i6 FUNGOID DISEASES 



make " healthy " growth on certain varieties of the 

 same species. The different varieties of wheat, 

 barley, potatoes, etc., show marked differences in 

 the power of warding off the attacks of their re- 

 spective diseases ; e.g. while one variety of potato ^ 

 forms a suitable feeding ground for Phytophthora, 

 and falls a prey to disease in consequence ; another 

 is unfavourable to its development and as a result 

 suffers less from the attack. This diversity is due 

 in a great measure to the difference in the relative 

 amounts of the various chemical substances within 

 the plant. Influence of plant on parasite is again 

 shown where a change of the host results in the 

 formation of a different kind of spore, as is well 

 instanced in Pticcinia graminis which forms on 

 wheat uredo- and teleuto-spores, while on the bar- 

 bery {^Berberis) aecidiospores are produced ; again 

 Cystopus candidus (producing white rust in cruci- 

 fers) on shepherd's purse forms only conidia, while 

 on some other plants belonging to the same order 

 another kind of spore is formed, viz. oospore. 



The above examples show that the host exerts 

 a very considerable influence on the parasite. 



Signs of Disease. — The various changes enum- 

 erated under ''Effect of Parasite on Plant" (p. 

 13) indicate the presence of some attack. These 



^ Grown under identical conditions, some varieties may have over 

 two tons of diseased tubers to the acre, e.g. " Up-to-Date," and others 

 not as many cwts., e.g. " King Edward VII ". 



