Resistance to Phytophthora infestans in tomato wild relatives

Late blight of tomato (Solanum lycopersicum L.) caused by the heterothallic oomycete Phytophthora infestans (Mont.) de Bary, is one of the most destructive and serious diseases of tomato in cool and wet environments. Tomato breeders have developed late blight-resistant tomato lines and cultivars based on Ph resistance genes derived from S. pimpinellifolium, but resistance can be short-lived because P. infestans is highly diverse and can readily develop virulence towards the Ph resistance genes. Studies were carried out to assess the resistance level of four tomato genotypes and 48 wild relatives of cultivated tomato to P. infestans. The highest late blight resistance was detected in S. habrochaites accessions LA1777, LA1352, LA2855, LA1347, LA1718 and LA1295, with disease severities ranging from 4.5 to 13.5%. Interestingly, tomato genotypes containing Ph-2 and Ph-3 had significantly lower disease severity indices compared with the susceptible control 'Super Strain B' when inoculated with a highly virulent isolate. However, when a different isolate was used in 2014, the Ph-2 and Ph-3 containing tomato genotypes were as susceptible as 'Super Strain B'.  The overall results demonstrate that LA1777, as previously reported, had a high level of resistance against all isolates of P. infestans and is a useful genetic resource for future tomato breeding programs. Key words: Tomato, late blight, Phytophthora infestans, disease resistance, Ph-genes, Solanum habrochaites.