TOBACCO DISEASES AND THEIR CONTROL. 3 7 



Control. — More facts in regard to the cause, overwintering-, and 

 dissemination of mosaic must be ascertained before control measures 

 especially applicable to this disease can be assured. In general, how- 

 ever, the measures which are applicable fall under the head of seed- 

 bed sanitation. The location of seed beds a. considerable distance 

 away from weedy patches of ground where the disease may be hiber- 

 nating in perennial plants, soil sterilization to kill weed seeds or any 

 other possible* infective material in the soil, and persistent destruc- 

 tion of solanaceous weeds around seed beds and in other locations are 

 recommended. Plant-bed frames or covers previously used with beds 

 in which mosaic plants appeared should not be again used without 

 sterilization. Special precautions should be taken to prevent the 

 introduction into the seed bed of any trash from the preceding- 

 year's crop. Before planting, the beds should be carefully examined 

 for mosaic plants, and even if only a very slight infection is notice- 

 able it is advisable not to use any plants from such beds if good 

 plants can be obtained elsewhere. TYhere infected plants appear to 

 be numerous in areas; of the bed, it is practically certain that a poor 

 crop will result from transplanting from such beds. Infection may 

 occur in the seed bed, however, without being noticeable. If it is 

 found that the symptoms show up in the field early in the season on 

 a large percentage of the plants, it is usually advisable to disk up 

 and replant. No possible means of controlling the disease in the 

 field are known. On the other hand, the disease does not ordinarily 

 spread in the field early enough to cause any considerable damage. 



FRENCHING. 



Description. — The disease of tobacco commonly known as french- 

 ing occurs in practically all tobacco districts of the United States 

 and in many foreign countries. It is often referred to under other 

 names, such as shoestring, sword-leaves, strap-leaves, rosette, and 

 polyphyllie. The disease is often mistaken for the mosaic disease and 

 confused with it on account of the leaf deformities and chlorosis pro- 

 duced which are also characteristic of mosaic. There is no relation 

 between the two diseases, however, from the standpoint of cause, as 

 sometimes seems possible when the two diseases occur simultaneouslv 

 on the same plant. 



The symptoms of frenching vary greatly in their intensity of ex- 

 pression. The earliest stages of the disease, especially in very young 

 leaves, are characterized only by a yellowish or chlorotic appearance, 

 with some increased thickness and brittleness of leaves. As the 

 growth of these leaves continues they will show an early tendency 

 toward narrowness and mottling. Later, the margins of the leaves 

 may curl downward and the younger leaves in the bud increase to 

 abnormally high numbers, though being usually closely bunched. 

 In later stages of the disease the leaves are frequently normal in 

 color but extremely narrow in proportion to their length, quite thick, 

 and ruffled at the margin (fig. 20). In the most severe cases the 

 number of these stringy leaves becomes extraordinarily high, often 

 reaching more than five' times the normal number on the stalk, which 

 is usually considerably shorter than normal, hence producing a 

 distinctive rosette appearance. More commonly, however, the disease 

 is limited to chlorosis of the top leaves. The losses from this disease 



