[2 MISC. PUBLICATION -4 4 0, U. S. DEPARTMENT OF AGRICULTURE 



CONTROL MEASURES 



It is not advisable to plant the same field to onions year after year. 

 Crop rotation is especially beneficial in regions where bulb rot is 

 likely to become severe. Affected stock should not be put in storage 

 or shipped long distances but should be put into immediate consump- 

 tion. If necessary to store diseased stock the development of decay 

 will be retarded by temperatures below about 45° F. 



(See 13, 14, 17, 56, 56.) 



GRAY MOLD ROT (NECK ROT) OF ONIONS 

 (Botrytis allii Mumi., B. byssoidea Walker, and B. squamosa Walker) 



OCCURRENCE, SYMPTOMS, AND EFEECTS 



Although gray mold rot causes practically no damage to the grow- 

 ing crop, it is one of the most serious troubles affecting onions fol- 

 lowing harvesting and during transit and storage. It occurs to some 

 extent in all onion-growing regions and frequently is the limiting 

 factor in determining the marketability of a crop. At times 50 per- 

 cent or more of a crop is lost through the development of neck rot 

 during storage. Apparently all varieties of onion, shallot, and garlic 

 are susceptible to this disease, but the white ones are the most seri- 

 ously affected. 



Infection usually takes place through the neck tissues of the bulbs 

 at harvesttime. This characteristic mode of infection has led to the 

 use of the term "neck rot" for describing this disease, although fre- 

 quently infections take place through wounds made in the base or 

 sides of the onions. 



The first indication of the disease is a softening and water soaking 

 of affected scales. Soon this tissue becomes grayish and finally gray- 

 ish brown as the mycelium of the fungus develops within (pi. 6, B. C, 

 E. H). Later, a dense layer of white to grayish mold develops over 

 the surface of the affected region. On the mold growth in the older 

 lesions on the outside scales and between the scales at the neck, fine 

 grayish-brown, powdery masses of spores are bome (pi. 6, G, H) . 

 This development of gray mold and grayish -brown spores has given 

 rise to the term "gray mold rot" as a designation for this decay, par- 

 ticularly when it occurs at the base or sides of the bulbs and not at 

 the neck. The decay develops laterally from the point of infection, 

 as well as down through the fleshy scales. The line of demarcation 

 between the diseased and healthy tissue is quite definite, although the 

 advancing edge of the lesion usually shows a slight water soaking of 

 the fleshy tissues. No odor accompanies this decay. In advanced 

 stages of decay the affected tissues become sunken and fairly firm. 

 In stored onions that have developed advanced decay at the neck, 

 there may be found firm black bodies (sclerotia) y 32 to 14 i R eh m 

 diameter on the decaying tissues (pi. 6, A, B). 



CAUSAL FACTORS 



Three closely related species of Botrytis cause types of gray mold 

 rot or "neck rot" that are somewhat similar. The rot caused by B. 

 alUi is probably the most common form (pi. 6, l?. H). It is charae- 



