1919. \ 29. 



THE FUNaOUS DISH.iSB3 OF R03E3 AND 

 THEIR TREATMENT. 



By James Birch Roree, 

 Mycologist, Board of Agriculture. 



With One Plate. 



On account of the increased interest in rose growing which has- 

 been fostered by the Horticultural Club during the past four or five 

 years, many enquiries have been received by the writer in regard to the 

 treatment of rose diseases. As a result, numerous specimens have 

 been examined in order to ascertain the causes of the diseases, and a 

 series of experiments carried out to find some practical and easy method 

 of combating them. 



Though there are several destructive diseases and insect pests of 

 roses in the colony, it must always be remembered that rose plants are 

 very susceptible to unfavourable soil and climatic conditions and their 

 sickly appearance is not always due to diseases or pests. 



These plants are rather particular as to soil requirements ; good 

 drainage and abundant manure being necessary for successful results. 

 It is fatal, however, to manure rose plants heavily if the ground is not 

 thoroughly drained, as this induces root rots of various kinds. It is 

 essential also that rose gardens be kept free from the roots of other 

 plants, which seem to have a very harmful effect on rose bushes. 



There are three fungous diseases of roses which occur wherever 

 these plants are grown, in addition to which there occurs in Trinidad 

 two diseases which have not been reported from other countries so far as 

 the writer knows. All these diseases do a great deal of damage to 

 rose gardens every year, and the black spot, leaf spot and mildew are so 

 common that healthy plants cannot be grown unless some means are 

 taken to combat these troubles. 



BLACK SPOT DISEASE. 



Perhaps the commonest of all the fungous diseases of roses is the 

 so-called " Black Spot " caused by the fungus Diplocarpon rosce, Wolf, 

 {Actinonema roses). This disease is characterised by more or less 

 irregularly circular, or oval brown, black spots upon the upper surface 

 of the mature or nearly mature leaves. The spots are quite small at 

 first, but rapidly increase in size during damp weather, several often 

 coalescing, so that one-third or one-half of the leaf surface may be 

 affected. As the spots grow older they become grey in the centre^ 

 while the adjacent parts of the leaf turn yellow. See upper figure of 

 plate. 



The leaves, even those which are but slightly afiected, fall from the 

 plant prematurely, so that sometimes the ground beneath the bushes i» 

 covered with fallen leaves. Naturally, the plant immediately puts out 

 new leaves, which under favourable weather conditions, become 

 infected in turn and fall to the ground ; thus the bushes become greatly- 

 weakened and begin to die back from the tip. 



