44 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE. 



than the normal leaf. As the decay proceeds the>e spots run together, 

 and the infected areas break up and fall out at the least disturbance 

 (PL XVI. rig. 1 ) . This trouble is most common in tobacco which is air 

 cured rather than in any system of curing using more or less heat. 

 Under favorable conditions for its occurrence large losses may result. 



Cause. — It seems fairly well established that at least some of the 

 symptoms described for shed-bum are due to microorganisms. 

 There is not good agreement, however, as to just which organism or 

 organisms are primarily concerned in all cases. The most recent 

 experimental evidence seems to indicate that several fungi are cap- 

 able of damaging the leaf in a similar manner. Species of Alter- 

 naria. Fusarium. Botrytis. and other fungi and bacteria are prob- 

 ably most instrumental in actual decay. 



Conditions favoring attack. — Tobacco leaves ordinarily are not at- 

 tacked by these organisms while they are still in the green or yellow 

 stage of curing. Xormally. in the air-curing process no danger of 

 damage exists until some time after harvest, or when the leaves are 

 turning from the yellow color to the final stage. At this stage any- 

 thing that is conducive to excessively high humidity may bring 

 about shed-burn, especially if at the same time the temperature is 

 quite high. The ideal conditions for air curing itself are a relatively 

 high temperature (80° to 100° F.) together with a relatively high 

 humidity (80 to 90 per cent). When the relative humidity rises 

 above 95 per cent, however, there is considerable danger, especially 

 if little or no circulation of air prevails. Under such conditions 

 little pockets of air form between closely hanging leaves and between 

 leaves and stalks in which the actual humidity reaches saturation, the 

 leaf surface becoming actually moist through precipitation of moist- 

 ure from the air or by excretions from the leaf which are not carried 

 off. thus producing a favorable medium for the growth of micro- 

 organisms which can enter the leaf tissue and consequently damage 

 the leaf. It follows, therefore, that the close hanging of large 

 tobacco or an improperly ventilated shed are conducive to shed- 

 burn. Foggy or damp weather at the critical stage of curing, es- 

 pecially if it persists for several days with a moderate temperature, 

 should be guarded against by control measures. 



Control. — The principles of control of shed damage are primarily 

 concerned with proper ventilation to secure adequate circulation of 

 air throughout the shed or raising the temperature within the shed 

 by artificial heat, thereby reducing the relative humidity of the air. 

 The former is dependent upon the proper construction of the shed, 

 together with proper management of the ventilating system. 



The quantity of moisture the air can hold varies greatly with the 

 temperature, The ratio of the amount of moisture actually in the 

 atmosphere at any time to the amount which can exist in the air 

 at that temperature is known as relative humidity. The capacity of 

 the air for holding moisture i= doubled for every increase of I - 

 to 2o c F. in temperature, and consequently its relative humidity 

 is reduced by half. Under normal conditions the temperature of 

 the curing shed rise- in the daytime and the relative humidity falls. 

 As the temperature falls at night the relative humidity rises often 

 above 95 per cent but again falls the following day. During rainy 

 or foggy weather the relative humidity, of course, remains hiuh. 



