THE SPREAD OF PLANT DISEASES. 131 



leaves or plants under the benches. If this is the only method 

 of disposal they might almost as well he left in place. It is 

 generally best to burn them. 



2. Destruction of Insects and MoUusks. — More practicable in 

 some cases than in others. 



(1) Field. — In the field we may make use of liand picking, 

 nets, resin wash, kerosene emulsion, and traps of various sorts. 

 Xo very satisfactory method of dealing with certain of these 

 carriers of disease has yet been worked out. The discovery of 

 cheap efficient ways of destroying these insects furnishes many 

 interesting problems for the economic entomologist. A combina- 

 tion of this method and the preceding (removal of diseased 

 material) is recommended for such diseases as the brown rot of 

 the potato and cabbage and the bacterial wilt of cucurbits. 

 Root aphides may sometimes be reached by tobacco dust dug 

 into the soil. Root nematodes specially infest certain soils, and 

 are difficult to combat. 



(2) Hothouse. — The troubles most prevalent in hotliouses are 

 aphides, scale insects, red spiders, root nematodes, roaches, and 

 slugs. Slugs may be handpicked at night. Roaches are easil}- 

 poisoned with a mixture of phosphorus and honey. Nematodes 

 in hothouses are seldom troublesome except when the potting 

 soil is full of them to begin with or when the plants have been 

 systematically overwatered, and in either case the remedy is 

 obvious. Red spiders are often very troublesome, especially 

 when the air of houses has been kept too dry. They are not 

 easily destroyed by the common insecticides, snapping their legs, 

 so to speak, at tobacco smoke and even at the deadly hydrocyanic 

 acid gas, of which they can endure more than the plants. The 

 best remedy is not to let the plants become infested, and the best 

 palliative is frequent douching of the plants with a line spray 

 of water. Plant lice are readily held in check by fumigation 

 with tobacco. This, however, must be managed with care, as 

 radishes, violets, and some other plants are peculiarly sensitive, 

 and a single careless fumigation might do more injury than a 

 dozen generations of aphides. I have seen the following arrange- 

 ment for fumigation in 'a large rose house in Washington, and 

 many of you are probably already familiar with it : Long, 

 narrow, shallow galvanized-iron pans half full of a very strong 

 tobacco water were distributed down the aisles of the house, and 

 into these about once a week was dropped a good-sized red-hot 



