126 DISEASES OF CULTIVATED PLANTS 



favoured by the weather conditions indicated above, takes 

 possession of the stem and leaves, which succumb within a 

 few days. A German investigator has given it as his 

 opinion that there is not a tuber free from disease, and the 

 sudden outbursts, favoured by suitable weather, support his 

 statement. 



Mycelium slender, aseptate, haustoria rare, conidiophores 

 solitary, or 2-5, emerging through the stomata, simple or 

 sparingly branched above, tapering upwards, with scattered 

 knots above, corresponding to the origin of the conidia, up 

 to 1 mm. high, forming a delicate white mould on the leaf; 

 conidia lemon-shaped, colourless, with a prominent papilla, 

 25-30X15-20 /., producing on germination 6-16 baciliate 

 zoospores. 



Sexual mode of reproduction arrested, hence there are no 

 oospores or resting-spores. The bodies described by Smith 

 as oospores of this species were proved to belong to a species 

 of Pythium. 



Much has been written on the subject of prevention or 

 cure of this disease, but actual results are poor, as would be 

 expected, now that we know that the great bulk of disease is 

 due to hibernating mycelium in the tubers, against which no 

 remedy is known. By means of this hibernating mycelium 

 present in the tubers, the disease is conveyed from one district, 

 and from one country, to another. Potatoes from a crop 

 known to be diseased should never be used for 'seed.' 

 Spraying with Bordeaux mixture has proved beneficial, 

 inasmuch as it prevents the amount of infection that would 

 be caused by conidia, but unfortunately it has no control 

 over the entry of mycelium into the young tubers. Bordeaux 

 mixture is also said to invigorate the foliage, and causes it to 

 continue in a healthy condition for a longer period of time 

 than when not sprayed. 



The many attempts to produce varieties of potato immune 

 to this disease leave much to be desired. 



De ])j.ry, /<>ur/i. Roy. Agric Soc Engl., 12 (1876). 



Jensen, Mem. Soc. Agric, 131 (1877). 



Massee. Kew Bull., No. 4 (1906). 



Prillieux, Malad. des Plantes Agric, \ p. 7S (1897). 



Ward, Diseases of Plants, p. 59. 



Beech seedling mildew. This disease is caused by 



