322 MAINE AGRICULTURAL EXPERIMENT STATION. I907. 



skin of the tuber. It was seen only on the Irish Cobbler variety 

 and was wholly confined to the early dug portion of the crop. 



The spots, which were black, varied from the size of a small 

 pea to one-half an inch or more in diameter, and showed up 

 in marked contrast to the clean, white surface of the potato. 

 Little or no shrinking of the tissues below the blackened area 

 was apparent. On cutting into a spot it was seen to extend 

 only through the skin and practically none down into the tissues 

 below. The appearance as a whole suggested the late blight 

 fungus, Phytophthora infestans, (also the cause of the common 

 dry rot), as a possible explanation of the cause. However, it 

 differed from the appearance of the ordinary dry rot enough 

 so that none of the shippers would agree that it was the same 

 thing. Furthermore, a careful search late in the preceding 

 August over some of the fields on which affected potatoes were 

 grown failed to produce a single characteristic specimen of late 

 blight on the foliage. Samples of the diseased tubers were 

 taken to the laboratory and placed in a moist chamber. Before 

 doing so, the whole tubers were carefully washed and dried, 

 and then cut across through one or more diseased areas, using 

 a flamed knife. After 2 or 3 days the cut pieces were exam- 

 ined and in every case the characteristic spore clusters of Phy- 

 tophthora infestans could be found along the cut margin of the 

 diseased areas. No other fungus was found associated with 

 the spotting. 



The potatoes on which the spotting occurred were dug early 

 in September. The tubers were not mature and were easily 

 skinned and bruised by the digger and in handling. Without 

 doubt the fungus was at that time present on the leaves in a 

 very slight degree. The spores falling on the bruised tubers 

 germinated, but on account of adverse temperature and moisture 

 conditions the mycelium passed at once into a resting state, 

 instead of penetrating farther into the tissues and producing 

 the usual characteristic dry rot of the tuber. 



Potatoes showing these spots as well as those showing the 

 ordinary dry rot should be rejected for seed. The use of 

 unsound tubers for seed is one of the sources, and possibly the 

 only source from which late blight is conveyed to the foliage 

 of the growing crop. 



