DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 533 



As the spores occur in the soil, it is useless to treat the onion seeds 

 with chemic bodies. The most successful method of prevention is to 

 transplant the seedlings into beds known to be free from smut. Some 

 growers place sulphur (100 pounds to the acre) and air-slacked lime 

 (50 pounds) in the drills as the seeds are planted. 



Orange {Citrus aurantium, L.) 



Black-rot {Alternaria citri). — Only navel oranges are subject to 

 black rot which is recognized by the premature ripening, large size of 

 the fruit and its deep red color. The fungus gains entrance through 

 the navel end, because there imperfections of the skin occur. There 

 soon arises a black area of decay under the peel which remains isolated 

 for some time without spreading, therefore, the disease is not very 

 virulent. In Alternaria, the conidiophores are in bundles, always 

 unbranched and short. The conidiospores are club-shaped to flask- 

 shaped, divided and united into chains by thinner cells. 



Fruit-rot (Penicillium italicum, Wehm.). — A large part of the decay 

 of the orange and other fruits of the genus Citrus is due to blue and 

 green molds. These molds usually cannot enter uninjured fruits, and 

 so their attacks usually follow a bruise occasioned by careless handling, 

 or when the fruit falls from the orange tree. Penicillium italicum seems 

 to be more common than the other species, P. digitatum. Pure cultures 

 of this fungus can always be secured from decaying oranges in the 

 market, which have the blue-green areas of rot just beginning to appear 

 upon them. These areas are usually blue-green in the center sur- 

 rounded by white areas which are grouped usually into little white 

 patches toward the vegetative margin and the whole superficial colony 

 surrounded by an area of soft watery rot. Sometimes, as the colonies 

 become older, P. digitatum mixes with P. italicum. 



The conidiophores are short (looyu), or very long (6oo/i) and black 

 in media containing sugar. They average about 250/Li in length. The 

 conidial fructifications are up to 300^ or more in length, consisting usu- 

 ally of a main branch and one lateral branch, each producing a whorl 

 of branchlets bearing crowded verticils of conidiospores, 12 to 14/x 

 by 3/11. The chains of conidiospores are cylindric to elliptic, slightly 

 ovate, clear green by transmitted light and measure 2 to 3/x by 3 to s/i. 

 Decay of this sort can be prevented by careful handling of the fruit in 

 field and packing house. 



