Evaluation of mulberry germplasm for resistance to powdery mildew in the field and greenhouse

Powdery mildew caused by the ascomycete Phyllactinia guttata (syn. P. corylea) is a major foliar disease worldwide of the unique mulberry (Morus spp.) for silkworm feed. Genetic resistance to powdery mildew, the most sustainable and economical strategy for disease control, is still elusive for tropical mulberry. About 147 germplasm sources, representing 18 countries of origin, were screened for resistance to P. guttata in six seasonal fields and greenhouse trials after exposure to natural and artificial inoculum, respectively. In the field, the level of plant responsiveness to disease was assessed from 30 to 62 days after pruning in each season as variations in the disease severity index (DSI), disease incidence (DI%) and area under the disease progress curve (AUDPC). These measures differed significantly among the germplasm. Of 147 germplasm sources, similar to 6.8% had useful resistance (two high and nine moderately resistant) to the powdery mildew pathogen on the basis of DSI. The AUDPC values were 13.5-fold higher in the most susceptible accession-(Philippines) than the least responsive (Vietnam-2). The results of DSI were strongly correlated with the obtained DI values (r = 0.92; P < 0.01) and AUDPC (r = 0.89; P < 0.01). Moreover, field screening results were highly correlated (R (2) = 0.839) with values from the greenhouse evaluation using artificial inoculum. However, the DSI values in field and greenhouse screenings for three sources (Non-nayapati, Nao-khurkul and Tista Valley) varied significantly. A relatively low disease reaction of 09 resources (Vietnam-2, Ankara and 07 others) using different assessment scales after natural and artificial inoculation prove, for the first time, that they have potential in breeding for resistance in tropical mulberry to powdery mildew.