This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
This disease of the potato is sometimes known to growers as "Little Potatoes," "Stem Rot," and "Rosette."
Considerable research work in regard to rhizoc-tonia was done by Prof. F. M. Rolfs when he was at the Colorado Agricultural College. Prof. A. Nelson also presents some interesting facts in "Bulletin 71' of the Wyoming Experiment Station. In the information which follows these two sources have been consulted freely.
This rhizoctonia fungus attacks the underground portions of the plant. It is a true parasite, living either in the internal tissues or upon the external parts. It attacks the stem at or just below the surface of the ground, destroying the bark in whole or in part. If the attack be a severe one it may result in the death of the plant, but if less severe it may induce a wet rot and thus result in the death of the plant, but if still less severe it may simply girdle the stem, the plant continuing to live and often producing as a result of the girdling an enlarged and apparently vigorous top. Owing to the fact that the girdling will prevent the return of the elaborated sap on the underground portions, there can be no tubers formed, or if formed they will be few and small. In many instances when the plant is thus prevented from forming the underground tubers it will throw out from the stem at points above the injury many short tuber-forming stems. These tubers are small and green, and of no value.
The tubers are also attacked by the fungus, and on the surface of these small, hard knots of mycelium, known as sclerotia, are produced. These appear as dark brown bodies, irregular in outline and varying from a mere speck to the size of a grain of wheat. These spots resemble dirt, but do not wash off readily.
Rhizoctonia. Showing development of "little potatoes" on the branch because of the effort of the plant to form tubers above the point of injury. - From Bulletin 70, Colorado Experiment Station.
Rhizoctonia. The illustration, from Bulletin 70 of the Colorado Agricultural Experiment Station, shows stems of a young potato plant that has been affected from the seed.
Prof. F. M. Rolfs found three distinct stages in the development of the disease in Colorado, as follows:
1. Rhizoctonia stage - the first or growing (vegetative) stage. Two kinds of hyphse occur - the light colored ones in the inner tissue of the host, which, if abundant, produce wet rot; dark colored ones in the outer tissues forming a close web of felted covering, which constitutes merely a girdle or band. If the last only is present, the plant is not killed, but may seem unusually healthy.
2. The Corticium stage. It had been supposed that the fungus produced no spores, but was perpetuated solely by the sclerotia, which are the closely compacted masses of the mycelium forming the dark scale-like or grain-like bodies on the tubers and stems of the host plants. At one stage in its development, however, spores are formed on short lateral branches arising from the hyphae of the rhizoctonia stage. These are so readily dislodged that their presence is easily overlooked when a microscopic examination is made. It is probable that the spores serve merely for the rapid dissemination of the disease during its vegetative period.
3. The Sclerotium stage. This is the period when provision is made for the perpetuation of the fungus. The sclerotia on the tubers of an infected crop, on the stems of the potato and weeds, carry the disease over from year to year.
To stamp out the disease the sclerotia must be killed, and this is done by the use of clean seed, rotation of crops, and treatment of infected seed the same as for scab.
Following is a description of some of the insect enemies: