POTATO WILT, LEAF-BOLL, AND BELATED 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 vagwn solani Burt (Hyyochnus 
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 simulating 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 well 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 all, 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. 
