Tab A - Page 29 
pathogenicity, to that host. It should be noted that pathogenicity is not a 
prerequisite for epiphytic growth on herbaceous plants. It Is possible that 
pathogenic capability gives some strains an advantage as epiphytes or that 
their epiphytic fitness Is a significant component of their host range under 
natural conditions. 
Due to their ice nucleating properties, epiphytic populations of INA + P_. 
s. syringae predispose frost-sensl tive plants to freezing injury at relatively 
warm subzero temperatures . This might be considered an "opportunistic - type of 
pathogenicity. (I) 
&. Frost-conditional Virulence 
Frost injury has often been reported as a predisposing factor for 
infection of some plants by certain P_. s_. syri nqae on several hosts (e.g. pear, 
apricot, peach). Ice formation is either required for or favors disease 
development in these cases. Most of those studies were conducted prior to the 
in/ 
discovery of ice nucleation as a property of P_. s_. syri nqae or the availability 
of ice nucleation deficient mutants of the bacterium. Evidence, although 
circumstantial, Is highly suggestive that ice nucleation activity is a 
frost-conditional virulence determinant of certain P_. s_. syringae under field 
conditions. Therefore, the INA” deletion mutants to be tested, in addition to 
being unable to cause frost injury, are expected to be less likely than the 
INA + bacteria to cause disease. (I) 
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/yon him 
