MARKET DISEASES OF FRUITS AND VEGETABLES 17 



CAUSAL FACTORS 



The spores of the causal fungus (Urocystis cepulae) are resistant to 

 cold and serve to carry the organism through the winter in the soil. 

 Diseased bulbs and sets are a means by which the fungus is carried 

 over the winter and are also an important source of inoculum when 

 they are set out in the field. 



Only very young plants are subject to infection. After the seed- 

 lings are about 3 inches high they become immune to further infec- 

 tion. The disease does not develop nor spread in transit or in 

 storage. 



CONTROL MEASURES 



Because infection proceeds from the soil and from diseased onions, 

 it is advisable not to return infested onion refuse to the soil. 



Onions are not susceptible to infection after they are 3 to 4 inches 

 high ; consequently, the transplanting of noninf ected plants even into 

 smut-infested soil is a practicable measure. Onions grown from sets 

 also are not susceptible and can be planted in smut-infested soils 

 without risk of a smutted crop. 



Where the use of sets or seedlings is impracticable, as in the larger 

 onion-growing sections of the North, smut can be controlled by the 

 application of a solution of formaldehyde ( 1 pint formaldehyde in 16 

 gallons of water) in the furrow with the seed. The diluted solution 

 should be applied at the rate of 200 gallons per acre, or 1 gallon to 

 about 185 feet of row. 



(See 1,2,39,4%, 50, 51.) 



SUNSCALD OF ONIONS 



Onions grown hi regions where the temperature is high and the 

 sunlight extremely bright are often severely affected with sunscald. 

 This injury takes place at harvest-time when the bulbs are exposed to 

 heat and bright sunlight. Immature and moist onions are generally 

 injured most severely. The tissues of the exposed area of the onion 

 are killed and become soft and slippery, but soon lose moisture rapidly 

 by evaporation, so that sunken leatherlike areas are produced, which 

 are usually bleached almost white. The scalded areas vary from 

 y 2 to iy 2 inches in diameter, depending on the exposure and the size 

 of the onion. 



Sunscald is often a serious factor in marketing onions, not only 

 on account of the blemishes but because various bacteria and other 

 organisms enter through the dead areas and cause decay during 

 transit and marketing. Bacterial soft rot is usually the most 

 important disease that follows sunscald. 



(See m.) 



WHITE ROT OF ONIONS 



(Sclerotium cepivorum Berk.) 



OCCURRENCE, SYMPTOMS, AND EFFECTS 



White rot has long been known in the British Isles, Holland, 

 and France, where it attacks onions and leeks; in Italy and Spain, 



303920°— 41 3 



