xxxv.] POTATO DISEASE, I. ITS ACTIVE STATE. 279 



of the Journal of the Royal Horticultural Society, from the 

 pen of the Rev. M. J. Berkeley. This paper also gives a 

 complete description of the potato fungus, a description 

 so complete and admirable that, even now, very few new 

 facts can be added to it. Since 1846, when that account 

 was published, many fresh observers have written on the 

 potato fungus, and some of Mr. Berkeley's original obser- 

 vations have been amplified, enlarged, and curiously con- 

 firmed. 



The potato disease is seldom seen in Europe before 

 July or August, although its appearance has been noted 

 on rare occasions in May and June. Mr. Jensen has 

 stated that the fungus of the potato disease cannot exist 

 in any country where the mean temperature exceeds 77 

 Fahr. for any length of time during the period when the 

 fungus generally perfects itself, and that in a temperature 

 of 34 it cannot produce either mycelium or spores. It is 

 generally first distinctly seen in the midland and southern 

 counties of England between the 20th and 31st of July. 

 It generally appears during close humid weather, when 

 there are mists in the fields in morning and evening, 

 and the days are hot, damp, and possibly stormy. Many 

 other fungi suddenly perfect themselves under exactly 

 similar meteoric conditions. It will be pointed out later on 

 why it is, as we think, that the potato fungus appears with 

 apparent suddenness under these conditions and at this 

 particular time of the year. In the meantime it may be 

 noted that the fungus generally makes itself manifest to 

 the less experienced observer as a fine white bloom on the 

 leaves, accompanied by dark putrid spots. The bloom is 

 sometimes more profuse on the lowermost leaves of potato 

 plants, not because the fungus has travelled up the stem 

 from the seed tuber, but because the air is more moist 

 and stagnant near the ground. The bloom, with its accom- 

 panying black disease blotches, soon travels to the stems, 

 and when at length the tubers are reached the exhausted 

 seed tuber (the weakest part of the plant) is commonly 



