The influence of long-term storage in liquid nitrogen on survival and pathogenicity of Phytophthora infestans isolates

A set of 14 aggressive Phythophthora infestans (Mont.) de Bary isolates with a wide range of virulence, was stored for eight years (2002-2010) in liquid nitrogen at -196°C (209 samples) and under paraffin oil at 7°C (70 test tubes). The survival rate of samples stored in liquid nitrogen was scored as 88%, 45 days after thawing. The revived isolates were passaged through potato tissues four times and tested for virulence on a set of 11 Black’s differentials. The mean number of virulence factors per isolate, of these P. infestans isolates evaluated before storage, was 7.4. The isolates stored in liquid nitrogen and under paraffin oil and passaged four times through potato tissues showed a similar mean number of virulence factors per isolate, 7.3 and 6.9, respectively. Isolates stored under paraffin oil showed no expression of factor avr5, but expression of this factor occurred in six isolates after liquid nitrogen treatment. The initial expression of this factor occurred in four isolates. Before storage, the average aggressiveness of the tested isolates was assessed as 1.7, on a 1-9 scale, where 1 means the most aggressive. After storage and after four subsequent passages through potato tissues, the mean aggressiveness of isolates stored in liquid nitrogen and under paraffin oil reached the level of 1.5 and 2.1, respectively. The mean aggressiveness was not significantly different from the level of initial assessment.