4 BULLETIN 1156, U. S. DEPARTMENT OF AGRICULTURE. 
production of very striking overgrowths, so that check rows commonly 
exhibit practically 100 per cent infection. 
Any variety with 20 per cent or more of tuber infection was classed 
as susceptible and forthwith discarded, but stocks which were 
infested to a lesser degree were grown another year before discarding. 
Where no infection occurred, the entire stock was saved for successive 
planting year by year until immunity was considered definitely 
established. Where a single hill or only two or three tubers showed 
wart- formation, as occurred in but very few instances, the infected 
hill was discarded and the test repeated with the remainder of the 
stock. 
It is to be noted that, owing to the limited occurrence of areas 
infested by wart our tests have not been carried on with the same 
degree of attention to large size and systematic duplication of plats, 
repeated through several years or in a number of localities, that has 
been characteristic of similar tests in England and Germany. 5 
Nevertheless, it is felt that a very high degree of reliability attaches 
to the results here reported. This question was discussed briefly in 
the preliminary report 6 to which reference has been made. The 
following considerations derived from more extensive experience may 
be cited in addition: 
Purity of slocks. — The material which has been available for these tests has been of 
exceptional quality as regards uniformity and trueness to type. For the most part 
these stocks have been grown by the Office of Horticultural and Pomological Investi- 
gations of the Bureau of Plant industry for a number of years at the potato-breeding 
farm of the United States Department of Agriculture in Aroostook County, Me., 
and mixtures have been entirely or practically eliminated. This has avoided at 
the outset most of the difficulty due to errors in variety names and to mixed types 
nearly always inherent in lots obtained from commercial seedsmen. Likewise, the 
seedlings upon which these tests have been made do not have the heterogeneous and 
uncertain element that prevails in material obtained from amateur plant breeders,, 
but are S3lected lines derived from the controlled crossing of known parents. 
Consistent results from trials. — All varieties for which a definite report of immunity 
is here made have been carried through two or more years, surviving tests conducted 
in at least three gardens, for the most part locat"d in different regions. A few others 
indicated as probably immune by the tests of the past season are entered in the tables 
as immune, but with a question mark. 
These variety tests were begun in 1919 under a cooperative agree- 
ment between the Bureau of Plant Industry and the Federal Horti- 
cultural Board of the United States Department of Agriculture and 
the Pennsylvania Agricultural Experiment Station. The same ar- 
rangement was continued in 1920 and 1921. A similar test was 
carried out in cooperation with the Agricultural Experiment Station 
of West Virginia. In 1920, feeling the need for further information 
as to the adaptability of some of the immune varieties as compared 
with that of the standard susceptible varieties grown in the wart- 
infested areas, the Pennsylvania State Department of Agriculture 
began tests of local varieties collected in the quarantined areas as 
compared with a number of immune stocks, the introduction of 
which it was thought would be desirable for purposes of regulated 
planting. These trials led to the collection of considerable data on 
the relative value of immune and susceptible varieties in districts 
situated climatically like those infested with the disease and furnished 
Appel, O. Uber die Anfalligkcit und Widerstandsfahigkeitiverschiedener Kartoffelsorten gegen Krebs. 
Arb. Gesell. Ford. Baues and Verwend. kartoffeln, Heft 15, 19 p., fold diagr. 1918. 
. Lyman, G. R., and others. Op. cit. 
