MARKET DISEASES OF FRUITS AND VEGETABLES 13 



terized by its compact gray-brown mycelium and abundant develop- 

 ment of spores, and often by the formation of crustlike sclerotia about 

 the necks of storage onions. The decay caused by B. byssoidea (pi. 6, 

 A, B, C) cannot easily be distinguished from the first-mentioned 

 form. It differs chiefly in the production of a greater abundance of 

 light-gray fluffy mycelium. The rot caused by B. squamosa has been 

 found on the market only on white onions. The mycelium and spores 

 are inconspicuous, and the first evidence of the disease is often the 

 appearance of very thin small black sclerotia adhering to the dry 

 outer scales at the neck of the onion (pi. 6, Z>, E, F). The decay is 

 much slower in development than that induced by the other two 

 organisms, and when advanced stages are reached the affected tissues 

 are darker brown than in either of the other types of gray mold rot 



Each of the neck rot organisms is able to grow throughout a wide 

 range of temperature. Below 37° and above 91° F. their growth is 

 very slow. At 68° to 77° most rapid growth is made in pure culture, 

 but inoculation experiments have shown that temperatures between 

 59° and 68° favor the development of decay. High humidity favors 

 infection and the spread of decay from one bulb to another by con- 

 tact in storage. Cool, wet or humid weather favors the development 

 and spread of the fungus in the field, particularly just previous to 

 and during harvesting time. Hot, dry weather not only checks the 

 growth of these fungi but it also causes the onions to mature more 

 rapidly, so that the necks cure quickly and become dry before infec- 

 tions take place. 



These fungi live from season to season as sclerotia in the soil and 

 as sclerotia and mycelium on infected bulbs. They also live on dead 

 onion leaves and other plant debris during the growing season. The 

 bulbs become infected by wind-borne spores at maturity and during 

 the harvesting and curing processes. 



CONTROL MEASURES 



If the weather permits a satisfactory maturing of the crop and 

 prompt thorough curing at harvesting time, the neck rots caused by 

 Botrytis species are easily controlled. When the onion necks are 

 moist and curing is slow and unsatisfactory, the application of warm 

 air by artificial means has been found advantageous in rapidly dry- 

 ing the neck tissues. 



Because colored varieties are less susceptible to neck rot than white 

 ones, a change in varieties may be advisable in some localities. 



Onions should be kept in a cool, dry storage. Even cold-storage 

 temperatures of 32° F. will not completely check the development of 

 gray mold rots, if the bulbs are already infected when placed in 

 storage. 



(See 18, 21, 1^1, Ifi, W* 50.) 



GREENING (SUNBURN) OF ONIONS 



One type of injury caused by sunlight is known as greening or 

 sunburn. It is caused by exposure of the bulb to moderate light 

 during the growing period or after harvesting. The sunlight causes 

 a development of green coloring matter in the exposed tissues but 



