20 PLANT DISEASES 



practice of using diseased canes for purposes of propaga- 

 tion. Bulbs or tubers infested with sclerotia naturally 

 produce diseased plants ; it is for this special object that 

 the sclerotia are present. Cuttings from diseased plants 

 are prone to perpetuate the disease, even if the particular 

 cutting used appeared to be free from the disease. To 

 understand this statement, it must be remembered that 

 when the mycelium of a parasitic fungus has entered the 

 tissues of a host, it usually spreads far beyond those points 

 where it announces itself by killing portions of the host, 

 and producing its fruit on the surface of such dead or 

 dying parts. Furthermore, mycelium possesses the power 

 of remaining latent for a long period of time, and of assum- 

 ing an active condition when favourable circumstances 

 present themselves. 



Finally, the seed of diseased plants often produces 

 diseased seedlings, as in the ' sleepy disease ' of tomatoes. 

 An instance of this kind came under my observation : a 

 fine hollyhock growing in my garden was severely attacked 

 by the hollyhock rust (Pucdnia malvacearum\ the calyx 

 and even the carpels being studded with the small brown, 

 wartlike pustules of teleutospores. The seed of this 

 diseased plant was sown in due course, and, in a large 

 percentage of the seedlings, the characteristic fruit of the 

 fungus appeared on the cotyledons. 



Cuttings from diseased plants should not be used for 

 propagation. Bulbs or tubers produced by diseased plants 

 perpetuate the disease, and do so with certainty if sclerotia 

 are present. Seed produced by diseased parents should not 

 be sown. Seedsmen should be required to give a guarantee 

 that seed offered for sale was obtained from healthy plants. 



