: Phytophthora: Its Biology, Taxonomy, Ecology and Pathology. The American Phytopathological Society. St. Paul, MN, 1983. (3) T. J. White et al. Page 315 in: PCR Protocol: A Guide to Methods and Applications. Academic Press, London. 1990.