114 I. WAHL 



extremely difficult^^. According to Stover^^: The majority of 

 root-infecting fungi do not have an obhgate type of dormancy 

 as defined by GottHeb. The 'dormancy' of most root-infecting 

 fungi is enforced by adverse environmental conditions.' Table 1* 

 redrawn from Stover's paper demonstrates the relationship 

 between the survival ability of the reproductive resting struc- 

 tures and plant rotation schedules employed for controlling 

 some of the economically important diseases produced by root- 

 infecting fungi. The role of microsclerotia in the persistence of 

 the Verticilliiim wilt fungus was demonstrated by Wilhelm^i. He 

 proved that the sclerotia of this pathogenic organism retain 

 viability for 13-14 years. 



Two sclerotia-producing pathogens. Sclerotia rolfsii and 

 Sclerotinia sclerotiorum cause serious losses to agricultural 

 crops in Israel. 



Reduction in yields inflicted by S. rolfsii becomes very pro- 

 nounced in summer. Stakman and Harrar^ stress the great 

 diversity in vitamin requirements for sclerotia production 

 among a relatively small number of single basidiospore isolates. 

 The vitaiTiins involved are: thiamin, biotin and nicotinic acid. 



Bedi"^-' "^^ studied sclerotial production of S. sclerotiorum 

 extensively. He demonstrated genetic variability among strains 

 of this organism as far as sclerotial formation is concerned. 

 Sclerotial formation may be stimulated by staling products 

 of the same organism by low temperature and by chemicals 

 such as uranium nitrate^. Sclerotial formation by a non- 

 sclerotial mutant can be induced by growing it along with 

 a sclerotia-producing isolate. Fully mature sclerotia floated in 

 water begin to germinate after 32 days while germination com- 

 mences after 16 days in sclerotia previously subjected to ultra- 

 violet radiation. 



* Table I is reprinted by permission of the copyright owners, the Regents 

 of the University of Wisconsin, from H. R. Stover, Plant Pathology: 

 Problems and Progress, 1908-1958, the University of Wisconsin Press, 

 Madison, Wise, U.S.A., 1959, p. 340. 



