110 DISEASES OF TROPICAL PLANTS CH. 



not so prominent, but the leaves have a tendency to 

 become black and to fall quickly. In either case the 

 plants are very much weakened, and become an easy 

 prey to the fungi already mentioned. In fact, the 

 fungi are already at work before the discolorations are 

 complete. 



It will be readily seen that the important factor in 

 the control of this trouble, or combination of troubles, 

 is to produce strong, vigorous plants. Such plants will 

 not suffer materially from any of the fungi men- 

 tioned. The use of potash fertilizers has given most 

 excellent results in the southern part of the United 

 States. 



Anthracnose of the Stem. - - This characteristic 

 anthracnose or rot disease is due to the fungus 

 Glomerella gossypii, (South worth) Edg. It attacks 

 the stem, leaves, and bolls, but does not cause the 

 characteristic cankers. The spots or points of attack 

 are at first of a dull, reddish-brown colour, and are 

 slightly depressed. As they approach maturity they 

 enlarge, the spores are formed in great abundance, and 

 give them the dirty grey or pinkish colour. The spores 

 are carried in various ways, and the disease spreads 

 very rapidly. It is very severe in plants which have 

 become weakened from some other cause. Wounded 

 plants are especially susceptible, and the spores 

 frequently enter through the scars which are left 

 by the fallen leaves. It also attacks the young plants 

 at or just below the surface of the ground, causing 

 them to wither and die in the same manner as when 

 attacked by the "damping off" organisms. This early 

 attack of the young plants is no doubt due to the spores 

 being carried over with the seed. 



The best precaution against this disease is to get 

 seed, if possible, from the fields which are not affected, 

 or to select seed from plants which are clean. At the 

 same time the debris from the old fields should be 

 burned, and the crops should be rotated so as to put 

 clean seed on the clean fields. 



