Vegetable Improvement Newsletter No. 3, February 1961

Compiled by H.M. Munger, Cornell University, Ithaca, New York

1. Preventing Fruit Rots in Cucurbit Breeding

W.C. Barnes

Clemson College Truck Experiment Station, Charleston, S.C.

One noteworthy improvement has been attained in the problem of fruit rot control in cucurbits since the report in 1960 Newsletter. Bird & Co. "Neponset" vapor barrier paper has been found to be superior to aluminum foil, laminated paper, etc. previously used. "Neponset" is an extra heavy paper coated with plastic-like materials that are not broken down in the field. It is one of the cheapest materials used and does not pocket from the weight of fruit as readily as most materials.

2. Techniques of Breeding Spinach in the Field

E.A. Borchers

Plant Breeder, Virginia Truck Experiment Station, Norfolk, Virginia

When individual female spinach plants are to be pollinated in the field with pollen from different male plants, precautions must be taken to prevent accidental cross-pollination. Spinach pollen is wind-disseminated, so the problem involved is one of excluding foreign pollen borne on air currents while at the same time providing favorable conditions for the desired pollen, when introduced, to function.

A procedure which has proven successful in eastern Virginia is to plant those lines in which sib-matings or crosses are to be made in each of two plots separated from each other by at least 400 feet. Selections are made in each plot and the remaining plants are destroyed. One planting is then designated as the female, or seed producing plot, and the other as the male plot, or pollen source.

As the flower stalks emerge and the sex of the plants becomes evident, all hermaphroditic and male plants in the female plot are either destroyed or carefully moved to the male plot before any pollen is shed. As the remaining plants in the female plot develop a considerable number of receptive stigmas, they are caged individually by placing an ordinary cylindrical section of stove pipe 10 inches in diameter and 2 feet in length over each. Additional soil is banked around the base of the cylinders for added support, but no covering is placed over the top. The desired pollen is introduced by carefully placing pollen shedding branches from the desired male selection on top of the female plant and shaking them three or four times during the next two days to disseminate the pollen. Three or four days after caging, the plant and cylinder are sprinkled with water to remove the remaining pollen,, the pollen shedding branches are carefully removed and buried, and the cage is washed in a barrel of water kept in the field for this purpose.

Plants pollinated in this manner have produced much more seed than those pollinated by using cloth cages which enclose the plant entirely, even when muslin cloth of fairly open weave is used. Poor seed set in the cloth cages appears to have been due to humid conditions that prevail when the plant is completely enclosed and which make the pollen sticky and interfere with its dissemination.

The metal cages have the advantages of being readily available, inexpensive, easy to install and readily cleaned after use. However, since they are not covered on top there is a chance that some pollen might escape and contaminate nearby plants. Some contamination is to be expected when the stove-pipe cages are used, but if care is exercised this can be kept to a small amount.

3. Incomplete Dominance of Scab Resistance in Certain Cucumber Crosses

H.M. Munger and R.E. Wilkinson

College of Agriculture, Cornell University, Ithaca, N.Y.

Bailey (Proc. A.S.H.S. 36:645-646, 1939) reported monogenic inheritance of resistance to scab (Cladosporium cucumerinum). His results indicated that dominance was incomplete, although more recent papers on scab resistance have mentioned only complete dominance.

In 1959 a group of F₂ progenies were tested for scab resistance and part of the survivors were put in the field. The offspring of these plants proved to include more than 50% homozygous progenies where only one-third would be expected if dominance were complete. Apparently there was unconscious selection for homozygous dominant plants. In some segregating progenies observed during the winter of 1959 to 1960 it appeared that the resistant plants could be separated into two groups on the basis of degree of resistance while in others they could not.

In progenies tested in the spring of 1960 for field planting, notes were made as to whether it appeared that homozygous resistant plants could be separated from heterozygous or not. These notes were correlated with the progeny tests conducted on the offspring of these plants. From seven F₂ progenies for which the notes indicated that selection for homozygosity was not good, twelve F₃s were homozygous resistant and twenty-three heterozygous. On the other hand, from eleven F₂ progenies in which the notes indicated that the plants selected were probably homozygous, thirty-four F₃s were homozygous resistant as compared with twenty-six heterozygous.

While selection for homozygosity of scab resistance is far from being completely effective it seems clear that the breeder can greatly increase his chances of obtaining homozygous progenies by making careful selection for seedlings most free of symptoms.

4. Scab Resistance and Length of Slicing Cucumbers

H.M. Munger and R.E. Wilkinson

College of Agriculture, Cornell University, Ithaca, New York

In several instances where the dominant gene for scab resistance has been added to a susceptible variety by repeated backcrossing, the resistant variety derived from this procedure has been shorter than the susceptible counterpart. Barham and Winstead (North Carolina Bul. 409) report that Fletcher and Ashe are shorter than Stono and Ashley from which they were derived by 5 backcrosses, and this observation has been made on these varieties in other states. At Cornell, homozygous scab resistant progenies derived from 4th and 5th backcrosses to Marketer have likewise been found shorter than Marketer. This is illustrated in the table below and has been evident from inspection of homozygous resistant versus susceptible progenies from the same backcross line.

In 1960 we grew ten F₂ progenies, part derived from 5 and part from 6 backcrosses to Tablegreen. Susceptible plants were eliminated in the seedling stage. At market maturity the fruits in this group of progenies looked virtually identical to those of Tablegreen both in average length and in variability with respect to length. Mature fruits were measured and classified as homozygous or heterozygous resistant according to scab tests run on their selfed progenies. As shown in the table below the ratio of length to diameter was virtually identical in the homozygous and as compared with the heterozygous resistant plants.

Apparently it is possible in some cases to add the gene for scab resistance to a slicing cucumber without altering the fruit length. As yet we have no explanation as to why this has not been possible in some varieties. Highmoor was the source of scab resistance for the backcross program involving Marketer, and SR 52-55, which derived its scab resistance from Highmoor, was the source for the backcross program involving Tablegreen. SR 52-55, as reported in Vegetable Improvement Newsletter No. 2, continues to look as long as its susceptible counterpart although we do not have actual measurements to substantiate this.

Ratio of Length: Diameter for fruits on number of plants indicated.

* 2 fruits per plant measured in F₂ of 5th and 6th backcrosses to Tablegreen
** 1 fruit per plant measured in F₃ of 4th and 5th backcrosses to Marketer
*** 5 fruits per replicate measured in 4 replicates. Susceptible Marketer and Fletcher were from commercial lots of seed; homozygous resistant Marketer a composite of F₃ lines derived from 4th and 5th backcrosses to Marketer.

5. Evaluation for Cucumber Beetle Resistance in Cucurbit Seedlings

Prem Nath and Charles V. Hall

Department of Horticulture, Kansas State University, Manhattan, Kansas

The spotted cucumber beetle (Diabrotica undecimpunctata howardi Barber) and the striped cucumber beetle (Acalymma vittata F.) are among the most injurious insect pests of cucurbits in Kansas. Various control measures have been used since they were reported as injurious pests, but none are completely satisfactory. Hence, efforts in this study were directed toward host plant resistance.

The level of resistance of four varieties each of cucumber (Cucumis sativus), muskmelon (Cucumis melo) and watermelon (Citrullus vulgaris) to the cucumber beetles was determined in the greenhouse between September, 1959 and July, 1960. Separate greenhouse experiments were conducted for the two beetle species.

A completely randomized design was used. Three replicates of each of the twelve varieties were included in each of the three experiments. Two varieties of seedlings were planted in each greenhouse flat, giving a total of six flats per replicate or a total of 18 flats per experiment. A screen covered greenhouse bench was used to confine the insects with the seedlings.

When the seedlings had two well-developed cotyledonous leaves, the beetles were released inside the cage. Soon after the release of insects it was noted that they moved toward the preferred varieties and started feeding. Overall damage was rated on the sixth day after release and the final observations were taken on the seventh day. The following rating system, which has been used in field experiments since 1956) was adopted for the greenhouse study. 0 = No injury; 1 = Slight injury; 2 = Moderate injury; and 3 = Severe injury. The following results were obtained.

Cucumber: The varieties used were Nappa 63, MR 7097, Model, and Palomar. There was little difference in resistance to the striped cucumber beetle. All the varieties were fairly resistant with Nappa 63 (0.58) being the most resistant.

Muskmelon: The varieties studied were Gold Cup 55, No. 6, Georgia 47, and Cranshaw. Gold Cup 55 (0.72) and No. 6 (0.99) were highly resistant to the striped species, Georgia 47 (1.53) fairly resistant and Cranshaw (2.29) was highly susceptible.

In an experiment with the spotted beetle these four muskmelon varieties were fairly resistant with the varieties No. 6 and Gold Cup 55 found to be entirely free of damage.

Watermelon: Hope Diamond, Charleston Gray, Blackstone, and Black Diamond were the varieties under study. No significant difference occurred in the level of damage by the striped beetle between the varieties Blackstone and Black Diamond but both varieties were highly susceptible with average ratings 2.86 and 2.99 respectively. Hope Diamond (1.77) was fairly resistant as compared to Charleston Gray (1.99) which was slightly resistant.

All the four varieties were susceptible to the spotted species with an overall mean damage of 2.67.

In general, for the spotted beetle the order of resistance from high to low was muskmelon (1.00), cucumber (1.5), and watermelon (2.67) whereas for the striped beetle the order was cucumber (0.94), muskmelon (1.38), and watermelon (2.50). Black Diamond, Blackstone, and Cranshaw were the most susceptible to the striped beetles among the twelve varieties of the three species studied, whereas Nappa 63, Gold Cup 55, and No. 6 were highly resistant. In studies with the striped beetle, there was little difference in the level of resistance between cucumber and muskmelon with the exception of Cranshaw which was highly susceptible.

6. New Hermaphroditic Inbreds of Cucumbers

Oved Shifriss

Department of Horticulture, Rutgers University, New Brunswick, N.J.

Hermaphroditic inbreds bearing small, barrel-shaped, hairy, fasciated fruits were developed several years ago (Weiz. Inst. Rept, 1956-57). Crosses between hermaphrodites and some monoecious varieties have led to the development of new hermaphroditic lines bearing long, hairy, non-fasciated fruits. The term "hermaphrodite" refers to a plant bearing perfect flowers exclusively. Seed will be available shortly.

7. Irradiation of Lima Bean Seed

L. C. Peirce

Iowa State University, Ames, Iowa

A 20 pound sample of Fordhook 242 seed was divided into seven equal lots for irradiation at the Brookhaven National Laboratory. The exposures consisted of three levels each of thermal neutron and X-ray. In an effort to discover quantitative improvement, the treated material was entered with an untreated check in a trial consisting of ten replicates. Measurements were taken of number of pods, seed size, germination, earliness, and qualitative mutations. The data reported herein concern only the number of pods per plant. A measure of variation was obtained by apportioning variance between replicates and within replicates for each entry. The following table shows the mean for each treatment and the corresponding coefficient of variation.

* Significantly different from check, 0.05 level.
** Significantly different from check, 0.01 level.

The effect of increasing dosage can be seen in a reduction of yield and in an increase in variation. Of particular interest was the X-10,000 treatment. The mean yield of X-10,000 differed little from that of the check, but variation was materially increased. This suggested that further improvement could be made by selection. Therefore, 55 plants of X-10,000 (about 10% of the surviving population) were selected for high pod set and bulked. Similar samples were drawn from two other treatments and the check. In addition, a random sample of the check was included for comparison. A replicated trial of these entries gave the following results.

No significant differences were found. In addition to variation induced by soil heterogeneity and competition, the appearance of deleterious mutations in this generation may have contributed to a masking of selection effect. Several samples therefore were drawn from these treatments for testing again in 1961. Apart from the quantitative test, one mutation affecting seed size was recovered. Most characteristics of the Fordhook 242 variety were retained in this mutant type, but further testing is required to determine its value.

8. Precocious Yellow, A New Fruit Color in Cucurbita pepo

Oved Shifriss

Department of Horticulture, Rutgers University, New Brunswick, N.J.

It has been demonstrated (J. Hered. 46:213-222, 1955 and Genetics 41:659-660, 1956) that the expression of factor B, for precocious yellow pigmentation and bicolor fruit, can be modified and stabilized in self-reproduction and that some BB lines bear uniformly yellow fruits. However, heterozygotes Bb of crosses between these BB lines and several bb varieties produce bicolor fruits. Recent investigation revealed that the variety, Improved Fordhook Zucchini, carries genes which stabilize the expression of B even in the heterozygous state. Specifically, when B is incorporated into the genetic background of Improved Fordhook Zucchini, both BB and Bb ovaries are uniformly yellow as soon as they can be seen with the naked eye. Thus, in the latter background factor B behaves phenotypically as an ordinary rather than as a pattern gene except that it acts at least two weeks earlier in the ontogeny of the ovary than gene Y. Seed of some BB lines is available for distribution.