Étude du potentiel phytoprotecteur des extraits de Thevetia peruviana (Pers.) K. Schum contre les populations de Phytophthora infestans (Mont.) de Bary sur la pomme de terre au Cameroun

Abstract

Late blight caused by the oomycete Phytophthora infestans remains a huge problem in potato production and one of the most formidable crop diseases worldwide. In Cameroon the characteristics of the populations of this pathogen remain very poorly known and control measures involve frequent applications of synthetic fungicides. The general objective of this work is to control the Phytophthora infestans populations in potato crop by using Thevetia peruviana extracts. Twelve localities in the three main potato-producing basins (Bamboutos, Menoua and Mezam) of Cameroon were selected in agroecological zone III. The actual epidemiological situation of late blight in these basins was perfomed. The morphological and morphometric characters (length, width, length/width ratio) of sporocyst from the different isolates were described. The presence of matingtypes and pathogenicity tests on detached leaflets of two potato varieties were evaluated. Aqueous and methanol of T. peruviana extracts were tested at different doses (6.25 μl/ml; 12.5 μl/ml and 25 μl/ml corresponding respectively to CE1, CE2 and CE3 for the aqueous extract and CM1, CM2 and CM3 for the methanol extracts) on the mycelial growth of three isolates of P. infestans (P2, P17 and P30) and sporocyst germination. Fungicidal and fungistatic activities as well as MIC50 and MIC90 were evaluated. In the field, two potato varieties (local (V1) and an improved variety "CIPIRA" (V2) were used. Four phytosanitary treatments (an aqueous extract (T1); a Bravo 720 fungicide combined with an insecticide le Décis 5 CE (T2); a Bravo 720 fungicide (T3); a methanol extract (T4) and a control (T0)) were tested according to a completely ramdomize design. Disease progression and tuber yield during two growing campaigns (rain season and dry season) were evaluated and compared according to the different treatments. Results showed that 100 % of the plots visited in the different localities during the surveys were infected by late blight with high incidence and severity, and their distribution was found to be very heterogeneous. The Bamboutos basin is the most affected by mildew. A total of 36 isolates was obtained. Sporangia shape of P. infestans were elipsoid, globose, elliptic, ovoid, lemoniform, oval to ellipsoid; pip form and subglobose. Sporangia sizes were highly variable and can be classified into two groups according to the length/width ratio (1.47 and 2.01). A1 and A2 matingtypes are present in Cameroon. All isolates showed pathogenicity to detached potato leaflets. The level of aggressiveness of three isolates (P2, P17 and P30) from the three basinsshows that the P2 isolate from bamboutos basin is the most aggressive. Aqueous (CE) and methanol (CM) extracts at 12.5 and 25 µl/ml doses significantly inhibited (100%) the evolution of mycelial growth diameter of all strains compared to the control, as did the chemical fungicide treatment in all the isolates tested. All extracts were fungicidal at 12.5 and 25 µl/ml while the 6.25 µl/ml dose was fungistatic for some isolates. The lowest MICs were 3.35 and 14.29 µl/ml for MIC50 and MIC90 respectively. The extracts were effective in inhibiting of sporocyst germination. Germination rates were very low (0 to 13.33 %) in the aqueous extract (CE) and methanol (CM) treatments at 12.5 and 25 µl/ml compared to the control (T0) where rates ranged from 86.66 to 100 %. Aqueous extract (T1) and methanol (T4) significantly reduced disease severity as did chemical fungicide treatments compared to the T0 control in all observation periods and during both growing seasons. The applied products (T1, T2, T3 and T4) reduced late blight severity with an average of 90 % in the dry-season compared to an average 75 % reduction in severity during the rain season compared to T0. These products significantly increased yield in number of tubers per plant and commercial yield in tonnes per hectare as did chemical fungicide compared to the control during both growing seasons. This study shows that late blight remains and continues to be the most important disease on potatoes. Sporocyst shapes and sizes of P. infestans in Cameroon are highly diverse; isolates are two matingtypes A1 and A2 in Cameroon and were found to be very aggressive. Extract tests carried out on late blight development were positive. The extracts can therefore be exploited for their antifungal properties in an integrated control program against mildew.