22 FUNGOID DISEASES 



this is due to their not drying so quickly after rain 

 as when more space is given. 



3. Continued Cropping, — -Where plants are grown 

 continuously on the same land, a suitable host is 

 always at hand for the disease-producing fungi, and 

 spread of disease is favoured thereby. 



4. Diversity in Host, — Even when crops are not 

 grown continuously, if w^eeds be allowed to flourish 

 in the interval, some of these mav act as host until 

 the same crop is repeated ; for it is known that 

 some fungi are capable of existing on many, or all, 

 of the plants of a certain order ; a good example 

 being found in Plasmodiophora brassicce, which is 

 capable of living, during the interval between the 

 turnip crops in the rotation, on cruciferous weeds, 

 should such be present. This fact is favourable 

 to the continuance and spread of disease. 



5. Badly Nonrished Crops. — Crops which are 

 ill-nourished usually succumb more readily to disease 

 than those in a robust condition ; sw^edes grown 

 where phosphate is deficient suffer more from the 

 attacks of Plasmodiophora than those having an 

 adequate supply of this substance. Potatoes ap- 

 pear to be more susceptible to the attack of certain 

 forms of leaf curl when low in vitality, 



6. Adaptability to Varied Conditions. — The fact 

 that many plant parasites are able to adapt them- 

 selves to the most varied conditions favours attacks. 

 When privations of any kind set in then resting 



