62 



James (J. Horsfall 



house. Comparable results have been obtained likewise by Killian (1923) 

 and by Bayliss-Elliott and Stansfield (1924). The two last-named authors, 

 furthermore inoculated plants successfully with pycnospores and ascospores. 

 The fungus undoubtedly is pathogenic, but apparently is an obligate 

 parasite like the rusts. 



Life history. According to Killian (1923) the ascospores, matured in 

 perithecia in dead clover leaves in the spring, constitute the inoculum 

 for primary cycles. ( 'onidia from these infections 

 serve as the inoculum for the secondary cycles. 

 Conidia have refused steadfastly to germinate 

 satisfactorily in the laboratory. In two trials no 

 germination whatsoever occurred in aerated dis- 

 tilled water after eighteen hours, but once about 

 3 per cent germination was obtained in a decoc- 

 tion of clover leaves. Most of the spores sank 

 to the bottom of the drop, although little differ- 

 ence in germination was noted regardless of 

 whether the spores were at the edge or in the 

 center of the drops, indicating that oxygen is 

 not the limiting factor. Germination of conidia 

 in the clover decoction is illustrated in figure 14. 

 Killian says that the conidium on the leaf 

 forms a short germ tube which penetrates the 

 cuticle directly at the point of contact with 

 the radial wall between two epidermal cells. 

 Bayliss-Elliott and Stansfield, on the other hand, 

 saw entrance through stomata as well as penetra- 

 tion directly through epidermal cells (1924:221). 

 The mycelium at first invades intercellularly and later intracellularly. 

 Within from six or nine clays the black stromata form and push through 

 to the lower leaf surface. 



In the autumn conidia cease to form and the stromata containing 

 pycnidia become shiny black and papillate. According to Bayliss-Elliott 

 and Stansfield, as well as Killian, spores from these pycnidia are capable 

 also of initiating infections. It is these stromata which later give rise to 

 perithecia during the course of the winter and thus complete the life cycle 

 of the fungus. 



Control 



Killian (1923) states that the fungus may live in the soil for five years 

 or more and infect clover crops. If this be true, then attempts at eradi- 

 cation by cultivation or rotation would be in vain. Cooke (1906:247) 

 holds that it would " probably be dispersed by spraying with dilute Bor- 



FlGURE 14. CONIDIA OF DO- 

 THIDELLA TRIFOLII EIGH- 

 TEEN HOURS AFTER BEING 

 PLACED IN A CLOVER DE- 

 CC CTION 



Note the gnarling of the germ 

 tubes on the spore at the upper 

 left denoting disintegration. 

 Spores placed in distilled water 

 refused to germinate 



