Feb. i, 192 1 Onion Smudge 687 



Penetration of underlying dry scales by the fungus causes similar spots, 

 which are commonly surrounded by yellowish borders. On the fleshy 

 scales the disease first appears as minute, sunken, yellowish spots which 

 gradually enlarge and often coalesce. As the disease progresses, the 

 black stroma of the fungus usually appears; and, with the collapse of the 

 host cells, spots very similar to those on the dry outer scales result. 

 When the dark-colored stroma does not develop before the scale has 

 entirely dried down, the affected portions appear as slightly raised, yel- 

 lowish spots, giving to white onion sets an unnatural color which is 

 almost as detrimental to their market value as the black, smudgy spots. 



The disease makes its appearance early in July under Wisconsin con- 

 ditions, the fungus living on the outer dead scales and increasing in 

 amount up to harvest time, when the outer two or three scales may be 

 affected. From this time on it penetrates farther into the bulbs, progress 

 depending upon environmental conditions. Badly diseased bulbs tend 

 to sprout prematurely in storage. In most severe cases the fungus pene- 

 trates the entire bulb and causes a complete collapse of the fleshy scales. 



The foregoing description applies to the disease as it appears on white 

 onions. On colored varieties (red, yellow, and brown) the fungus is con- 

 fined, with rare exceptions, to the neck of the bulbs where there is 

 little or no pigment in the tissue, and the symptoms in these cases resem- 

 ble closely those on the corresponding parts of the white varieties. 



On shallots the disease appears as smudgy spots very similar to those 

 on onion and is confined to the outer leaves or scales. On leeks similar 

 symptoms prevail. 



OTHER DISEASES LIKELY TO BE CONFUSED WITH SMUDGE 



Onion bulbs as they mature are subject to attack by a number of 

 fungi which develop saprophytically on the dead outer scales and pro- 

 duce symptoms which may easily be confused with those of smudge. 

 The most common of these are two species of Macrosporium (Macro- 

 sporium porri EH. and M. parasiticum Thum.) (33), and a species of 

 Phoma, probably Phoma alliicola Sacc. and Roum. (24). The Macro- 

 sporiums produce irregular, dark green spots which are due to ram- 

 ification of the mycelium through the dead scales, but which lack the 

 stromata and more or less regular outline of the smudge spot. In a 

 moist atmosphere the fungi fruit and develop a dark green mold due to 

 the production of conidia (PI. 81, F, G). In rare instances black peri- 

 thecia of M. parasiticum are found on the outer bulb scales. Phoma 

 produces small black pycnidia which are often difficult to distinguish 

 macroscopically from the stromata of the smudge fungus. It is com- 

 monly associated with M. porri (PI. 81, H). These two fungi commonly 

 attack both white and colored varieties, and in the latter case the pig- 

 ment in the outer scales is usually destroyed, giving a symptom which 

 is known in the trade as "onion blotch." 



