DISEASES OF CULTIVATED PLANTS 571 



rise to the name of "black shank" among the growers. These dark 

 lines or blotches often appear upon etiolated portions of the stem 

 and are almost black in color. In very severe cases the tip of the 

 sprout wilts and dies. No appearance in the field has so far been 

 observed that would distinguish hills diseased with black rot from 

 those attacked by some other of the numerous rots; but the dark 

 sunken areas on the potato and the black discolorations of the sprouts 

 can scarcely be confused with any other sweet-potato disease. This 

 infection may take place either through the medium of spores or 

 by the growth of mycelium from the diseased areas themselves. Dis- 

 eased sprouts planted in the field produce diseased roots which may 

 spread the disease to other hills either through the soil directly or by 

 means of the numerous fibrils from other plants. These infected 

 areas, although perhaps inconspicuous at first, grow steadily in 

 diameter not being checked by digging, and when the potatoes are 

 stored for keeping continue to grow in the root and at the same 

 time to produce the various forms of spores. These reproductive 

 bodies when supplied with sufficient moisture are capable of infect- 

 ing, unaided, sound potatoes through their eyes. Thus one diseased 

 potato when stored in a bin of healthy ones is capable of infecting 

 all those in the bin and causing them to rot in a short time. 



1. The most important precaution to be taken in combat- 

 ing the disease is to plant only perfectly healthy seed in the hotbed, 

 even if it is necessary to import such. This preventive measure is 

 most essential, as diseased seed will give diseased sprouts, which in 

 turn will grow a crop of worthless potatoes. 



2. The selection of healthy sprouts is plainly necessary in 

 case the fungus gets into the hotbeds, and under no circumstances 

 should diseased plants be put into the field. The test of using cop- 

 per fungicides in the hotbed has not been made, but from analogy 

 seems to promise assistance. If the fungicide is used the shoots 

 should be kept green with it until pulled. 



3. Fields which have become so impregnated by the disease 

 that they refuse to grow profitable crops had best be added to the 

 regular farm rotation. This method will, if continued for several 

 years, allow the accumulated infective material to burn itself out by 

 consuming all available food material in the soil. 



4. Decaying roots and the refuse after digging should be 

 carefully removed from the field and burned, as such debris adds 

 to the food of the parasite. 



5. The use of large quantities of barnyard manure probably 

 favors the development of the trouble, since it adds greatly to the 

 decaying vegetable matter of the soil. Where the use of commercial 

 fertilizers can be made to take the place of manure it will certainly 

 be desirable to make the change. 



6. Although no experiments have yet been completed upon 

 the matter, it is probable the spread of the disease in the bin may 

 be checked by dipping the roots in one of the copper mixtures, pre- 

 ferably the ammoniacal solution, before storing for the winter. What 

 effect tobacco smoke or the fumes of sulphur would have in checking 



