Potato late blight caused by Phytophthora infestans (Mont.) de Bary is one of the most investigated plant pathogens. However, problems with disease control have increased in recent years, which plant pathologists have explained by its ability of forming oospores in potato leaves. Consequently, the P. infestans population over the world has become more aggressive and virulent and late blight on potato stems is common. Oospores could be found in field conditions in potato leaflets with two or more separate spots. In Latvia the P. infestans mating type A2 was found in the 1980s. A laboratory experiment on the formation of oospores in Latvia was started in 2002. The total number of collected samples was 215. The samples were collected from potato trial fields, conventional fields and from private gardens in different regions of Latvia. Many potato leaflets with two spots were observed at the end of the growing season when the disease severity exceeded 50%. The results showed that the formation of oospores on potato leaflets in the field took place in the P. infestans population in Latvia. Oospores were found in 80-94% of the investigated cases. The occurrence of oospores means that plant rotation and certified seed material are important measures for potato late blight control and the usage of fungicides may not be effective.
