Feb. i, 1921 Onion Smudge 705 



six days after inoculation. This condition is shown in figure 2. In other 

 cases where sporulation is postponed through lack of proper environ- 

 ment the stroma continues its growth more slowly and eventually in- 

 volves a larger portion of the scale. The cuticle, however, remains 

 intact on the exterior and normally is not ruptured until the palisade 

 layer of conidiophores is formed. A cross section of a scale which had 

 been held in poorly ventilated storage several months is shown in Plate 

 83, B. Acervuli were produced upon exposure to proper conditions for 

 sporulation. Note that the cuticle is still present outside the extensive 

 stroma, except where it has been ruptured by the sporiferous hyphae. 



FACTORS IN THE PRODUCTION AND PROGRESS OF THE DISEASE 

 OVERWINTERING OF THE CAUSAL ORGANISM 



The experiments already reported on the effect of desiccation and 

 freezing upon conidia indicate only a remote possibility that the fungus 

 lives through the winter in this form under Wisconsin conditions. The 

 stromata, on the other hand, are capable of withstanding protracted 

 periods of drouth or freezing temperature. In order to confirm the 

 supposition that the fun- 

 gus actually overwinters W^ ^ — ^U ST^^ 

 and is widely disseminated ^Mfc^^ ^ ggg^P WWB I^^ 

 in this latter form, four lots * ■■■— id 



rt -i • r -11 11 Fig. 9. — Cross section of onion scale naturally infected with 



of heavily infected bulbs ^ ,, , t ■ u u n. r a 1 • 



J Collelotrichum ctrcmans, showing the mycelium developing 



Were placed OUt Of doors at first just beneath the cuticle and later penetrating the sub- 



Madison, Wis., On Decem- cu ^ arwa » ( Camera^lucida sketch. X 450. (Thisphase 



' ' is illustrated further in PI. 83, A). 



ber 7, 191 5.. One lot was 



left in an instrument shelter near the surface of the ground, and the 

 remaining lots were buried in the soil at depths of 2, 4, and 6 inches, 

 respectively. Spore masses were present on this material at the begin- 

 ning of the experiment, and germination tests showed a high percentage 

 of the conidia to be viable at this time. 



On January 22, 191 6, examination of spores from the bulbs placed 

 in the instrument shelter showed that they had completely lost viability 

 by that date. The four lots of bulbs were examined on April 12, 191 6. 

 Those which had been buried in soil readily produced conidia in abun- 

 dance upon exposure to humid conditions at room temperature. The 

 material kept in the instrument shelter had dried out considerably 

 during the winter and, though much slower to respond, eventually 

 proved to be viable by the production of spores. A similar experiment 

 conducted during the winter of 191 6-1 7 yielded confirmatory data. 



It is to be expected that infected scales from the crop of the previous 

 season furnish a source of abundant inoculum for initial infection of 

 the growing crop. This, combined with the fact that in most onion- 

 growing sections it is the common practice to grow this crop successively 



