MYCOLOGICAL SECTION. 123 



sufficient moisture in the air, the conidia or summer spores (see plate) 

 are produced by myriads upon the diseased vines. These sjjores, 

 blown about by the wind or carried by insects, birds, or other ani- 

 mals, find lodgment on healthy plants. Dry air soon destroys them, 

 but in drops of dew or rain they germinate readily, so that many 

 thousand new centers of infection may begin in the course of a single 

 day. The sudden rot of the green tissues of the plant, so often ob- 

 served when the weather is warm and wet, is due to the rapid j)ropa- 

 gation of the fungus by means of these summer-spores, although the 

 direct breaking down of the tissues is generally accomplished by bac- 

 teria and Ascomycetous fungi. 



Undoubtedly the mycelium of Phytophihora may grow down 

 through the stalks and thus reach the tubers, but this lias never been 

 proved. The tubers are ordinarily infected by the conidia, which 

 may be washed into the soil by rains or carried down by small ani- 

 mals. A wet rot of the tubers does not always follow their infection. 

 Very often the only indications of disease are some superficial dis- 

 colorations of the tuber. These brown or black sj^ots contain the 

 mycelium of the fungus. At first these spots are scattering and near 

 the surface of the tuber, but during the winter they may increase in 

 number and size as the mycelial threads penetrate farther and 

 farther into tlie sound tissue. I have found the minute threads of 

 this fungus penetrating the sound tissues for some distance (one cen- 

 timeter or more) beyond the discolored parts of the tuber, and, by 

 exposing these white and apparently sound portions to moist air for 

 some days, have obtained conidiophores and conidia (see plate) with 

 which I have caused the rot in other tubers previously sound. This is, 

 however, no new experiment ; others have obtained the same results. 

 That in damp, warm cellars or pits the summer spores may be devel- 

 oped on the surface of infected tubers, and that the neighboring 

 sound ones may thus be infected, is no longer a matter of specula- 

 tion, but has been settled in the affirmative by observation. 



The Phytophihora lives from year to year by means of its peren- 

 nial mycelium lodged in the tuber. It is possible that the fungus is 

 also propagated by winter spores, oospores, but this not certain. 

 Spores have been figured and described as the winter spores of this 

 fungus, but the evidence cannot be accepted as conclusive. The 

 parasite finds its way into the fields " along with the planted tubers, 

 either concealed as mycelium within their tissues or adhering to the 

 surface of the tubers in the form of recently developed conidia acci- 

 dentally lodged thereon. It has been shown that the mycelium may 

 grow from the tuber in the growing shoots and reach the surface 

 and fructify in this way, or may develop conidia in the soil upon the 

 surface of the tuber, whence the spores may be brought to the surface 

 and find their way to the leaves and stems by the aid of earth-worms 

 and various burrowing insects. In favorable weather the propaga- 

 tion of the fungus by summer spores is so rapid that only a few 

 original centers of infection are requisite in order to speedily infest 

 h whole field or district. 



Certain conditions favoring rot are beyond control of the farmer. 

 He cannot prevent warm, wet weather; but, by planting upon dry 

 ground and by using the greatest care in the selection of tuljers for 

 planting, he may greatly diminish the severity of the rot. No tubers 

 having dark spots or blotches upon their surface, or which look brown 

 or black in places when cut open, should be planted. Whether early 

 or late sorts will rot worse depends chiefly on the character of the 



