TOBACCO DISEASES AND THEIR CONTROL. 11 



the soil remains wet for a considerable time, the parasite may again 

 obtain a foothold and make a fairly rapid development, with accom- 

 panying decay of the plant, before being again checked by unfavor- 

 able conditions. 



Control. — The control of this disease naturally centers around the 

 use of disease-free seedlings for transplanting. When pulling plants 

 all those infected should be discarded. Soil sterilization and other 

 measures should be used to prevent damping-off in the seed beds 

 where sore-shin is of common occurrence. The disease is not usually 

 evident sufficiently early to permit replanting. Since wet, poorly 

 drained soils favor the disease, they should be avoided if possible, 

 and fields frequently giving a high percentage of the disease may 

 be advantageously rotated. 



BLACK-SHANK. 



Description. — A serious disease of tobacco which has long been 

 known in Sumatra and Java as " lanasziekte " or " bibitziekte " is 

 now said to be found in the shade-growing sections of Florida and 

 Georgia. This disease may attack the plants in the seed beds, caus- 

 ing a rot similar to damping-off. It is in the field, however, that the 

 greatest damage is done. The symptoms may be a general wilting 

 of the plant, following marked signs of decay at the base of the 

 stalk and extending upwards as far as 24 inches. In this respect it 

 resembles to some extent severe cases of sore-shin previously de- 

 scribed, but it more often resembles Granville wilt. The rot may 

 also extend into the pith and roots of the plant. This disease also 

 attacks the leaves, producing large brown blotches, especially after 

 rains. The loss may frequently be very severe, involving most or all 

 the plants in the field (PI. Ill, fig. 1). The introduction and 

 spread of this disease into tobacco districts of this country should 

 be guarded against. 



Cause. — In Java and Sumatra considerable work has been done 

 upon this disease. It has been shown to be due to a fungus which 

 has been named Phytophthora nicotianae. Experiments in this 

 country indicate that the same fungus is concerned. This parasite 

 is closely related to the fungus causing our common potato late- 

 blight, which produces a serious disease of potatoes under favorable 

 conditions for its development. The mycelium and spores of the 

 fungus may be spread in various ways, and apparently in the case of 

 tobacco they may persist in the soil for at least two years. 



Conditions favoring the disease. — Aside from wet or humid 

 weather little is known concerning the conditions favoring the 

 disease. It is not unlikely that high temperatures are also favorable. 

 Varieties and strains differ markedly in susceptibility. 



Control. — Seed-bed sanitary measures heretofore described (p. 4) 

 are important. Plants showing signs of this disease, of course, 

 should not be transplanted, and infected seed-bed areas should be 

 sterilized or avoided. When fields have once become infested it is 

 unsafe to plant them to tobacco again. Especial care should be taken 

 to prevent the transfer of infested soil from the diseased areas to 

 new fields, by men, animals, tools, or other equipment. If the disease 

 should become annually serious in any section the development of 

 disease-resistant strains offers a promising method of control. 



