IOWA ACADEMY OP SCIENCE 
53 
Brassica nigra and a single bed of Raphanus sativus were found infected. No 
very robust growth of the fungus was observed this season. Of the five speci- 
mens (N. Y., one; Del., one; Ind., three) examined the only oospores found 
were intermingled with those of Alhugo Candida on a pod of Raphanus. 
Regarding Grape Mildew, Parlow^® says, “The fungus, like all the other 
species of the genus, flourishes best in moist warm weather, but seems more 
tolerant of dryness than any other Peronospora with which we are acquainted.” 
To quote from the paper of Mr. Guy West Wilson^®: 
“Two important and interesting problems in the history of Peronosporales 
are (1) the interrelation of meterological conditions and the abundance and 
development of these fungi and (2) the location of centers of seasonal distribu- 
tion. These problems necessitate a series of observations extending over a 
term of years in a given locality and at the same time require the co-operation 
of observers in localities widely separated in space and climate. 
A phase of the subject has been treated by Drs. Orton and Selby in their 
investigations of the centers of seasonal distribution. 
These observations have been confined almost entirely to economic species 
such as Pseudoperonospora cuhensis and Phytophthora infestans. From these 
studies on the first species the oospores which have so persistently eluded the 
collector it appears that this spore form may be elided or only produced at 
irregular intervals as a means of rejuvenation. The fungus is perennial in 
Florida and from this center is carried northward by the conidia each season. 
From a study of Peronospora in Iowa in 1887, it appeared that the species 
are suited to a moist season. In general the mildews were found in early 
spring and after this through the long dry summer, in limited quantities upon 
plants growing in damp places. During the season from March to August had 
only 8.32 inches of rainfall. 
The Powdery Mildews are much more abundant in a comparatively dry 
season than in a very moist season. For this reason the Powdery Mildew of 
the Lilac, Cherry, and Grape are much more common in Iowa during the latter 
part of the season than during the early part. The years 1895, 1899, 1901 were 
noted for the abundance of Powdery Mildews. 
Dr. Halsted^^ in a discussion of the relation existing between late fall and 
potato rot, states that in 1897 and 1899 were two seasons particularly destruc- 
tive so far as potato rot was concerned, in the eastern states. In 1897 the out- 
break of potato rot was very unusual and carried away the larger part of the 
potato crop. He gives the following table: 
Year _ ___ 
1888 1889 
1890 
1891 
1892 
1893 
1894 
1895 
1896 
1897 
Av. 
Rainfall 
3.50' |l0.19 
5.62 
5.30 
4.03 
2.72' 1 
1.66 
4.24 
5.50 
11.42 
5.42 
He states further that the fungus disease was not destructive in 1889. “It 
seems to me that the Phytophthora or late blight is quite dependent upon an 
abundance of moisture in midsummer, and if this relation is noted sufficiently 
the time may come, it may be predicted with reasonable certainty, that a wet 
the American Grape-Vine Mildew. Bull. Bussey Institution 1876 : 415-425. 
^oGuy West Wilson. Notes on Peronosporales for 1907. Proc. Iowa Acad, of Sci. 
1908:85. 
«Torr. Bot. Club 25:161. 
