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44 Tltrirty—F0urth Aomual Report
Lettuce Investigations. Investigations upon the failure of
lettuce to head have been continued, the varieties and strains ’
used in this work being Hanson (most largely) together with
i Hothouse, Hittinger’s Belmont and New York. The progeny
· of the selected individuals from those plants which headed in
i the greenhouse in the winter of 1020-21, were plant>cd in the
field in the spring of this year and again in the greenhouse in
, the fall. Marked differences occurred between the various
t strains in extent of heading and considerable variation was
  noted in individual plants within a strain. The results indicate
  that progress may be expected by continued selection. All
plants grown in steam sterilized soil, formaldehyde sterilized
soil, baked soil and sterile washed sand to which a nutrient
solution was applied have shown a considerable amount of
root—rot and have eventually developed tip burn.
So far as soil is concerned, the best results have thus far
been obtained on soil eomposted from sod and manure drench-
ed with formaldehyde, and in virgin bluegrass soil from which
· the surface layer of sod had been removed. \Vell-formed,
solid heads developed from all plants on these two soils, but
eventually root—rot and tip burn of the inner leaves occurred
in every ease, tho not as a rule rendering them nnsalable.
_ \\ihen grown on a soil composted of sod and manure and not
  treated with formaldehyde, less than 50 per cent of the plants
  headed. lt can be stated definitely that the methods of soil
i` sterilization used in these experiments have influenced to a
, marked degree, the factors causing failure to head. Tip burn
I and other physiological disturbances have been delayed by the
Q soil treatments, but eventually have appeared in every crop.
lb seems from the results thus far obtained that the cause of
the root-rot and the associated disease, tip burn, is a seed-
· borne organism which may be prevalent in garden soils and
refuse, but apparently is not present in virgin soil. Similar re-
sults have been obtained with Grand Rapids lettuce, a standard
loose leaf variety, in the effort to secure a strain combining
freedom from tip burn and vigor of growth.
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