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THE COMBINATION OF THERMOTHERAPY WITH SENSITIVE DETECTION METHODS MAY REDUCE CASSAVA MOSAIC DISEASE DISSEMINATION
THE COMBINATION OF THERMOTHERAPY WITH SENSITIVE DETECTION METHODS MAY REDUCE CASSAVA MOSAIC DISEASE DISSEMINATION
S RANOMENJANAHARY 1 , J RAMELISON 1
1 FOFIFA, Laboratory of Plant Pathology, P0 Box 1444 Ambatobe 1 Antananarivo (101) Madagascar
Background and objectives
Due to the lack of outlets, farmers usually plant cassava cultivars sensitive to mosaic disease whose yield and quality are well adapted for their own consumption. So, the use of infected cuttings obtained from a previous crop is largely responsible for the virus dissemination. Actually, severe infection is observed in 70% of farmers' fields. Growing plants are weak and stunted with small and distorded leaves. This involves the shortage of vegetative material and subsequently the deficiency of production. Although, the demographic growth in Madagascar is high (2,3%), there is some emergency to care about food security. To avoid starvation, a short term action is undertaken. It consists in providing farmers with virus free planting material from appreciated clones. Clean cuttings are obtained by applying thermotherapy to systemic resistant stems issued from infected and non infected plants.
Materials and methods
Fourteen out of 138 clones in the germplasm evaluation at Kianjasoa, mid-west of the Island, which showed cassava mosaic disease (CMD) symptoms were used for the study. Cuttings from the 14 clones were planted in 12 m2 plots with 4 replications at random. The 20 cuttings of each plot contained 10 cuttings issued from diseased stems planted adjacent to another 10 from apparently healthy stems of the same clone. The sprouting percentage and disease infection were evaluated. Estimation of disease intensity was done 3 months after planting using the scale of 0-5 . The first year, pathogen identification was done using the ELISA method. Polyclonal anti-serum specific to ACMV provided by IITA was used. The second and third years, the same experiments were carried out but stems were treated with hot water: 47 C for 30 minutes. To elucidate previous results a more sensitive serological test, Triple Antibody Sandwiches ELISA and a more precise method as PCR were used. One isolate extracted from very severe infected leaves of malagasy sensitive cultivars was identified at the SCRI, U.K using pairs of primer specific to African Cassava Mosaic Virus (ACMV) and to East ACMV. A set of polyclonal and monoclonal antibodies were also provided by the Institute to carry out the experiments in Madagascar.
Results and Conclusions
From the first experiences, depending on the level of resistance of studied clones, the ability to provide apparently healthy stems is variable. The more the cultivars are sensitive, the less the systemic resistant stems appear. Good sprouting, mild symptoms and high yield have positive correlation with the clones' resistance to the virus. The polyclonal anti-serum specific to ACMV allowed to identify the causal agent only on 6 clones with severe infection noted 4 on the scale. Although, three very sensitive clones and five with very low infection had no reaction to the anti-serum. The inefficiency of the anti-serum used to detect the lower level of infection lead to other more advanced methods. From the second and third experiences, the prophylactic heat treatment improved the plant tolerance. In general, this treatment allows the rapid growth and strengthens the vigour of those apparently healthy stems until the last stage of vegetation. Thus, plants acquired early adult resistance. Among the treated clones, some of them appeared free of disease or with very mild infection which is the case of those attacked by ACMV. Tolerant ones remained with mild symptom caused by EACMV. Some gain of resistance were observed on highly sensitive clones.