TOBACCO DISEASES AND THEIR CONTROL. 31 



commonly very small dark specks (spores) can be seen, are typical 

 of the disease. Considerable variation in the shape, size, color, and 

 relative size of the colored areas occurs, however, and correct diag- 

 nosis of the disease is dependent upon the occurrence of the spores 

 of the causal organism on the spot (PL IX, fig. 1). The diseased 

 spots ordinarily do not break and fall out of the leaf in dry or 

 cured tobacco. While the disease is not usually regarded as serious 

 and in fact may be regarded as desirable in mild form, yet in iso- 

 lated cases the damage from it is reported by individual growers to 

 be relatively very high. 



Cause. — The frog-eye disease of tobacco has not been carefully 

 studied, but sufficient work has been done to show that it is caused 

 by the fungus Cercospora nicotian-ae, to which it has long been 

 attributed. The long, narrow, septate spores appear in small 

 bunches on the surface of the central area of the spot and can readily 

 be seen with a small hand magnifying lens. Practically nothing is 

 known about the overwintering and spread of this particular dis- 

 ease, but from closely related diseases we know there is nothing un- 

 usual in overwintering in the spore form and subsequent dissemina- 

 tion from primary infections by wind and rain. 



Conditions favoring the disease. — Moisture is essential for the ger- 

 mination of the spores and subsequent leaf infection. Frog-eye is 

 especially likely to develop or spread following long periods of rainy 

 or damp weather, or even following heavy dews, which remain on the 

 basal leaves for long periods in the case of large tobacco. Mature 

 leaves appear to be more susceptible than unripe leaves, but no par- 

 ticular difference in varietal susceptibility seems to have been observed 

 in this country. 



Control. — Practically nothing is known about satisfactory means 

 of controlling this disease, and since it occurs infrequently to a 

 serious extent no effort, so far as known, has been attempted in that 

 direction. While it seems likely that the soil may be the common or 

 only source of infection in the spring, this is not established with 

 certainty. Until this fact is established recommendations for con- 

 trol practices can not be satisfactorily made. Where the disease oc- 

 curs on primed tobacco, something can be done in the case of heavy 

 outbreaks by attempting to keep ahead of the disease by harvesting, 

 but this is practicable, of course, to only a limited extent. 



BLUE-MOLD. 



Description. — In March, 1921, there appeared in the seed beds of 

 the shade-growing district of Gadsden County, Fla., and Decatur 

 County, Ga., a disease not previously found on tobacco in this coun- 

 try. This disease proved to be one commonly called " blue-mold " 

 in Australia and Malaysia, where it had often appeared as a more 

 or less serious trouble of tobacco. The disease spread very rapidly 

 in the seed beds in the Florida-Georgia section and for a time 

 seemed to threaten the industry, but fortunately it disappeared 

 rapidly, doing little or no damage in the field, owing, possibly, to a 

 change in weather conditions. The disease did not reappear in 1922, 

 although judging from the experience of other countries, it is not 

 unlikely that it may reappear at any time to a more harmful extent. 



