POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 41 



by Dr. H. A. Edson, of the Bureau of Plant Industry. It is, how- 

 ever, important to mention that types of potato disease are not 

 infrequently encountered which simulate in one character or another 

 the leaf-roll, the curly-dwarf, and sometimes blackleg, but which 

 is believed to be associated with Rhizoctonia, although it must be 

 admitted that the proof is somewhat scanty. 



This fungus is almost ubiquitous on potato tubers in its sclerotial 

 form; small black mycelial masses superficially attached to the epi- 

 dermis without evidence of parasitism may be found on tubers from 

 every State. In other cases a russet scab or cracking is attributed to 

 the same fungus, and lesions are formed on the underground stem and 

 stolons. The fruiting stage, Corticium vagum solani Burt (Hypochnus 

 solani Prill), is formed on the green stem above ground and is merely 

 a superficial nonparasitic layer over healthy tissues. 



The reaction of the potato plant to Rhizoctonia infection depends 

 upon the part attacked. If this be the stolons, the young tubers are 

 cut off, and this process, taking place in the heavy irrigated soils of 

 the West, is held by Rolfs (1902, 1904) to be the cause of that type of 

 potato failures in which large overgrown vines produce few or only 

 small tubers. If the lesions encircle and girdle the main stem near 

 the soil line, the result will be the formation of numerous aerial tubers 

 (PL XV, fig. 1) formed as a result of the destruction of the phloem 

 and the prevention of carbohydrate translocation. The same result 

 would follow mechanical girdling. This type of injury sometimes 

 results in a leaf-roll that is hard to distinguish from the genuine leaf- 

 roll until the plant is pulled and the stem injury noted. Such plants 

 were conspicuous in the Red River Valley in Minnesota in 1913. 

 There may have been a complication with blackleg there, but there 

 was no leaf -roll. In the San Luis Valley of Colorado, also, the Rhizoc- 

 tonia injury is reported by Edson and Wollenweber to take a form 

 strongly simulatmg leaf -roll. 



Rhizoctonia lesions on the young hypocotyl, such as are figured 

 in Plate XV, figure 2, cause a dwarfed growth described by Selby 

 as rosette. The condition figured by him closely approaches curly- 

 dwarf, and the question is weU worth raising in the case of stunted 

 plants bearing Rhizoctonia lesions whether their vigor had not been 

 impaired prior to infection. 



One can pass through potato fields in Ohio and Wisconsin, for 

 example, and on pulling the small, weak, or rosette plants find 

 many, but usually not aU, with these stem lesions. So far as the 

 writer knows, no one has planted the tubers from such hills to learn 

 whether the weakness is transmissible. The case for Rhizoctonia is 

 weakened, however, when one finds the stem lesion on vigorous, out- 

 wardly healthy hills as well as on the rosette examples. The subject 

 clearly needs further investigation. 



