Kentucky Agriculmral Erpcrinz.cnt Staifion 29
·· Back crossing the Fi generation with the susceptible
Tw parent having the disease qualities has been found valuable in
¤· quickly isolating resistant strains of a desirable type of to-
l1€ i baceo. This is particularly valuable when Turkish tobacco
fd- is used as the resistant parent. The first cross has usually been
in made in the field and the back cross in the greenhouse during
on the winter. The next summer it is possible to obtain heterozy-
ut gous resistant plants of the desired type. Further tests of
me the resistance of the commonly grown black tobacco varieties ,_
of indicated that in a season like 1923 in heavily infected soil
40 they would be nearly a complete failure.
l¤· ‘ Tobacco Brown Root-Rot. This root disease of tobacco
ith is prevalent in both the burley and dark tobacco sections of
ul' Kentucky. It is characterized in some cases by completely A
he stunting the plant as in a severe case of black root-rot and in i
GU others by rotting only a sufficient number of small rootlets so
OU that altho the plant makes a fairly normal growth, yet it is
Hd quickly affected by periods of drouth when it may wilt badly.
of One strain of black root resistant burley (VV) has been found ,
lH· to be particularly susceptible to this trouble as indicated by its ,
2l1€ decidedly inferior growth on corn land at the Experiment
is Station in comparison with other varieties and· strains, and l
the by the fact that in comparison on farms, it has been decidedly T
of inferior in growth in about 5 per cent of the cases, to the T
local variety. ,
wk The cause of this disease has been studied and isolations
OX. from the rotting edge of small rootlets usually give Fusaria,
ins but very often another organism has been isolated which thus
ial. far has remained sterile in culture. It is characterized by
hat producing a brilliant wine-red color on "Bacto" agar in con-
in trast with the fusaria which rarely produce color on this
tins medium. In this medium, it also produces very small black
ids, sclerotia and the mycelium is from white to dark brown.
oils Inoculation tests on young seedlings of tobacco, red
.ose clover, alfalfa, alsikc clover, radish, cabbage and onions, grow-
it, ing in test-tubes on nutrient agar, show conclusively that ]'>l'flC—
tically all species of Fusaria isolated from the roots of tobacco