108 I. WAHL 



a bridge between the appearance of susceptible crops. In some 

 cases activation of the organisms is timed properly and secures 

 persistence of the pathogen. In others, genetic and ecological 

 factors may offset this timing. 'Nature's imperfection is here 

 man's opportunity' -. Such an imperfection may appear in two 

 ways : obligate dormancy may operate against the ability of the 

 fungus to utilize suitable media that are discontinuous in their 

 sequence, or on the other hand, response to stimuli in the absence 

 of congenial hosts may endanger the survival of the pathogen. In 

 this respect, the so-called 'decoy plants'- which stimulate the 

 activation of the resting bodies but do not support their further 

 perpetuation and propagation, may greatly contribute to reducing 

 the potential inoculum in the soil. Introduction of these decoy 

 plants into crop rotation affords an effective means for controlling 

 root rots. Macfarlane^ showed that some plants resistant to the 

 clubroot disease agent Plasmodium brassicae may cause ger- 

 mination of its resting spores and thereby reduce their population 

 in the soil. Not less interesting is the fact that oospores of the 

 parasite respond to stimuli of plants taxonomically remote, such 

 as Tropaeolum majus. Reseda odorata, Papaver rhoeas, Lolium 

 perenne and others. 



RESTING STATES IN FUNGI 



(a) Resting spores 



Of all resting structures produced by the fungus, resting spores 

 have been most thoroughly investigated. It seems to be a matter 

 of general agreement^' ^ that one of the features some spores 

 which exhibit long dormancy have in common is their associa- 

 tion with cytological processes, such as nuclear fusion, or 

 fusion and division. Oospores of Phycomycetes, ascospores of 

 various fungi, teliospores of smuts and rusts, and basidiospores 

 in many Hymenomycetes serve as illustrations. 



Thick-walled resting spores of Phycomycetes formed by sexual 

 reproduction account for the protracted survival of the fungi in 

 soil in the absence of a susceptible host. According to Schaflfnit^ 



