ENVIRONMENTAL FACTORS 65 



In contrast to those just mentioned, there is that general sub- 

 division of diseases which we have designated as leaf spots, stem 

 rots, and fruit decays. The fungi producing these affections fre- 

 quently, though by no means always, kill the tissues as they pene- 

 trate the host. In other cases they enter and produce disease only 

 when the affected parts have suffered some injury, overstimulation, 

 or drying out. In the case of fruits they are proverbially destructive 

 when the fruits approach maturity. In other words, a very large 

 number of the fungi here included are not in very close associa- 

 tion with living tissues, and are, from several points of view, 

 hemiparasitic. It is to be emphasized, however, that many of our 

 disastrous fungous 'diseases are included in this subdivision. Now 

 in the case of the strictly parasitic fungi already referred to, it is 

 very doubtful if vigor of the host is alone sufficient to prevent 

 disease. In fact, some of the most vigorous varieties (whether 

 judged by vegetative or fruiting achievements) have been particu- 

 larly susceptible to certain diseases. Before adopting the view that 

 all ills flee before vigor we must make ourselves clear as to what 

 vigor means. If it is synonymous with resistance to disease, then 

 of course all plants subject to disease under any conditions are 

 nonvigorous. Many wild or native prototypes of certain highly 

 responsive, cultivated varieties when grown side by side with the 

 latter may show more, or may show less resistance to disease, 

 wholly independent of robustness. It does not at all hold that 

 factors which favor the fullest development of the host may not 

 also encourage the fungus. Moreover, factors unfavorable to the 

 host may be similarly unfavorable to the fungus. 



Phytophthora infestans, Plasmopara Viticola, and Cystopus can- 

 didus, on some of their hosts, — the potato, the grape, and the 

 shepherd's purse respectively, — would seem often to be most 

 effective when the host is growing vigorously. Ward has suggested 

 from experiments with the rust on brome grass that any hindrance 

 to free nutrition of the host is likewise a means of inhibiting the 

 fungus. 



In the case of fungi whose weapons for attack are most effect- 

 ive where the plant is least alive, so to speak, — as when the 

 leaves have been injured by drought or other causes, or when the 

 fruit is maturing, — it is then clear that any environmental factor 



