48 James G. Horsfall 



and 1928. Dr. Charles Chupp in a conversation in the summer of 1928 

 stated that late blight appeared a little late that year because the nights 

 had been too warm for good germination of the sporangia. Powdery mil- 

 dew of clover also appeared in epiphytotic form about ten days later 

 in 1928 than in 1927. It appears probable that hold-over mycelium in 

 the overwintered leaves may not be stimulated to form conidia or the 

 conidia may not germinate well except under conditions of cool nights and 

 warm days. It is a common opinion among greenhouse men that powdery 

 mildew of roses is worse where there is a draft, and, according to Salmon 

 (1907), gardeners believe that strawberry powdery mildew becomes more 

 prevalent when a cool period is followed by warm weather. 



Some authors (Van Hook, 1906:249; Crawford, 1927:0) maintain that 

 Erysiphe polygon! on peas is carried in the seed. This may be true for the 

 clover mildew as well, but no experimental data are available on this 

 point. Infected seed might readily explain the widespread appearance of 

 the apparently new strain of powdery mildew over the United States 

 during 1921 and 1922. 



In 1927 an attempt was made to germinate conidia in aerated distilled 

 water in the laboratory; it was a dismal failure, for only a few short or 

 otherwise distorted germ tubes were obtained. The spore material for 

 subsequent experimentation was obtained by washing the conidia from the 

 mildewed leaves into Syracuse watch glasses with a gentle stream of water 

 from a wash bottle, to minimize injury and to get only the most mature 

 spores. The spores were then transferred with a platinum loop to small 

 drops of water on chemically clean slides which in duplicate were placed in 

 moist chambers at room temperature and were left for thirty hours. 



In one experiment conidia dusted dry on duplicate slides and placed in 

 moist chambers were compared with conidia in drops of aerated distilled 

 water. A small difference in favor of wet slides was noted, but, even under 

 wet conditions, the germination hardly exceeded 10 per cent. 



Since many chemicals are reputed to stimulate the germination of 

 refractory spores, an experiment was laid out to test the effect of these 

 upon the germination of the conidia of E. polygoni. The following treat- 

 ments were used: (1) a quantity of spores were placed in 1-per-cent citric 

 acid and were removed at five-minute intervals; (2) spores were exposed to 

 ether in a moist chamber for ten minutes; and (3) spores were germinated 

 directly in the following solutions: 0.5-per-cent ethyl alcohol, buffer solutions 

 of pH 5 and pH 7.2, clover extract, distilled water with clover leaves in a 

 moist chamber, and distilled water alone. All the moist chambers were held 

 under identical conditions in the laboratory. The germination in most of 

 the solutions, including the buffer solutions, ether, alcohol, monobasic 

 potassium phosphate, and in water (checks), was approximately 5 per cent, 

 the same percentage as in the preceding experiments. The average 



