Tab A - Page 41 
bud break. Populations of Ice-nucl eatlon-actlve-bacteria have been shown to be 
largest during April through May, coinciding with the period of maximum frost 
hazard to pear In this location. Populations of Ice-nucleatlon-actlve bacteria 
decreased after late May, declining with the onset of hot, dry weather to 100 
cell s/g by late summer. 
It should be noted that not all INA* bacterial cells are active as Ice 
nuclei foci while on plant surfaces. Nucleatlon frequency Is not an Intrinsic, 
Invariable property of the bacteria, but a property that varies, depending on 
the temperature and other factors. In an experiment by Ors. llndow and 
Panopculos with pear, only about one _P. syrinqae cell In 1000 acted as an ice 
nucleus at -5° C while on plant surfaces. The frost sensitivity of most plants 
l 
can be explained by the fact they harbor very large numbers of 
J 
ice-nucleation-active bacteria. Ice nucleation strains of both £. s_. syrinqae 
and E. herblcola can be detected on most plants, but P. s. svrinqae is the pre- 
dominant species on the majority of plant species investigated In California. 
( 0 ) l 
p 
.w 
Observations of natural ly-occurring bacterial populations on potato plants 
near Tulelake In Northern California are consistent with the pear studies. 
INA + populations were very low (30 cells/gram fresh weight) shortly after 
emergence of the plants, but Increased rapidly during periods of cool wet 
.weather.- INA + bacteria nevertheless generally comprised less than 0.01 percent 
of the total bacteria on leaves of potato plant's. Ourlng the 1979 growing 
season, strains of P_. syrinqae , E. herblcol a , and P_. fluorescens were found In 
nearly equal numbers on potato leaves. The number of Ice nuclei active at -5° 
C was substantially less than 1.0 percent of the number of INA + bacteria during 
38 
[ 235 ] 
