PARASITIC WEEDS 95 



arvensis) is found on a variety of weeds as well as on clover, 

 including dock, dog's mercury (Mercurialis perennis), sow- 

 thistle, charlock, black nightshade (Solatium nigrutri), nettle, 

 creeping thistle, wild lettuce, knotgrass, and mayweed. 1 



Dodder seeds retain their power of germination for several 

 years, even when buried in the soil. The germination of the 

 seeds is spread over a long period, the largest percentage 

 coming up during the first month after sowing, while the 

 others appear later on in gradually decreasing numbers. 

 Some of the seeds are " hard " and it is these which form a 

 dangerous stock in the soil for several years after infection. 

 Although dodder seeds will lie dormant in the soil for so long, 

 Italian investigators claim that they are not able to withstand 

 the action of farmyard manure. If they are buried in a heap 

 of manure most of the seeds lose their power of germinating 

 within about a month, only a very small percentage retaining 

 their vitality over longer periods. It is considered, therefore, 

 that there is comparatively little danger of spreading dodder 

 infection by means of farmyard manure, provided the latter is 

 well made and properly treated. 



Eradication of Dodder. In this country clover dodder (C. 

 trifolii) is the species that causes most trouble. As prevention 

 is better than cure every effort should be made to sow seed 

 that is absolutely free from dodder seeds. With the excep- 

 tion of an American species (C. gronovti}, which has extra 

 large seeds resembling in size those of clover, the ma- 

 jority of dodder seeds can be effectively removed from clover 

 and lucerne by screening with sieves of suitable mesh. A 

 large proportion, but not all, of the dodder seeds may also be 

 destroyed by dry heating, as they lose their power of germina- 

 tion at temperatures that are harmless to the associated clover 

 seeds. The most effective temperatures seem to be : 2 



149 F. for 2 hours. 



158 F. for 30 minutes or for I hour. 



167 F. for 30 minutes or for I hour. 



If infection does occur and the patches are observed before 

 they have spread too far, the affected part of the crop should 

 be dug up, covered with chaff, sprinkled with paraffin and 

 burnt on the spot. The dodder should not be torn out with 

 a rake, as this serves to spread the trouble. If infected plants 



1 D'Ippolito, G. (1913), Le Stazioni sperimtntali agrarie italiane, XLVI, 

 pp. 540-549- 

 'Ibid. 



