PARASITISM AND RESISTANCE 375 



production of a disease. In some cases it is known that a fungus may 

 enter resistant or immune plants, as well as susceptible ones, but find the 

 conditions unfavorable for its establishment and further development. 

 Through wounds. A number of fungi apparently are unable to pene- 

 trate a healthy plant except through wounds. These may be insect 

 wounds, broken branches of trees, broken roots, etc. In addition, some 

 fungi which are capable of entering the host by other means may also 

 penetrate through wounds. Phymatotrichum omnivorum, the cause of 

 numerous root rots, commonly enters roots through wounds, although 

 these are not necessary. Fusarium, causing dry rot of potato, apparently 

 enters the tubers only after they have been wounded. Likewise most 

 of the wood-rotting Basidiomycetes enter the host only through wounds, 

 principally at broken or dead branches and at pruning or lightning and 

 fire scars. Here, the air-borne basidiospores must find suitable moisture 

 for germination and for penetration of the wood. Endothia parasitica 

 is said to enter the chestnut tree only through wounds that extend through 

 the corky layer. Ceratostomella ulmi is transmitted by the European 

 bark beetle, which introduces the spores into its feeding wounds. Bruises 

 and wounds of fruits and vegetables are common ports of entry for numer- 

 ous rot-producing fungi, such as Rhizopus nigricans on sweet potato. 

 Monilinia fructicola on stone fruits, Penicillium expansum on apple, and 

 P. italic^im. and P. digitatum on citrus fruits. 



PARASITISM 



A discussion of the action of the parasite after it enters the host is so 

 closely correlated with the defense of the host that it is difficult to discuss 

 each topic separately. For the sake of convenience, however, it seems 

 desirable to discuss some of the outstanding effects of fungi upon their 

 hosts and the methods by which the parasites obtain their food under a 

 separate heading of parasitism. 



Parasitism in plants. Parasitism may begin as soon as a fungus hypha 

 enters the host. The primary consideration is the securing of suitable 

 nutrients and water by the fungus. This may be accomplished by two 

 general methods, (1) by killing the cells of the host and obtaining food 

 from the dead cells, or (2) by establishing a close nutritional relationship 

 with the living host cells and absorbing the soluble nutrients without 

 causing necrosis. The fungi falling in the first group are the destructive 

 parasites, while those belonging to the second group have been called the 

 balanced parasites (Bessey, 1935). The latter group includes those fungi 

 known at present as obligate parasites (such as the Uredinales, Erysiphales, 

 and Peronosporaceae), and some other fungi (such as the Ustilaginalesand 

 Taphrina) which in their hosts obtain food only from living cells. 



The destructive parasites, as a whole, are strong producers of enzymet; 



