8 MISC. PUBLICATION 4 4 0, U. S. DEPARTMENT OF AGRICULTURE 



enter through the neck. Entering this way either in the field or after 

 topping, the decay progresses downward from the neck and often is 

 confined to one or two scales in the interior or to the central scales 

 (pi. 3, B). When this occurs the only external sign of the disease 

 is the loss of firmness, which can be detected by pressing the bulb. 

 This often forces a mushy, foul-smelling mass out at the neck. Any 

 injury on the onion, such as sunscald, freezing, and wounds due to 

 careless handling, may open the way for this rot. 



See Celery, Bacterial Soft Rot, page 46. 



(See 34.) 



BLACK MOLD ROT OF ONIONS 



(Aspergillus niger Van Tiegh) 



OCCURRENCE, SYMPTOMS, AND EFFECTS 



The black mold rot occurs on all varieties of the onion. Although 

 it attacks bulbs in the field it does not affect the yield. If rainy 

 weather prevails when the onions are pulled, the disease may seriously 

 blemish the stock, even though only the outer dry and dead scales 

 are involved. Under moist conditions in transit or storage the fungus 

 grows actively and rotting results; consequently the disease is of 

 considerable importance from the market point of view (pi. 4, C). 



Black mold rot may be a serious factor as a blemish or as a decay 

 of onions from any section of the country. However, it seems to be 

 a market factor more frequently in Texas and California stock than 

 in onions from other sections. 



The chief sign of this disease is the presence of black, powdery 

 spore masses of the fungus on the outside scale or between that and 

 the next inner one. When these masses occur between the scales 

 they tend to follow the veins (pi. 4, A. B). Sometimes there are no 

 apparent lesions and the fungus itself constitutes the only blemish. 

 At times the tissues show symptoms, such as sunken discolored areas, 

 which underlie the powdery fungus masses. Under dry conditions 

 the affected tissues are dry and papery and sometimes highly colored ; 

 under moist conditions they are semiwatery. 



CAUSAL FACTORS 



The pathogen, Aspergillus niger, is one of the soil fungi that live 

 on dead or dying plant material. From the soil it spreads to the 

 growing onion, attacking the outer scales and neck tissue. In transit 

 and storage it may spread from onion to onion by contact through 

 bruises and wounds, or by dissemination of the spores by air currents 

 and by mechanical means. 



Observations in the field, in storage, and in transit indicate that 

 moisture and high temperature favor the growth and spread of the 

 fungus. It grows slowly at 55° F. and makes most rapid develop- 

 ment at about 95°. 



CONTROL MEASrUF.S 



Protection from moisture in the field during and after pulling and 

 during transit and storage is advisable. The disease develops and 

 spreads less rapidly in cold storage than in common storage. 



(See 20, i". 50.) 



