MOSAIC DISEASE OF TOBACCO. 115 



after j^ear without attention to this point the bed usually becomes more 

 seriously infected. 



One of the cheapest methods for the control of this disease in the seed 

 bed, where it can be advantageously carried out, is to change the location 

 of the beds to soil on which no tobacco has been grown, and to avoid the 

 use of stems and tobacco water. Occasionally, however, some sHght in- 

 fection will occur even here, but as a rule not to any great extent. If 

 proper attention is paid to watering, ventilation, etc., Httle trouble of 

 this character is to be expected in new seed beds. 



It has been shown in Connecticut and elsewhere that a thorough ster- 

 ilization of the seed bed by steam at a boiler pressure of from 70 to 90 

 pounds is also a satisfactory method for the control not only of fungous 

 diseases but weeds also, and the same holds true for the mosaic disease. 

 The writer has seen this tried a number of times with excellent results 

 where the above-mentioned pressures have been used. Some growers, 

 however, seem to be of the opinion that the prime value of steaming is to 

 kill weed seeds, and so use low pressures. While low pressures will kill 

 weed seeds, it is questionable if they will sterilize the soU sufficiently to 

 kill the spores of fungi or render inactive the causal agent of the mosaic, 

 although under laboratory conditions it is rendered inactive at tempera- 

 tures of about 80°C, equivalent to 176°F. In some of our experiments 

 conducted some years ago it was strongly indicated that improper partial 

 sterihzation would not entirely rid the soil of the causal agent of mosaic. 



It might be stated here that, in many cases where the growers have 

 reported failure in the control of diseases after steam sterilization, inquiry 

 has usually brought out the fact that too low pressure was used, and as 

 a result thorough sterilization was not obtained. Another source of fail- 

 ure of beds after sterihzation with steam, under high pressure, has been 

 that the grower has not paid sufficient attention to watering. This mat- 

 ter should be closely attended to, as a sterilized bed, particularly on light 

 soils, dries out very quickly, and needs much more attention than is usually 

 given a bed under ordinary conditions. If the watering is neglected there 

 is very often a severe checking of the germination of the seed, and in some 

 cases a partial loss of the bed. 



Formahn sterilization may also be used, and is quite as satisfactory, 

 especially when used on light soils. On heavy soils it is not quite so con- 

 venient to apply, however. Where formahn is used the beds cannot be 

 sown until all the formalin is out of the soil, which usually takes from ten 

 days to two weeks. This very often is too long a delay, particularly where 

 spring sterihzation is practiced. 



It has been pointed out that the workmen may be a rather important 

 factor in transmitting the disease (page 88), and in cases where at trans- 

 planting time diseased seedhngs are handled it has been recommended 

 by Clinton^ that the hands be thorouglily washed in soap and ■water 



' G. P. Clinton: Chlorosis of Plants with special reference to Calico of Tobacco. Conn. Agr. 

 Exp. Sta. Rept., 1914, p. 417. 



