PROSPECTIVE OF TRANSGENIC CASSAVA LINES RESISTANCE IN EFFECTIVE MANAGEMENT OF CMV AND CBSV CAUSED VIRAL DISEASES OF Manihot esculenta Crantz IN KENYA

Abstract

Cassava (Manihot esculenta Crantz) is an essential food crop in Kenya. Despite its importance, the national average yield of 3 tonnes/hectare is among the lowest globally, primarily due to the lack of high-quality and disease-free planting materials. The most devastating viral diseases in Sub-Saharan Africa are; Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD). The viruses are spread by whiteflies and infected planting material. Recent studies have revealed that CMD and CBSD cause up to 100% yield losses. Proposed strategies for minimizing these loses include planting resistant varieties and large-scale surveillance. This study sought to establish the sources of cassava seed among farmers, type of viruses and associated cassava disease levels in selected counties of Kenya. Further, the study evaluated the diversity of viruses, viral load, resistance and yielding efficacy levels of 27 transgenic cassava lines, engineered towards management of CMD and CBSD diseases of cassava under a confined field trial at Alupe, Busia County. A diagnostic survey was carried out in Cassava growing counties of Kenya, followed by a randomized complete block in design experiment that evaluated the transgenic lines under a confined environment. Real time Quantitative PCR, Sequencing, modelling, TAS and DAS ELISA techniques were used to answer the objectives. To establish an infection under the confined trial; leafy and hardwood cassava stem cuttings which were CMD and CBSD-infected from growing counties were used as infectors in the confined experiment at Alupe-Busia. The diagnostic survey results showed that all the sampled counties had CMD and CBSD diseases. Out of 240 samples that had CMD symptoms at the field; 132 (55%) tested positive for CMVs, 29(12%) tested positive for CBSVs, whereas 79(32.9%) were negative of the two diseases. For the 207 samples that had CBSD symptoms; 108 (52.2%) tested positive for CBSVs, 7(3.4%) tested positive for CMVs and 92 (44.4%) were negative. The CMD incidence levels ranged from 27.6% at Kisii County to 60.34% in Busia County. Whereas, for CBSD it ranged from 26.02% in Kilifi County to 38.74% in Busia County. For phylogeny analysis, two isolates from Kenya (BUS-STR and KAK 16) clustered in Group I, while the others clustered in Group II. The Kenyan novel ACMV isolates from Busia and Kakamega were more similar to Ugandan isolates, suggesting common ancestry.CBSD resistant transgenic lines; 19, 22, 56, 398, 402, 404, 497, 498 and 500 exhibited symptoms of CBSD and CMD within the first four months after planting with varying incidence levels. For CMD resistant transgenic lines; 74, 115, 141, 145 and 157 showed CMD and CBSD symptoms within first four months after planting. Hence, were purposively not selected for viral load, resistance and yield efficacy levels evaluations in objective three and four. Among the CMD viruses detected in the transgenic lines samples; EACMV was the highest at 87.5%, with EACM Ug most dominant strain at 85.71%. For CBSD; UCBSV was dominant at 79.1% followed by CBSV at 62.5%. The viral load varied among the CMD transgenic lines with line 167 having the highest at 4697.736, whereas line 166 had the lowest at 167.466. On the other hand, CBSD transgenic lines 407 and 506 having the highest and lowest at 8.167 and 0.2135 respectively. Focusing on resistance levels; CMD transgenic line 166 had 64.43%, with non-transformed line 60444 having the lowest at 24.44%. For CBSD transgenic lines; 501 had 82.84%, with 60444 having 59.70%. For the yielding efficacy levels; CMD transgenic lines 166 and 129 had 13.53%, with 133 having the lowest at 4.05%. For the CBSD transgenic lines; 501 had efficacy of 57.14%, whereas, 60444 had the lowest at 9.02%. These, findings confirm the significant presence of CMD and CBSD viruses in major cassava-growing regions of Kenya. The resistance of the transgenic lines varies in a continuous manner among the lines. Hence, providing an opportunity for optimization in efforts to manage CMD and CBSD effectively.