^o Journal of Agricultural Research voi. vi.no. i 



CoNiDiAL PRODUCTION. — Under favorable conditions conidia are 

 produced apparently much more readily by young than by old leaf- 

 spots. For instance, as will be seen in Table IV, leaf spots (*) ^ 2 to 4 

 days old showed a marked increase in conidial production from July 23 

 to 25, while during the same period spots (**) 14 to 24 days old in most 

 cases showed a decrease. However, spots (***) 2 to 3 weeks old on 

 green leaves showed an increased production from August 18 to 25, the 

 conditions being favorable, and some (f) even produced a second and 

 third crop, although usually but one crop (ft) is produced and this while 

 the spots are comparatively young. It was also found that under favor- 

 able conditions a spot (fft) may produce abundant conidia on both 

 surfaces in one day. Usually the maximum production is reached within 

 10 days after the spots appear (fig. 6), and sometimes under very favor- 

 able conditions the production may increase after this period (fig. 6, 

 curves D and E, July 17 to 23), but the older spots do not always respond 

 to favorable conditions in this way (fig. 6, curves C and F). In no case 

 was a new growth of conidia observed on spots on yellow or dying leaves 

 on green plants in the field. The fungus seemed to lose its vigor much 

 sooner on such leaves than on green leaves which remained attached 

 to the crown at harvest time. From the standpoint of control of the 

 disease this is a very important point, from the fact that at harvest time 

 the green leaves, on which the fungus is vigorous, are removed with 

 the crowns and stored in the silo, while the yellow and dying leaves, 

 on which the fungus may be too weak to overwinter, break off and 

 remain on the ground. 



During the greater part of August and September, when the precipita- 

 tion was light (fig. 7), many of the conidia had a shrunken appearance 

 and were massed together on the leafspot areas (Table IV,§). When 

 placed in water, these conidia did not germinate; consequently this 

 desiccation of the conidia may also be an important factor in connection 

 with the vitality of the fungus on the host. 



The position of the leaf on which the spot studied was located was also 

 found to be an important factor in conidial production, an abundance of 

 conidia being frequently observed on leaves protected from the sun, 

 while at the same time few were observed on those exposed to the sun 

 the greater part of the day. This difference in production is thought to 

 be due mainly to the difference in humidity of the protected and the 

 exposed locations. 



A study of the comparative production of conidia on the upper and 



the lower surfaces of the spots was also made, the conidia on the spots 



included in series E, K, N, and S (fig. 6) being tabulated for this purpose. 



Generally a more abundant conidial production was found on the 



lower than on the upper surface (fig. 8), and this was due apparently to 



I The asterisks (*). daggers (t), and section marks (§) refer to particular leaf spots in Table IV. 



