70 WEEDS OF FARM LAND 



treatment by rolling, harrowing, and the treading of sheep 

 was more efficacious in eradicating moss. 1 



(c) Destruction of Weeds by Parasites. Weeds, in common 

 with other plants, are liable to attack by various insect and 

 plant enemies, and some attempt has been made to turn this 

 susceptibility to account in their eradication. Comparatively 

 little work has yet been done in this direction, but definite 

 experiments have been made in New Zealand and Italy and 

 probably elsewhere. Creeping thistle (Cirsium arvense], even 

 in this country, is very often attacked by rust, which powders 

 the leaves and stems with dusty brown spots and greatly 

 weakens the plants. Cockayne 2 states that if the rust is to 

 be made an adequate means of control it is necessary to in- 

 crease infection considerably beyond that occurring naturally. 

 It is possible to cultivate the spores of the rust on thistle leaves 

 under artificial conditions. These rusted leaves are crushed 

 up in water to liberate the spores and the liquid is sprayed on 

 to the thistles in their early stages of growth. Under these 

 conditions it is said that infection is virulent and the disease 

 spreads rapidly. 



Italian investigators point out that such common weeds as 

 thistle, sowthistle, bindweed, poppy, bladder campion, hawk- 

 bit, charlock, willow-weed, and broomrape are under natural 

 conditions frequently attacked by parasites, and they suggest 

 that with due encouragement a useful means of weed eradica- 

 tion lies at hand. Much care would have to be taken, however, 

 if any great extension of this method were attempted. Many 

 of the weed parasites may be capable of carrying on existence 

 on other hosts, and crops might easily be attacked by the 

 disease employed to kill the weeds. Also, many of the 

 fungus diseases of plants go through two phases of existence 

 on two utterly different hosts, like the wheat rust, which passes 

 one phase on wheat and the other on such plants as the bar- 

 berry. It would therefore be necessary to investigate the life 

 history carefully in every case, in case the parasite used for kill- 

 ing the weeds should be one that spent another life phase on a 

 useful crop. This danger is a very real one, and some workers 

 suggest that it would be more advisable to encourage insect 

 pests which hinder the spread of weeds by riddling the seeds 



1 A. D. H. (1900), " Moss in Pastures," South-Eastern Agric. Coll. Journal, 

 No. g, pp. 7*-72. 



2 Cockayne, A. H. (1915), " Californian Thistle Rust (Puccinia suaveolens)," 

 New Zealand Jour. Agric., XI, No. 4, pp. 300-302. 



