500 Scientific Proceedings, Royal Dublin Society. 



a pure culture of P. cryptogea from Tomato produced exactly the same 

 type of disease. 



Summing up, therefore, it may be stated that P. cryptogea is the cause of 

 a disease of Tomato and Petunia plants, and in all probability causes a similar 

 disease in Asters and Wallflowers. 1 



VIII.— Source of Infection and Control of the Disease. 



All our observations point to the conclusion that the plants become 

 infected through their roots from the soil. This is further borne out by the 

 experience that when Tomato seeds were sown in the soil in which a diseased 

 plant had been growing all the seedlings became affected with the Phytoph- 

 thora disease when they were about two inches high. Seeds from the same 

 source sown in sterile soil produced healthy seedlings. 



At one of the nurseries visited it was found that it was the practice to 

 raise Tomato seedlings in a compost made up of equal parts of virgin loam 

 and leaf-mould, without the addition of any manure. The mould usually 

 employed was derived from decomposed beech leaves (Fagus sylvatica). 



Only small quantities of this loam and leaf-mould which had not previously 

 been used were still available. Equal amounts of the two were mixed and 

 filled into two seed pans. One of these was sterilised by exposure to live steam 

 for a period of two hours at a time on three successive days, while the other 

 was untreated. Tomato seeds were sown in each pan, and the result was 

 that the seedlings which developed in the untreated mixture of loam and 

 leaf-mould became diseased, while those in the sterilised mixture remained 

 healthy. One of the diseased seedlings was removed and placed with its 

 roots in sterile water, when the characteristic sporangia of the fungus 

 developed. 



It was suspected that the fungus might be present in the leaf -mould and 

 not in the loam. An experiment was carried out to settle the matter, but 

 the result was a negative one, for, in this case, no diseased seedlings arose 

 either in the loam or the leaf-mould. This is probably explained by the fact 

 that only a very small quantity of the leaf-mould and loam was still available 

 for the trial, and in order to get enough material to fill even a three-inch pot 

 the leaf-mould and loam had each to be diluted with a considerable amount 

 of previously sterilised soil. 



In this connexion it is at any rate interesting to observe that the fungus 

 was proved to be pathogenic to beech. A beech-nut was planted in soil, and 



1 At the time of going to press we have received specimens of diseased cinerarias also 

 attacked, apparently, by this fungus. 



