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XXXIV. 



THE BIONOMICS OF THE CONIDIA^ OF PHYTOPHTHORA 

 INFESTANS (MONT.) DE BAEY. 



By PAUL A. MUEPHY, B.A., A.E.C.Sc.I, 

 Assistant in the Seeds and Plant Disease Division, Department of Agriculture 

 and Technical Instruction for Ireland. 



[Read December 20,' 1921. Published Fehkuarv 2, 1922.] 



In previous papers (18, 19) - accounts were given of field experiments in 

 Canada and in Ireland bearing on the occurrence of blight in potato tubers, 

 particularly after digging. It was shown that the bulk of the infection in 

 the case of potatoes which develop blight in storage is contracted when the 

 tubers are being dug. Proof was given that contact of the tubers with 

 partially blighted foliage results in serious rot in storage. Evidence was 

 also presented to show that soil contaminated with eonidia shed from the 

 leaves continues capable of inducing blight in freshly dug tubers which are 

 brought into contact with it for a period of at least ten days, and probably 

 longer. It was mentioned that the part played by contaminated soil was 

 fully established by the results of laboratory investigations. The present 

 paper deals with this phase of the subject. 



I. — Vitality of Conidia. 



Vitality of conidia in soil out-of-doors. — As it is generally accepted, 

 following de Bary (4), Jensen (11), Hecke (10), McAlpine (14), Jones, 

 Giddings and Lutman (12), and Melhus (17), that the life of the conidia in 

 comparatively dry air is measured in hours rather than days, an attempt was 

 made to determine the effect of keeping the conidia in soil.^ A preliminary 



^ The word " conidium " is used throughout to describe the asexual reproductive body 

 which behaves sometimes as a conidium and sometimes as a zoosporangium. 



^ The numbers in brackets refer to the list of literature cited at the end of the paper. 



3 Jensen (11) found by experimental methods that conidia may survive in soil for five 

 days. Apart from this experiment, no other reference to the subject has been found in 

 the literature, with the exception of certain observations of de Bary (3) on the vitality 

 of conidia in soil which had carried a blighted crop ; of Melhus (16) on the behaviour of 

 conidia produced on the surface of infected tubers in the soil ; and of Peters, as reported 

 by Karsten (13), on the continued vitality of infecting material placed on the soil. 

 Although the matter was not followed up, the conclusions reached point in the same 

 direction as our own results, particularly in the last two cases. See also Appel (1). 



