292 ASSOCIATIVE EFFECTS AMONG FUNGI 



duction of mature lesions. Evidence strengthening this supposi- 

 tion is found in the frequent occupancy of lesions by secondary- 

 invaders. 



Wolf (1916) found fungi belonging to Gloeosporium, Fu- 

 sarium, and Phoma associated with citrus canker, whose primary 

 cause is Phytomonas citri. Of these fungi a species of Phoma 

 was noted to be capable of secreting cellulase, invertase, diastase, 

 and maltase, and from this fact it was concluded that this Phoma 

 is actively associated with processes involved in the destruction of 

 citrus tissues. 



GENERAL CONSIDERATIONS 



The numerous observations cited in the foregoing account may 

 be assumed to prove the obvious fact that fungi interact, but the 

 assumption is not warranted that certain combinations are always 

 antagonistic or stimulatory, as the case may be, under all condi- 

 tions. Combinations that are antagonistic in culture may not be 

 so under natural conditions, as Broadfoot's (1933) experiences 

 with Ophiobolns gramims and certain other soil-borne organisms 

 indicate. Whether an associative interaction is beneficial or in- 

 jurious may prove to be a matter of adjustment of climatic, 

 edaphic, and biotic factors whose balance is delicately poised. 



The possibility that fungi occur within plants that appear to 

 be entirely normal is worthy of consideration, and it is indicated 

 that systematic attempts should be made to isolate fungi from 

 "normal tissues." If this were done, it should not come as a 

 surprise to discover that certain fungi may prove capable under 

 some environmental conditions of producing serious diseases, 

 under others of being benign, and under still others of inducing 

 no evidence of abnormality. No doubt many of the fungus asso- 

 ciations in the soil are intricately complex. Whether stability is 

 ever attained among soil fungi or whether a condition approxi- 

 mating such a vegetational climax as a prairie or a hardwood forest 

 ever obtains among fungi is extremely doubtful. 



Attention has been centered in this account on the effect of 

 one fungus on another, to the almost complete exclusion of the 

 interaction of bacteria, protozoa, and green plants with fungi. 

 Much has been learned from studies of these problems, but these 

 topics are regarded as outside the scope of the present summary. 

 Interactions between parasites and saprophytes on living host 



