160 Diseases of Economic Plants 



The round, brownish, purple-bordered spots turn ashen 

 in the center, and, when mature, often become so thin and 

 brittle as to drop out, leaving ragged holes. The sporiferous 

 hyphae upon the spot centers may be seen with a good lens. 



These spots frequently destroy the greater part of the 

 green tissue of the leaf, and thus its value to the plant as a 

 sugar producer, or even cause the leaves to die, in which 

 event they blacken and remain standing nearly upright upon 

 the crown. The death of the older leaves causes the crown 

 to elongate, suggesting the name "pineapple disease." 

 Dry weather followed by a damp spell favors the disease. 

 Infested refuse in manure may serve to spread the leaf-spot 

 but beet tops made into silage do not carry contagion. 



Sprayed plants in New Jersey gave a much cleaner foliage 

 and yielded 480 pounds of root as against 380 pounds from 

 the unsprayed plants from an equal area, a gain of nearly 

 26 per cent. While partial control may thus be had by 

 spraying, the cost may not, however, be justified. Crop 

 rotation and deep fall plowing should be practiced. 



Curly-top. ^^^' ^''^' -°^ — The first symptom appears in the 

 inner leaves, which curl inward from the entire margin toward 

 the midrib. Soon the veins become knotted on the dorsal 

 surface. Later, the entire plant is stunted and shows similar 

 changes; whole leaves curl tightly and the petioles remain 

 short and become bent. The roots develop an abnormal 

 number of fine rootlets, leading to the common name ''hairy- 

 root." (See also p. 162.) They are also tough and some- 

 times present a blackened area or cavity within. Curly-top 

 occurs annually on sugar beets in Colorado, Utah, Idaho, and 

 California, and in fact in all states west of Nebraska where 

 sugar beets are grown. In some years thousands of acres are 

 completely destroyed, the total loss during the last sixteen 

 years being estimated at $16,000,000. Even though the beets 

 grow to fair size, impurities lower their value for sugar 

 production and their seeding power is injured. The disease, 

 however, is not seed-borne. As to the cause, but little is 

 definitely known beyond the fact that this disease can be 



