562 INSECTS AFFECTING VEGETATION 



more susceptible varieties, however, the stem end may be rotted to 

 a depth of one inch or more. 



The Disease an Internal Infection of the Tuber. With this in- 

 ternal infection of the new crop we have come to one of the most im- 

 portant facts regarding the disease. The depth of infection varies 

 from that of a very shallow, light yellowing of the vessels at the 

 stem end, so inconspicuous as to be entirely passed over by the lay- 

 man, to that of a pronounced blackening of the vessels to a depth of 

 one-half inch, or indeed later it may extend clear through the tuber. 

 When the infection is very light the presence of the parasitic fungus 

 can be determined only by microscopic examination, or preferably 

 by means of artificial cultures. This internal infection may be very 

 marked and yet in no way impair the superficial appearance of the 

 tubers, nor even in keeping qualities when the potatoes are placed in 

 proper storage. Indeed, it is this inactive or dormant internal infec- 

 tion during storage which is the means of bringing about the wide- 

 spread distribution of the disease. Under improper storage much of 

 this infection becomes so active as to cause a high percentage of dry- 

 rot, an extreme condition in which the potatoes are of no value for 

 food or seed. The importance to the grower of being able to recog- 

 nize this internal infection cannot be over-emphasized. With the 

 seed lies the only possible opportunity of controlling the dissemina- 

 tion of the disease. The appearance of this infection in the tubers 

 varies slightly in different varieties. As a rule the ring of vessels 

 becomes deeply blackened wherever the fungus penetrates. Some- 

 times, however, the fungus is not confined strictly to the vessels, but 

 may penetrate at random throughout the flesh of the tuber. This 

 peppered appearance was noted in several of the early varieties and 

 was identified as the same disease by means of artificial cultures. 



A Cause of Dry-Rot in Storage. Since the recognition of the 

 fusarium as a cause of blight in the field, and of an internal infec- 

 tion of the tubers, pathologists have lost no time in learning of the 

 progress of this internal infection in storage. As a dry rot the disease 

 makes its greatest headway under high temperatures in the presence 

 of considerable moisture. For these reasons the storage of potatoes 

 in warm cellars under dwellings is a practice certainly not to be rec- 

 ommended. The writer carried out experiments on the progress of 

 the disease with selection, as one picker can gather 30 to 40 per cent 

 of the hills for seed purposes while the other gathers the remainder. 

 The wagon box may be divided to accommodate the two divisions. 

 Immediately upon drying the potatoes selected for seed should be 

 placed in the best available storage. 



Treatment Previous to Planting. Previous to seed treatment, 

 the tubers should be inspected and if the seed is carrying a shallow 

 infection, this should be cut away. This can be done without injur- 

 ing in the least the potatoes for seed purposes. 



There are two satisfactory methods of treating seed potatoes. 

 The older method is carried out by placing the seed in a solution 

 made by using one pound of formaldehyde (40 per cent grade) in 

 30 gallons of water. The seed remains in this solution one and one- 



