Use of thermotherapy and tissue culture to free diseased cassava cultivars from cassava mosaic disease.

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Four (4) field-infected local cassava (Manihot esculent Crantz) cultivars “Afisiafi”, “Amakuma”, “UCC-Bankye” and “Esi-Abaya”, showing differential symptoms of African cassava mosaic virus (ACMV) disease on leaves were assessed for index of severity of symptoms (ISS) for the presence of the disease on shoots to determine their infection status. Different sizes of meristem explants ( 0.2-1.0mm, 1.0-2.0mm and 2.0-3.0mm) were excised from developing shoots of each of the four accessions grown at 37°C for four weeks and cultured on modified Murashige and Skoog ( 1962) basal medium supplemented with 100mgl-1 Inositol, 30g of sucrose and 3.5g of phytagel. DNA extracts of leaves of emergent shoots from heat-treated plants were also tested for presence of ACMV by polymerase chain reaction (PCR) using specific primers designed previously to detect ACMV. Mosaic disease incidence and severity of symptoms were highest (100%) on leaves of plants from “Esi-Abaya” and “Amakuma”, while “Afisiafi” and “UCC-Bankye” showed comparatively less disease incidence of 85 and 80% respectively. Similarly, the index of severity of symptoms on all plants (ISSAP) and diseased plants only (ISSDP) were significantly higher (3.30) on leaves of “Esi-Abaya” and least (2.0) on the leaves of “UCC-Bankye”. The leaves of “Amakuma” obtained from cuttings without heat treatment showed mild symptoms of the disease when assessed by index for severity of symptom (ISS). In contrast, leaves from thermotherapy treated shoots were all free of the symptoms of the disease on all the four cultivars. The medium and large explant sizes (1.0-2.0mm and 2.0-3.0mm) showed the highest survival rate, developing into virus-free plantlets in all the cultivars studied. Also, PCR analysis did not reveal any ACMV symptom in leaves of thermotherapy treated shoots regenerated in vitro from the different explant sizes used, while in vivo shoots without thermotherapy showed presence of the virus by amplification of the 3 primer pairs used for the evaluation.
A thesis submitted in partial fulfilment of the requirements for the award of Master of Science degree in Agronomy (plant breeding).