BACTERIAL, WILT OF CUCURBITS. 41 



SUMMARY. 



As a result of our studies on bacterial wilt of cucurbits during the 

 period 1914-1918, considerable new information regarding the disease 

 and its relations to insects has been obtained. 



The disease occurs in 31 States, including the territory from Ver- 

 mont and Canada to Florida and west to Minnesota, Nebraska, Colo- 

 rado, and Texas. It also probably occurs in parts of California. 

 Of our common domestic cucurbits the disease affects cucumbers, 

 cantaloupes, summer and winter squashes, and pumpkins, but not 

 watermelons. 



The severity of the disease varies widely in different seasons and 

 localities from an occasional wilted plant up to a destruction of 75 to 

 95 per cent of the crop. There is very little direct relation between 

 percentage of infections and severity of the disease and ordinary 

 weather conditions in the North, but there is a direct relation to prev- 

 alence of cucumber beetles and condition of vigor in the host plant. 



Careful and extensive experiments have shown that infection does 

 not come through soil or seed; that the squash bug (Anasa tristis), 

 squash lady bird (Epilachna lorealis), melon aphis (Aphis gossypii), 

 potato flea-beetle (Epitrix cucumeris), and honeybee (Apis mellifera) 

 are not wilt carriers ; but that the striped cucumber beetle (Diabrotica 

 vittata) and the 12-spotted cucumber beetle (D. duodecimpundata) 

 are both summer carriers and probably the only means of summer 

 transmission of the disease in the localities studied. Infection 

 through the breathing pores of the plant does not occur, introduc- 

 tion of virulent bacteria into the interior plant tissues being neces- 

 sary for infection. 



It has been definitely proved that bacterial wilt of cucurbits does 

 not winter over in the soil, and all seed tests have given negative re- 

 sults. The disease has been carried experimentally for six weeks by 

 striped cucumber beetles hibernated artificially in cold storage. We 

 have thus far been unable to hibernate the beetles for a longer period. 



Considerable circumstantial evidence, however, points to cucumber 

 beetles as winter carriers. For example, among the first collections 

 of beetles in the spring a small percentage were found to be wilt car- 

 riers. The earliest cucurbits were still very small and no wilt was 

 anywhere in evidence. Futhermore, a careful record of spring se- 

 quence of wilt was kept in all field and garden plats of cucurbits in 

 two localities and in all cases the leaves first showing wilt in the spring 

 had typical cucumber-beetle injuries; the wilt had plainly started 

 from these injured points, and there was a tendency for wilt to spread 

 in groups around the original cases, in each new plant starting from 

 beetle-injured leaves. In many of the cases such beetle injuries had 

 not become general at this time throughout the fields. 



