ABSTRACT

Studies on the root rot disease of cocoyam (Xanthosoma sagittfolium (L.) Schott) caused by Pythium myriotylum Dresch, were carried out in the laboratory, greenhouse and experimental fields at Umudike, South-eastern Nigeria. Disease surveys conducted in three major cocoyam growing areas (Igbariam, Lowa and Umudike) in 2009 and 2010 cropping seasons showed percentage disease incidence and severity up to 47% and 3 respectively. Eight fungal isolates viz: Aspergillus niger, Batryodiplodia theobromae, Fusarium solani, Pythuim myriotylum, Rhizopus nigricans, Rhizoctonia solani, Scieratium rolfsii and Trichoderma harzianum were isolated either from infected cocoyam roots or the rhizosphere soils of infected cocoyam plants. Only P. myriolylum proved pathogenic on healthy cocoyam seedlings during pathogenicity test. Cocoyam cultivar NXs 003 was found to be resistant to infestation by P. myriolylum while cultivar NXs 001 gave the highest disease values of 69.0% and 5% (for disease incidence and severity respectively). Trichoderma harzianum one of the isolated rhizospheric fungi was found to inhibit the mycelial growth of P. myriolylum. Percentage mycelial growth inhibition of up to 66.50% was recorded at 100% concentration of the 20 day-old culture filtrate of T. harzianum, four days after inoculation. All the synthetic fungicides evaluated for their efficacy in mycelial growth inhibition in the laboratory were not significantly (p>0.05) different from each other. 'Percentage inhibition was up to 75% and above at concentrations of 5 — 25mg/i. Ethanol extractions and 40% concentration of crude hot water extracts of ginger (Zingiber officinale) rhizomes gave up to 90.33% and 85.03% inhibition of mycelial growth of P. myriotylum respectively. In the greenhouse, there were significant (P < 0,05) differences among the organic matter amendments in their disease incidences and seventies. The pig dung manure gave the lowest disease incidence and severity of 15% and 1.00 respectively while the least performing amendment was rice mill waste with disease incidence and severity of 52% and 2 respectively only slightly lower than the unamended control with 57% and 3. the assessment of effect of different fungicidal treatments showed that Ridomil gave the least disease incidence of 15% while Kocide gave least disease severity of 1.0 and highest plant height of 44cm. The order of performance was pig dung> cow dung> chicken manure> corn residue > rice mill waste > unamended control. In the field there were significant (p<O.05) differences among the synthetic fungicides evaluated in the plant height, percentage disease incidence, weight of corms and cormels per hectare at 15 and 28 weeks after inoculation. Ridomil gave the highest mean value of 0.191kg (1.3 13 t/ha) and 0.262kg (1,794 t/ha) for corm and cormel weight respectively. Results of the various disease management strategies adopted in this investigation against the pathogen (P. myriotylum) directly or the disease generally has given hope to the sustainable production of cocoyam in South-eastern Nigeria. KEYWORDS: Xanthosoma sagitqfolium, Pythium myriotylum, disease incidence, mycelial growth inhibition, botanicals, fungicides.