HYPOBIOSIS IN FUNGI 109 



potato crops were attacked by the parasite Synchytrium endo- 

 bioticum on field plots that had been kept fallow and free from 

 weeds for more than ten years^. Oospores of Peronospora 

 schleideni germinate only after several years^. Gibbs^ reported 

 the survival of resting spores of Plasmodiophora brassicae for 

 five years. According to Walker^, soils are known to be infested 

 for ten years and longer by this parasite in the absence of the 

 host plant. For these reasons crop rotation is of limited value in 

 controlling this disease. Stakman and Harrar^ report that 

 teliospores of Puccinia graminis formed in the late summer do 

 not germinate until spring thus securing the winter survival of 

 the parasite. Murphy^^ reported that some physiologic races of 

 oat crown rust dilfer markedly in their precocity of telial 

 development. Telial formation is hastened by adverse growing 

 conditions^i. Wahl and Tobolsky (unpublished data) proved that 

 oat stem rust, race two, produces telia readily on coleoptiles of 

 a number of oat varieties, while races one, six and eight are 

 devoid of this ability. Hingorani^^ proved that resistance of 

 teliospores of P. graminis avenae to activating stimuli varies with 

 physiological races. Teliospores of some races of Ustilago 

 striiformis-stripe smut of several grasses requires after-ripening 

 periods varying from 110 days to as long as 265 days^. 



Complications in effective disease control caused by pro- 

 longed spore dormancy can be well illustrated in the case of 

 wheat dwarf bunt incited by the fungus Tilletia controversa. 

 While the ordinary bunt or stinking smut may be successfully 

 controlled by chemical treatment of the seed, this does not hold 

 true for dwarf bunt, since the spores of this fungus can remain 

 in obligate dormancy in the soil for over two years^"^. It should 

 be stressed that spores produced independently from the afore- 

 mentioned cytological processes may also act as resting bodies 

 to assure the persistence of the pathogen in an adverse environ- 

 ment. Parkin found that Botrytis, Trichoderma and Stemphylium 

 are capable of forming true chlamydospores under inimical 

 conditions, and he concludes that the development of chlamydo- 

 spores is associated with the ability to remain viable under 



References p. 116 



