Impact of seed potatoes infected with Phytophthora infestans (US-1 or US-8genotypes) on crop growth and disease

As the surface area with late blight symptoms of seed tubers increased, the incidence of plant emergence decreased. This relationship was more dramati c with the US-8 genotype than the US-1 genotype of Phytophthora infestans, the causal agent of late blight. However, when seed tubers were inoculated with both US-1 and US-8 isolates, the plant growth responses were generally intermediate to those obtained with the two genotypes alone. More resistan t cultivars (eg. Dorita, Island Sunshine, and Sebago) had greater plant eme rgence and less seed rot than more susceptible cultivars (eg, Green Mountai n and Shepody) but the response differences between resistant and susceptib le cultivars was less clear with US-8 as most cultivars had 100% seed rot. Seed handling treatments affected crop growth and disease risk; as the inoc ulum levels and in some cases, the pre-planting storage period increased, p lant emergence decreased generally due to greater seed rot incidences. Of t he one registered seed treatment fungicide and several non-registered and/o r experimental fungicides tested, only two significantly improved plant eme rgence. During three years of study, "diseased" seed treatments generally led to se ed rot except in 1996 when one plant in one experimental plot of infected s eed had subsequent spread of the disease to the foliage. Furthermore, withi n one week, a field epidemic developed outwardly from the single infected p lant due to the occurrence of favourable weather conditions. Further resear ch opportunities are discussed relative to the impact of infected seed on c rop growth and disease risk.