14 MASS. EXPERIMENT STATION BULLETIN No. 283 



EFFECTS OF FUNGICIDES ON SPORULATION, GERMINATION, 

 AND INFECTION, AND ON CUCUMBER PLANTS 



Effects of Sulfur and of Copper-lime Dust on Sporulation 



The effectiveness of fungicides against the downy mildews and many other 

 fungi is usually explainable by the fact that they are toxic to the spores of fungi 

 and thus prevent spore germination and infection. Relatively little attention 

 has been paid to the effect of fungicides on the sporulation of such fungi. 



Cucumber plants which had been inoculated, but on which the fungus had 

 not as yet sporulated, were wetted and dusted either with sulfur or with copper- 

 lime dust.' These plants were then placed in moist chambers together with other 

 inoculated plants with the disease in the same stage of incubation, but without 

 fungicides. The fungus sporulated normally on the leaves of plants without fungi- 

 cides. There was some, but less sporulation on leaves of plants with copper dust. 

 There was no sporulation on leaves of plants which had been dusted with sulfur. 



In later experiments, detached cucumber leaves, showing the leaf spots 

 symptomatic of infection by P. cubensis, but with no sporulation of the fungus, 

 were wetted and divided into three groups of 50 leaves each. One lot was dusted 

 with sulfur, one with copper-lime dust, and one received no fungicide. All were 

 kept over night in a moist chamber at 18° C. and then examined for sporulation. 

 Per unit area of leaf surface, conidia present expressed as relative numbers were 

 as follows: on leaves without fungicide, 100; on leaves with copper-lime dust, 

 29 to 80; on leaves with sulfur dust, to 5. 



In these experiments, therefore, sulfur was much more effective than copper- 

 lime dust in preventing sporulation. Some of the effect of sulfur on this fungus 

 and the disease which it causes is due to its action in inhibiting sporulation, but 

 it is evident that this effect alone is not enough to prevent infection of plants 

 by a fungus the spores of which may be wind-borne from other plants which have 

 not been sulfured, unless the fungicide is also effective in preventing spore germi- 

 nation. Such prevention of sporulation can prevent later infection only when 

 there is no other source of inoculum from plants not sulfured, which may be the 

 case in greenhouses here in fall and winter after all field-grown cucumbers have 

 been killed by frost. This may explain in part the observation of Stone (43) 

 that sulfur vaporized on heating pipes at least helps to prevent downy mildew of 

 cucuml)er in greenhouses. 



Effect of Sulfur on the Germination of the Conidia 



Cucumber leaves infected with P. cubensis and bearing the conidioj)hores 

 and conidia of this fungus were cut in half, and half of each leaf was dusted 

 with sulfur. These and the unsulfured halves of the leaves, in neither case 

 wetted, were placed in moist chambers (Wardian cases) at temperatures from 

 22° to 26° C. After intervals of time varying from 13^ to 5 hours, conidia were 

 removed and placed in water at a temperature of 18° C. Under these conditions, 

 more than 60 per cent of the conidia from loaves not sulfured germinated. The 

 percentage of sulfured conidia which germinated is shown in Table 4. 



Exposure to sulfur for 5 hours at 22" C. or for -iVz hours at 24° C, with the 

 sulfur in contact with the conidia and without i)rocii)itat(Hi moisture, j)revented 

 the later germination of these conidia. 



'The sulfur used in tliesc and later experiments was 300-mesh (98 per cent) dusting sulfur, and the 

 ooppcr-Iinic dust was one guaranteed to contain not less than 19 per cent monohydrated copper 

 sulfate. 



