264 PHYSIOLOGIC SPECIALIZATION AND VARIATION 



a s;iven species, it would be anticipated that new forms are being 

 produced constantly. There is abundant evidence that this is the 

 situation, especially among rusts and smuts [Christiansen and Stak- 

 man (1926), Newton and Johnson (1927), Christiansen (1929), 

 Flor (1932)]. 



Sectoring. Sometimes differences appear as sectors in the col- 

 onies on Petri dishes. These sectors may appear as wedge-shaped 

 areas of different color, of different compactness of mycelial 

 growth, with less profuse sporulation, or of some other very 

 marked difference. Christiansen and Stakman (1926) and Stak- 

 man, Tyler, and Hafstad ( 1933) noted sectoring in Ustilago zeae. 

 In their report Stakman, Tyler, and Hafstad (1933) recorded the 

 isolation of 14 variant lines of U. zeae from a single monosporidial 

 cell that sectored in culture. Each line maintained distinctive 

 cultural characteristics for 5 years. Evidently each was a distinct 

 biotype, and each arose as a mutation. 



Dod^e ( 1931) isolated an albino strain of Nenrospora sitophila 

 that produced few conidia. Johnson and Yalleau (1935) isolated 

 from a sector an albino strain of Thielaviopsis basicola. Leonian 

 (1930) observed sectoring in Fusarium moniltforvie, and it appears 

 to be of rather common occurrence among Fusaria in the section 

 Elegans. Hansen and Smith (1932) recorded sectoring in Botrytis 

 cinerea, and Wolf and Wolf (1939) in Botryosphaeria ribis. Pes- 

 talozzia funerea sectored, giving rise to conidia with only a single 

 seta [Christiansen (1932)], which is characteristic of the genus 

 Monochaetia. 



The occurrence of dissociation with the production of albinistic 

 mutants has been noted in Brachysporhnn trifolii [Bonar (1922)] 

 and among sclerotia in Botrytis cinerea. 



Brierlev (1920) and Christiansen (1922) secured evidence that 

 some of the mutations of Helminthosporiwn sativum were more 

 virulent, and others less virulent, than their parents. Ustilago 

 zeae from purplish sectors was more virulent than that from tan 

 sectors [Christiansen and Stakman (1926)]. Newton and John- 

 son (1927) isolated a bright orange and a greyish strain of Pnc- 

 cinia graminis tritici from form species 9. Both seemed identical 

 in pathogenecity, however, with the normal form. 



Sectoring among fungi has been compared with "bud sporting" 

 among seed plants. The causes of sectoring are not understood, 

 although certain factors are known to exert an influence. These 



