Leaf blight and corm rot of taro (Colocasia esculenta (L.) Schott) caused by Phytophthora colocasiae Raciborski is the most disparaging disease of taro inducing severe losses in yield and quality of taro corms. Investigations were carried out in Umudike in 2016 and 2017 to determine the pathotypes present in that locality with a view to screening them on available taro genotypes for resistant cultivars. Symptomatic taro plant samples were collected from Taro Project Farm of the National Root Crops Research Institute, Umudike. Leaf tissue sections were excised from the periphery of lesions on symptomatic leaf, surface sterilized in 10% Sodium Hypochlorite solution for 2 minutes, rinsed in three changes of sterile distilled water and plated on Water Agar (WA). Mycelia emerging from diseased tissues after incubation were aseptically transferred to Corn Meal Agar (CMA). A total of 15 fungal isolates under 6 groups of species were obtained and these isolates were transferred to Carrot Agar (CA) from time to time until pure cultures were obtained. Based on varied cultural patterns, morphological characteristics and microscopic examination, the isolates were identified as Phytophthora sp, Fusarium sp FSSC, Fusarium solani, F. oxysporum and F. equiseti. Pathogenicity test confirmed that only Phytophthora sp induced leaf blight on detached leaf of taro plant. Molecular characterization of the isolates via Internal Transcribed Spacer (ITS) sequence analyses confirmed that the already implicated Phytophthora species was Phytophthora sansomeana. This is apparently the first report of P. sansomeana as a taro leaf blight pathogen in addition to the already known P. colocasisae.
Leaf-blight disease of Colocasia caused by Phytophthora colocasiae Raciborski has been a serious impediment to cocoyam production in Nigeria. Disease severity and susceptibility of the two most cultivated local varieties “Ugwuta” (Colocasia esculenta var. antiquorum) and “Nkashi Colocasia esculenta var. esculenta) were investigated. Disease severity was visually estimated as the percentage leaf surface affected by blight, lesion or lesion-related chlorosis for each leaf of a plant using a seven-point scale of 0, 5, 10, 25, 50, 75 and 100% in three locations: Ede-Oballa, Nsukka Urban and Obukpa. Susceptibility was assessed on 2 months old potted plants of each variety inoculated with a 7-day old culture of P. colocasiae. Diameters of lesions on inoculated leaves were recorded from the 3rd - 8th day after inoculation. Data on severity were subjected to ANOVA and susceptibility of the varieties was compared with t-test. Results revealed significant LSD=4.96 (0.05) and varying degrees of leaf blight severity among varieties and locations. Variety antiquorum had significantly higher severities of 42.08, 46.40 and 47.42% at Ede-Oballa, Nsukka Urban and Obukpa respectively, compared to 34.85, 36.55 and 28.19% recorded by var. esculenta at these locations, respectively. Similarly, var. antiquorum had greater lesion diameter ranging from 0.65±0.07 cm - 3.70±0.14 cm and average diameter of 2.4±0.16cm compared to var. esculenta which had 0.41±0.14cm - 3.12±0.19 cm and average diameter of 1.80±0.16. This research has shown that varieties and locations affect the severity and susceptibility of Phytophthora leaf blight disease. This could be a guide to farmers having known that var. esculenta is less severe to Phytophthora leaf blight disease.
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