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



The signs of the disease are especially distinctive in one respect, 

 namely, in the appearance of the white to light-purple or bluish 

 mildew, -which is usually found on the lower surface of the leaf. 

 The leaf tissue is broken down by the mildew and subsequently turns 

 brown and dries up in patches of irregular size and shape, frequently 

 limited by the veins of the leaf (PL IX. fig. 2). In severe cases the 

 seed bed as a whole has a patchy and scalded appearance. Only the 

 leaves appear to be attacked. 



Cause. — Blue-mold is caused by a fungus belonging to the group 

 of downy mildews, some of which are well-known parasites on other 

 crops. The tobacco mildew is known as Peronospora hyoscyami, 

 having been first found on black nightshade, a plant related to 

 tobacco. The fungus produces spores in great numbers. These are 

 powdery and light and easily carried by the wind through the beds, 

 which accounts for its rapid spread through beds when once infected 

 if favorable weather conditions prevail. These ordinary spores are, 

 however, very short lived, although other longer lived spores may be 

 produced under certain conditions and carry the fungus over un- 

 favorable periods, thus accounting for it* overwintering when this 

 occurs. 



Conditions favoring the disease. — Parasites of this class are usually 

 very sensitive to environmental conditions and consequently are quite 

 sporadic in their occurrence. Moist Avarm weather and sometimes 

 alternating cool nights and warm days are most favorable for in- 

 fection. In the Florida-Georgia section the outbreak was seemingly 

 associated with several days of heavy dew and fog in the morn- 

 ings. "With the cessation of these dews and fogs the disease became 

 less conspicuous and finally disappeared entirely. 



Control. — As soon as signs of this disease appear in a seed bed the 

 plants in the infected area, together with the surrounding bordering 

 plants, should be destroyed at once, preferably by applying form- 

 aldehyde solution (1 to 25). This should be followed up by burn- 

 ing over or other means of eradication. All the bed should then be 

 sprayed daily with Bordeaux mixture (2-2-50) in an attempt to hold 

 the disease in check, destroying with formaldehyde solution any new 

 infection which may appear. 



In the event of failure to control the disease in the seed bed it is 

 inadvisable to transplant diseased plants to the fields if it is at all 

 possible to obtain healthy plants elsewhere. Although the disease 

 appears to be primarily a seed-bed trouble, it is not unlikely that 

 it may prove equally serious in the field, weather conditions being 

 favorable. In case of repeated occurrence? of the disease, general 

 seed-bed and field sanitary measures, such as have been previously 

 described, should be undertaken, but until more experience with the 

 disease is had in this country, further recommendations can not be 

 given with any certainty as to their value. 



OTHER PARASITIC LEAF-SPOT DISEASES. 



Although a considerable number of other parasitic leaf-spot dis- 

 eases are reported from foreign countries, they do not seem to occur 

 in this country. 



In the early nineties three different fungi were reported as causing 

 different leaf spots in North Carolina. A "brown rust'' concentri- 



