EFFECT OF INOCULUM SOURCE ON ROOT AND TUBER INFECTION BY POTATO BLEMISH DISEASE FUNGI

The incidence of potato pathogens on healthy roots of micropropagated (MP) and seed tuber (ST) plants was examined on successive dates during the growing season in two field experiments. Microplants were grown in a glasshouse for 4-5 weeks in perlite or peat-based substrates, and exposed or not to natural inoculum before planting in the field. The seed tubers originated from stocks of visually clean or moderately blemished tubers and were surface-sterilized or not before planting. Polyscytalum pustulans and Helminthosporium solani only infected roots of ST plants and inoculated MP plants. The incidence of P. pustulans was affected by seed tuber-borne inoculum and, in 1 year, by the substrate. H. solani was detected infrequently on roots. Rhizoctonia solani was present at low frequencies in most root samples, and more ST than MP plant roots were colonized, there were no substrate effects. In 1 year, increased inoculum levels increased root infection, but only in MP roots. Colletotrichum coccodes occurred at high frequencies and was most common in roots of ST plants. Progeny tubers showed some treatment effects when tested in September and after storage for 6 months, but there were no consistent relationships between root and progeny tuber infection.