16 MISC. PUBLICATION 440, U. S. DEPARTMENT OF AGRICULTURE 



it into the field. The disease appears shortly before harvest. The 

 spores, which are borne in abundance in the aeervuli, are carried in 

 drops of water to other scales where they germinate in a few hours, 

 starting new lesions. If the weather is warm and moist, the fungus 

 enters the onion scale and produces a new crop of spores in a few days. 

 If favorable conditions continue, a relatively small amount of disease 

 in the field before harvest will supply enough spores to blemish a crop 

 severely during harvesting or while the crates are stacked in the field. 

 The fungus grows throughout a wide temperature range (36° to« 

 89° F.) but develops best at about 78°. The spores germinate at 

 temperatures ranging from 40° to 89°, the optimum lying between 

 68° and 78°. Infection may occur at 50°, but development is very 

 slow below 68°. The disease develops and spreads most rapidly in 

 the field when the soil temperature ranges between 68° and 86° and 

 when there is an abundance of rainfall. Under favorable conditions 

 the spores germinate and penetrate the scales in less than 24 hours. 



CONTROL MEASURES 



Except for planting colored varieties, which are highly resistant, 

 no measures have been worked out thus far for control of the disease 

 in the field. Control of the disease in the harvested crop depends 

 upon protection of the onions from rain, rapid and thorough curing, 

 and subsequent storage in a well-ventilated room at about 33° to 36° F. 



(See 4, 29, 33, 41, 45, 46, 52, 53.) 



SMUT OF ONIONS 

 (Urocystis cepulae C. C. Frost) 



OCCURRENCE, SYMPTOMS. AND EFFECTS 



Smut occurs in most of the important northern onion-growing sec- 

 tions, including New York, New Jersey, Ohio, Wisconsin, Illinois, 

 Iowa, and Oregon. It apparently does not occur in the southern 

 onion sections even though sets that come from infested areas in the 

 North are used in planting. 



Smut is important on onion, leek, and shallot primarily from the 

 point of view of production, because it causes poor stands and stunts 

 the growth of affected plants. The bulbs of affected onions usually 

 are so small that they are unmarketable, although at times they grow 

 large enough to sell. Smutted bulbs shrink excessively in storage. 

 Green onions and onion sets affected with smut are occasionally found 

 on the market. 



The disease appears as dark-colored, slightly raised streaks or 

 blisters on the leaves and young bulbs. These pustules are filled with 

 greenish-black, powdery masses of spores. Sets may be greatly 

 shrunken, with the whole exterior covered with blisters. On such 

 specimens, and on others that survive and get into trade 1 channels, the 

 lesions consist of slightly raised brown to black pustules, which are 

 most prevalent near the base of the bulb. 



The term "smut" is often incorrectly used by the trade to indicate 

 smudge and black mold rot. 



