674 INSECTS AFFECTING VEGETATION 



other parasites such as in the case of the club-root fungus of cab- 

 bage and related plants. In these cases some soil treatment must 

 be applied to field areas. In the case of the onion smut it is suf- 

 ficient to apply a formalin drip which will fall with the seed and 

 disinfect the soil layer in proximity to the seed. This is effective 

 because the smut fungus can penetrate the germinating onion plant 

 only in the earlier stages of growth. On the other hand with cab- 

 bage club-root, where plants are transplanted from the beds in 

 which they are grown, some general method of soil treatment which 

 involves tne soil mass is more effective. In this case it is the ap- 

 plication of stone lime or caustic lime in liberal quantities. These 

 examples are only illustrative of general conditions to be met 

 with. In the case of potato scab, it is found necessary both to 

 disinfect the seed where scab is present, even to a limited extent. 

 and to plant upon new soil not infested with the scab organism. 

 Potato rosette is certainly an acid loving parasite. 



Under the diseases described for each plant, methods of avoid- 

 ance and treatment are indicated and the diligent student will 

 find other instances of similar character mentioned therein. 



SOIL TREATMENT IN THE FORCING HOUSE. 



About our great centers of population have grown up conspicu- 

 ous developments of the forcing house industry; large areas are 

 covered with glass and these hothouses are maintained at the nec- 

 essary temperatures for the production of the special green crops 

 found profitable. At the same time the soil of the hothouse beds 

 is very liberally manured and enriched further by applications of 

 commercial fertilizers. Not only do these applications of manure 

 tend to enrich the flora of the soil and to introduce particular root 

 parasites, but the decay of the organic matter of the manure also 

 tends to produce humus acids in considerable quantities. Along 

 with these are brought parasitic eelworms which are peculiarly 

 fatal to curcurbitaceous plants, to violets and to some other hot- 

 house crops. Next to the eelworms the most conspicuous organism 

 in our area is the sterile fungus Rhizoctonia, whose resisting forms 

 or eclerotia may be readily introduced in manure. To meet this 

 indoor soil infection, special methods of soil sterilization had to be 

 developed, since soil rotation is practically impossible in the forc- 

 ing house. 



These methods of treatment are in brief, steaming of the soil 

 to render it practically sterile, so far as these parasites are concerned, 

 and a method of formalin drenching. This method of steaming is 

 ideal, or almost so, in its results upon sandy or loamy soils, but 

 often entails unfavorable changes in heavy silty or clay soils. 

 Essentially, it consists in burying a series of perforated pipes in 

 the soil and at intervals, covering the surface of the beds and pass- 

 ing live steam in sufficient volume into the pipes. These pipes 

 are prepared in sets with cross heads and boiler connections and 

 are perforated at desired distances. The surface of the bed is cov- 

 ered with canvas and the steam passed into the system for such 

 period as is required to heat the soil to a temperature from 180 to 



