Young, P. A.
Southern blight, caused by Sclerotium rolfsii, has caused great damage to tomatoes in the last 50 years in the southeastern part of the United States. Results of tests at Jacksonville, Texas in 1949 to 1954 failed to show a variety of tomato with strong resistance to southern blight. To try to develop such a variety, Manalee tomato seeds were sent to the Atomic Energy Commission at Oak Ridge, Tennessee early in 1956 and were treated with high-intensity gamma rays from Cobalt 60 or with thermal neutrons practically without fast neutrons. It was hoped that increasing the rate of mutation would lead to the discovery of a tomato with strong resistance to southern blight. It may be interesting to note that about one natural mutant tomato plant appears per 50,000 plants at this laboratory.
The R1 generation was grown in flower pots in the fall and in the greenhouse late in 1956. The R2 generation of 2685 plants was grown in fields in the spring of 1957 (Table 1), and the following notes were recorded:
GR1936 One R1 plant has prominent yellow curled leaflets.
GR1928 Some R1 seedlings showed temporary dwarfing.
GR1929 Many plants showed dwarfing, sterility and elongate
curled leaflets in the R2 generation. The leaflets of
two plants had parallel veins and yellow blotches.
GR1930 A bud mutation on one R1 plant made an abnormal branch
with very slender leaflets about 1/4 normal size with
non-necrotic tan areas near their centers; two trusses
of flowers on fruits.
GR1931 Irradiation killed most of the seeds. In the R2
generation, some seedlings had white-purple stems and
cotyledons and died within about 3 days. Many yellow
seedlings lived about a week. Some dwarfed chlorotic
seedlings had yellow blotches in their leaflets; they
remained diiarfed during a month or longer. Several
large sterile plants had yellowish-green top leaflets
GR1931D segregated with 31.7% of the plants with entire
leaflets like those caused by the e-allele. These plants
were vigorous and fertile.
GR1935 Several large plants in the P2 generation had chlorotic
top leaflets.
GR1937 Many R2 plants were chlorotic or yellow dwarfs.
GR1938 The radiation killed most of the seeds. Many R1
seedlings finally emerged and grew 1/2 to 1 inch tall;
they were yellowish, had poorly developed cotyledons and
died in 7 to 10 days. Some other seedlings grew onlv
about 3 inches tall. Five plants grew normally. A few
R2 plants showed chlorotic dwarfing; one plant had
downward curled leaves.
Scllerotium rolfsii growing on oats was used to inoculate 1015 R2- tomato
plants in 84 randomized groups on May 21, June 10 or June 23, 1957. All of
these plants died with southern blight within 7 to 14 days.Summary: In a test reported here, all of the chlorotic and dwarfed mutant tomato plants resulting from irradiation were sterile. A mutant type with entire leaflets was vigorous and fertile. Irradiation of tomato seeds did not result in any detected plants with resistance to southern blight.
Table 1. Mutations induced in Manalee tomatoes by thermal
neutrons and gamma rays.
Populations Numbers
__________________ of plants Percentage
Accession Number _________ of mutations,
No. GR of Treatment R1 R2 R2
________________________________________________________________
1936 8 Neutronsl, 15 min. 21 180 0
1928 5 Neutrons, 30 min. 14 168 0
1929 4 Neutrons, 60 min. 22 71 4.2
1930 7 Neutrons, 2 hr. 21 279 0
1931 7 Neutrons, 4 hr. 11 494 18.2
1932 2 None (checks) 24 47 0
1933 3 4000 r, gamma 11 47 0
1934 7 8000 r, gamma 26 141 0
1935 13 16,000 r, gamma 27 275 1.5
1937 17 32,000 r, gamma 35 936 1.8
1938 5 64,000 r, gamma 5 280 0.7