458 DISEASES OF CULTIVATED PLANTS 



American potato scab (Oospora scabies, Thaxter) usually 

 attacks potato tubers when young, forming rough patches on 

 the surface known as ' scab.' When just dug up, a very 

 delicate greyish mould is present on the diseased patches, 

 which, however, soon dries up and entirely disappears. Beet, 

 swedes, carrots, and cabbages appear to be also susceptible to 

 the disease, and should not follow a crop of potatoes, as the 

 germs remain in a living condition in the soil for some years. 



Vegetative hyphae rarely i //. thick, curving irregularly, 

 septate or falsely septate, branching; aerial hyphae at first 

 white, then greyish, evanescent, breaking up into bacteria-like 

 segments after producing spirillum-like ' spores ' by the coiling 

 of their free extremities ; forming a firm, lichenoid pellicle 

 on nutrient jelly, and usually, when growing in contact with 

 air, producing a deep, black-brown discoloration of the 

 substratum. 



I have but rarely observed this disease in this country. 

 Professor Arthur states that steeping potatoes intended for 

 1 seed ' in a solution of half a pint of formalin in fifteen gallons 

 of water for two hours is a complete specific. It would be 

 wise, when circumstances permit, to use healthy ' seed ' 

 potatoes. 



Arthur, Indiana St. Bull., No. 65. 



Thaxter, Ann. Rep. Conn. Agric. Exp. Sta., 1890. 



Defoliation of conifers. Professor Oudemans has shown 

 that the defoliation of various conifers, Abies excelsa, A. pin- 

 sapo, A. nordmanniana, A. douglasii, etc., is brought about 

 by a very minute and inconspicuous fungus which he has 

 named Oospora abietum. A single row of very minute, 

 greenish-grey, fluffy tufts emerge through the stomata on each 

 side of the median vein on both sides of the leaf. The 

 mycelium present in the tissues of the leaf rob it of food, 

 and thus bring about its premature fall. 



Tufts minute, emerging from the stomata ; conidiophores 

 short, unbranched ; conidia elliptical, ends rounded, con- 

 tinuous, 10-12x6-7 //,. 



It is recommended that fallen leaves should be collected 

 and burned. This is, I am afraid, an unpractical method. 



Oudemans, Comp. Rend. Acad. Roy. Set. des Pays-Bas, 

 seance de Jan., 1897. 



