Assessment of som (Persea bombycina) clones for resistance against Phyllosticta persae

Som (Persea bombycina) is an economically important tree used as food source by the muga silkworm, Antheraea assamensis. Phyllactinia leaf spot (PLS), caused by Phyllosticta persae, affects all som cultivars during June to September (rainy season) and is responsible for foliage losses up to 26%. Information about the effect of this disease at the farmer level and host resistance is lacking. PLS severity was assessed in six major sericulture areas of Assam, India. Furthermore, eight clones were evaluated for resistance in experimental field trials. Disease incidence and severity index (DSI), area under disease progress curve (AUDPC), apparent infection rate, lesion diameter and frequency all indicated differences among clones. DSI and AUDPC values were 3.6 fold and 4.2 fold higher in the most susceptible accession M-8 compared to M-5. High broad sense heritability (h(2) = 0.80) of DSI suggested an additive nature of resistance. Correlations of DSI (r range: 0.55 to 0.91) with other parameters provided a good empirical evidence of obtained PLS responsiveness. M-5 showed partial resistance on the basis of all measures and appeared as the most diverse accession based on Euclidean distance. These results suggested that clone M-5 may be a potential source of resistance for use in PLS breeding programs.