EXTERMINATION OF WEEDS IO3 



relative humidity, high temperature, high wind and NO 

 dew. 



"At Hunter the work started off in the same way as 

 at Warren, but the timely appearance of dew turned the 

 scale and we succeeded where failure seemed certain. 

 - "Here are the observations : White incrustation ap- 

 peared on sprayed leaves in three to five minutes, and in 

 some instances white pellicles of sulphate could be seen 

 scudding along the ground. At the end of twenty-four 

 hours white incrustation undiminished and weeds show- 

 ing no signs of distress. But let the dew fall and the 

 white incrustation disappears like magic and the plants 

 show signs of mortal injury, and in a few hours small 

 plants shrink to bunches of shapeless tinder and large 

 plants droop and brown and shrivel." 



Fungi Destructive to Weeds. In various parts of the 

 world certain fungi are known to be quite destructive to 

 weeds. In the East a rust (Puccinia suaveolens) destroys 

 large quantities of Canada thistle. In other places ex- 

 tensive patches of foxtail are exterminated by the downy 

 mildew of the foxtail (Sclerospora graminicola). Foxtail 

 smut (Ustilago neglecta) destroys a great deal of the seed 

 of this weed. Related smuts, as quack grass smut and 

 one found on stink grass, are also very destructive to 

 weedy grasses. The spot disease of crab grass (Pyri- 

 cularia gricea) is very destructive to crab grass ; so is the 

 spot disease of smart weed (Septoria Polygonorum). Many 

 other fungi might be named in this list. There is no 

 doubt that these fungi are important factors in the de- 

 struction of weeds. 



