Greenhouse and laboratory screening methods for assessing potato foliage for resistance to late blight were compared using 15 cultivars and advanced breeding selections with known field response to late blight. Screening methods included greenhouse inoculation of plants in several age classes, and laboratory assays of detached leaflets, leaf disks, and stem cuttings. Greenhouse inoculation of plants 7 to 11 weeks after planting, near the time of flowering, corresponded best to results obtained in field evaluations, but there were significant differences in disease severity between separate greenhouse tests. This is consistent with variation in late blight severity on a year-to-year basis when cultivars are compared in the field. The greenhouse inoculation method allowed for testing of several components of partial resistance, such as infection efficiency and lesion growth rate, which may exist for each cultivar. Laboratory assays proved less reliable than greenhouse assays for overall ratings of partial resistance, but could be useful for measuring specific components of resistance. Screening evaluations for late blight resistance should include standard cultivars with known reaction to Phytophthora infestans to reference the disease potential within the screening evaluation.

In Ghana, cassava (Manihot esculenta), yam (Dioscorea spp.), sweet potato (Ipomea spp.) and cocoyam (Xanlhosoma and Colocasia spp.) fonn the major root and tuber crops. They contribute more than 60 percent of the daily caloric intake of majority of Ghanaians (Ofori, 1991). Root and tuber crops especially cassava have been playing significant role in ensuring food security in Ghana (Dapaah, 1991). Fanners who cultivate these important food crops are faced with the problem of getting high and stable yields of good quality roots/tuber per unit of area, time and labour input. These crops have less resistance to pests and disease pressures such as viruses, anthracnose, nematodes, tennites, beetles and millipedes. These therefore contribute to both major preharvest and postharvest losses of food from susceptible varieties of root and tuber crops (Asiedu, 1997, Mercer et al., 1993, Afuakwa, 1996). The project has an overall goal of improving root and tuber crop production in Ghana. It consist of four main sections~ these are 1. Varietal breeding and gennplasm collection, 2. sustainable crop production, 3. Integrated pest management and 4. socioeconmics.