80 MASS. EXPERIMENT STATIOX BULLETIN 203. 



REL.\Tr\'E Susceptibility of Varieties. 



In the seed-bed the different varieties of tobacco seem to show no dif- 

 ference in susceptibility to the disease, but under field conditions the 

 spread of the disease seems to be governed somewhat by the manner and 

 t}-pe of growth of the variety. Shade Cuban seems to be but little 

 affected with the trouble after setting in the field. Here the protection 

 afforded by the tent undoubtedly has something to do -n-ith it, as the 

 rain and wind is broken; also the open habit of growth, with the leaves 

 relatively far apart, minimizes the chances for a rapid spread. The same 

 is true of the Havana to a less extent, the leaves of this variety, also, 

 haA-ing an upright habit of gro-wth, and touching but Httle. 



The practice of priming the leaves of the Shade Cuban and some of the 

 Havana is also an important factor in reducing the spread of the disease, 

 as the first priming is made early, and this includes usually the infected 

 leaves. In this way, as wiU be noted under the subject of removal of 

 diseased leaves, a large amount of field infection probably is prevented. 

 Perhaps Broadleaf suffers most of aU the varieties, as, when the leaves 

 get their growth, they droop and touch each other and also leaves of 

 near-by plants, and thus infection occurs more easily. 



During this past season a great deal of late infection in the field has 

 been observed, i.e., many fields of Broadleaf showed practically no infec- 

 tion prior to topping, but after a few days, as the plants were maturing, 

 showed a rapid spread of the disease. Cases have been called to the 

 attention of the -miters where the tj^pical halo spots were found on all 

 leaves of the plant. During this past season there was a large amount of 

 "rust" on Broadleaf in some sections, and this has been confused by 

 growers and others with true wildfire. The WTiters wish to point out the 

 possibHit}' that part of this spotting was caused by excess of nitrogen and 

 deficiency of potash, not sufficient in the case of the latter to show sjonp- 

 toms of potash hunger, but enough to cause a rusting. This rusting occurs 

 on some of the tobacco in Carohna, and has been demonstrated to be a 

 result of the above-mentioned conditions; and this riiay be true of some 

 of the spotting of Broadleaf, and possibly the other varieties, since very 

 little potash has been used during the past four or five years. There is, 

 however, no experimental evidence to warrant this statement as a fact 

 so far as our Massachusetts conditions are concerned. 



Removal of Diseased Pl.^-ts or Leaves from the Field. 

 When the disease has become established in the field, spraying opera- 

 tions are not practicable. The only promising method of control at this 

 stage seems to be the elimination of diseased plants or leaves. If infection 

 in a field is found to be pretty general early in the season, it is perhaps 

 best to remove all the plants and replant from a bed known to be free 

 from the disease, if such can be found. In two fields containing thirty- 



