688 Journal of Agricultural Research voi.xx, No. 9 



Onion smut is sometimes confused with smudge, especially when the 

 former occurs on mature bulbs. In such instances, however, smut usu- 

 ally causes slightly raised, linear lesions which on colored varieties are 

 commonly accompanied by more or less destruction of pigment. The 

 exposure of the powdery spore mass upon breaking of the lesion estab- 

 lishes the identity of the smut fungus. 



ECONOMIC IMPORTANCE 



The importance of smudge as a detriment to the onion crop may 

 properly be considered from three standpoints — (i) that of reduction of 

 market value as a result of marred appearance, (2) that of actual shrink- 

 age of the bulbs in storage, due to fungus invasion, and (3) that of 

 increased sprouting of onion sets during storage. Thaxter (33) calls 

 attention to the reduction of market value caused by smudge, citing 

 an estimate by one grower of an actual loss of several thousand dol- 

 lars to his crop in one season on this account. There is little doubt 

 that marked spotting by this disease hampers greatly the disposal of white 

 onions, since they are usually grown at a greater expense than colored 

 varieties for a fancy trade which is prone to discriminate against dis- 

 figured stock. Under prolonged storage smudge causes a distinct shrink- 

 age of the bulbs and promotes premature sprouting. These last two 

 factors are not usually of material importance on large bulbs, but they 

 are of much significance with respect to onion sets. The latter are usu- 

 ally harvested in August and September and kept in storage until March. 

 The small bulbs are thus subjected to fungus invasion for several 

 months, and data presented later in this paper show that in badly dis- 

 eased sets the shrinkage may be doubled by smudge during this period. 



Sets which sprout badly during storage are a total loss to the owner, 

 since they will not stand shipping and must be discarded. Much of the 

 sprouting of white sets in storage is due to severe attacks by smudge. 

 Experimental data in support of this statement are given later in this 

 paper. 



It will be seen, therefore, that smudge is of greater importance than 

 would be suspected from casual observation. In the Chicago district 

 alone, where approximately 1 ,000,000 bushels of sets are grown annually, 

 the aggregate loss due to shrinkage in weight and sprouting probably 

 runs into many thousands of dollars. 



CAUSAL ORGANISM 

 MORPHOLOGY 



The morphology of the causal organism has previously been discussed 

 by Berkeley (4), Thaxter (33), Stoneman (32), Stevens and True (30), 

 and Kempton (16). 



Mycelium. — The mycelium ranges from 2 to 8 microns in width, is 

 septate and branching, varying widely with age as to color and size. It 



