Breeding Melon for Resistance to
Lettuce Infectious Yellows Virus

James D. McCreight

U.S. Department of Agriculture, Agricultural Research Service, U.S. Agricultural Research Station,
1636 East Alisal Street, Salinas, CA 93905

Additional index words. Cucumis melo, Bemisia tabaci biotype A, sweetpotato whitefly, Bemisia tabaci biotype B, silverleaf whitefly, Crinivirus genus, Closteroviridae family, ELISA

Abstract. Lettuce infectious yellows virus (LIYV) is a member of the Crinivirus Genus, Closteroviridae Family, a new group of whitefly-transmitted viruses that induce yellowing symptoms on wide range of cucurbit and noncucurbit hosts. LIYV is transmitted efficiently by the sweetpotato whitefly, Bemisia tabaci Gennadius (SPW). PI 313970 stood out from the other potential sources for its consistently mild symptoms in response to infection by LIYV in a field test in 1990. Resistance to LIYV in PI 313970 was confirmed in greenhouse tests using controlled inoculations with the SPW. Uncertainty of symptom expression requires use of ELISA assays of inoculated plants, and may require serial transfer via sweetpotato whiteflies from inoculated melon plants to chenopodium for verification based upon symptom expression and ELISA assay.

I thank J.E. Duffus, H.-Y Liu, G.W. Wisler, and A.A. Cortez for providing virus isolates, antisera for ELISA, and sweetpotato whiteflies; J.A. Principe for technical assistance in the field tests at Brawley; and P.L. Fashing for technical assistance in the field tests at Brawley and greenhouse tests at Salinas. Mention of a proprietary product in this paper does not constitute endorsement of the product by the USDA.

 

The presence of the sweetpotato white-fly (SPW) has been known in Europe and North America for over 100 years as noted in a recent review by (Henneberry et al., 1998), but it was not recognized as a pest until 1948 when it was found to transmit cotton leaf crumple virus (Dickson et al., 1954). The first incidence of SPW affecting a cucurbit in the U.S. was in 1977 when squash leaf curl was observed on Cucurbita maxima in the Imperial Valley (Cohen et al., 1987; Flock and Mayhew, 1981). Squash leaf curl was endemic in the Imperial Valley and other parts of the Sonoran Desert through the 1980s. Later, melon leaf curl, and watermelon curly mottle also transmitted by SPW were described (Brown and Nelson, 1989; Duffus et al., 1985).

Lettuce infectious yellows (LIYV), transmitted by SPW, was first observed in 1981 in the Imperial Valley on lettuce (Duffus and Flock, 1982; Duffus et al., 1986). Melon was also adversely affected by LIYV although the yellowing symptoms are not obvious until late in plant development when fruit are setting. Fall melons

(generally planted in July to August) were a major source of LIYV for the winter lettuce crop (planted September to November) in the southwest United States.

The silverleaf whitefly (SLW), Bemisia argentifolia Bellows & Perring appeared in the low deserts encompassed by the Coachella and Imperial Valleys of California, and the Yuma Valley of Arizona in the 1990­91 winter season, and had essentially displaced the SPW by fall of 1991 when it caused severe feeding damage to melons and lettuce as well as other vegetable, agronomic and fruit crops (Brown and Costa, 1992). Although SLW is capable of transmitting LIYV under experimental conditions, the success rate is extremely low (Duffus, 1995). Displacement of SPW by the SLW virtually eliminated LIYV as a serious threat to melon crops in the Coachella and Imperial Valleys of California, the Yuma Valley of Arizona, and other parts of the Sonoran desert in the U.S. and Mexico.

 

 

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Research on resistance to LIYV was initiated before the displacement of SPW by SLW. Mayberry and Johnson (1982) reported the first field test of melon cultivars for their reaction to LIYV infection. WMR 29 and 'Harvest Pride' were found in their test to have no or very little yellowing in response to natural infection in the Imperial Val ley. All of the cultivars in their test exhibited yellowing on 10% to 30% of the foliage. McCreight (1991, 1992) reported results of field tests in 1988 and 1989. These tests included melons breeding lines and Plant Introductions (PIs) with known resistances to other diseases, some lines from the Middle East. Snake Melon from Egypt had the mildest symptoms in the 1988 test. PI 313970, MR-1, and PI 124111 were found to be asymptomatic in the 1989 test. In contrast to the Mayberry and Johnson test, WMR 29 exhibited moderate symptoms in the 1988 test and relatively mild symptoms in 1989. Although the mean ratings for 'Top Mark' in the 1988 and 1989 tests were similar to one another (6.5 and 6.2, respectively), the mean ratings for the other entries varied and rankings were not consistent. Symptoms of sudden wilt were present in the 1988 test and may have confounded LIYV evaluations whereas the 1989 test was virtually free of sudden wilt. PI 313970 had not been included in the 1988 test.

Here I report the results of a field test in 1990 for resistance to LIYV, and confirmation of resis

tance to LIYV in PI 313970 in controlled greenhouse tests. Displacement of SPW by SLW requires that continued work on resistance to LIYV be done using greenhouse inoculations. Some problems of greenhouse testing for LIYV are also presented.

Materials and methods

Field test, 1990. Twenty melon cultigens were direct-seeded in a field on 22 Aug. 1990 (Table 1). Plots were 27.5 ft (8.4 m) long on 80-inch (2-m) centers (Table 1). Four seeds were planted in each of 10 hills per plot; hills were spaced 2.5 ft (7.6 dm) apart. Seedlings were thinned to two per hill at the one to two leaf stage of growth. There were three replications. As in the 1988 and 1989 tests, infestation by viruliferous SPW was relied upon for LIYV inoculation.

Plants were evaluated on a plot basis 8 weeks postplanting for LIYV symptoms on crown, middle and terminal leaves. LIYV symptoms were evaluated on a 1 (asymptomatic) to 9 (completely yellowed) scale. Plant condition was rated on a 1 (dead) to 9 (vigorous and flowering). Plant size was rated on a 1 (runt) to 9 (large sized plants that cover the entire bed) scale. Numbers of fruit were counted. Data were analyzed using SAS ANOVA. Least squares means are presented; protected lsds (a = 0.01) were used for means separation.

Table 1. Least squares means of LIYV symptoms in 20 melon cultigens 8 weeks postplanting, Brawley, Calif., 1990.

LIYV symptoms on foliagez Plant Fruit

Cultigen Crown Middle Terminal Conditiony Sizex (no.)

PI 313970 2.0 a 1.0 a 1.0 a 9.0 a 6.7 cd 0.0 c

MR-1 7.0 b 4.0 abcdef 2.0 abc 4.3 defg 5.0 ef 4.7 bc

PI 124111 7.3 b 5.0 bcdefg 2.3 abc 3.7 fghi 4.3 f 2.7 bc

V038w 7.3 b 5.3 bcdefg 2.3 abc 4.7 defg 5.3 def 10.3 a

PI 414723 7.7 b 4.7 abcdefg 1.3 a 6.0 cde 7.0 bc 10.0 a

Snakemelon-1v 8.3 b 4.3 abcdef 1.3 a 6.7 bc 7.0 bc 9.7 a

PMR45 8.3 b 6.0 defg 1.7 ab 4. 0 fghi 4.3 f 2.7 bc

Top Mark 8.3 b 5.0 bcdefg 1.3 a 5.0 cdefg 4.7 ef 6.3 bc

PMR Honeydew 8.7 b 7.0 efg 3.3 abcd 4.0 fghi 5.3 def 5.7 bc

Snakemelon-2v 9.0 b 5.0 bcdefg 1.7 ab 6.3 bcd 7.0 bc 3.7 bc

PI 124112 9.0 b 5.3 bcdefg 2.3 abc 6.3 bcd 5.7 cdef 5.7 bc

WMR 29 9.0 b 5.7 cdefg 3.7 bcde 3.3 ghi 4.7 ef 11.0 a

Aly-14v 9.0 b 1.7 ab 1.7 ab 7.0 b 8.3 ab 5.7 bc

Aly-15 9.0 b 3.0 abcd 1.0 a 7.3 ab 8.3 ab 3.7 bc

Aly-4 9.0 b 2.3 abcd 1.0 a 8.0 ab 9.0 a 5.0 bc

Aly-6 9.0 b 3.3 abcde 2.3 abc 5.0 cdefg 6.7 cd 7.3 bc

Aly-7 9.0 b 7.0 efg 7.0 f 3.7 fghi 6.0 cde 11.7 ab

Greenflesh Honeydew 9.0 b 8.3 g 4.0 bcde 2.7 hi 4.7 ef 7.7 bc

Sharlyn 9.0 b 5.7 cdefg 2.3 abc 3.7 fghi 5.0 ef 6.7 bc

V001w 9.0 b 7.7 fg 2.7 abcd 3.3 ghi 5.3 def 17.0 a

zRated on a 1 (symptomless) to 9 (completely yellowed) scale.

yRated on a 1 (dead) to 9 (vigorous, flowering ) scale.

xRated on a 1 (runt) to 9 (large sized plants that cover the entire bed) scale.

wBreeding line for verticillium wilt resistance.

vSeeds received from Aly M. Ibrahim, United States­Saudi Arabia Joint Commission on Economic Cooperation (JECOR), National Agriculture and Water Research Center, Riyadh, Saudi Arabia.

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Greenhouse tests. PI 313970, 'Top Mark', 'PMR 5', C-87 and C-105 were evaluated in three controlled inoculation tests using SPW as the vector. PI 313970 seeds used in these tests were from our controlled pollination increases of this PI in 1966 and 1991. 'PMR 5' seeds used in these tests were from our controlled pollination increases in 1993. 'Top Mark' seeds were from one or more commercial sources. C-87 and C-105 seeds were received from M. Gómez-Guillamón (CSIC, Algarrobo-Costa, Spain), who found them to be potential sources of resistance to beet pseudo yellows virus (Duffus, 1965).

Seedlings were individually inoculated 2¥ in plastic sleeve cages using 40 SPW each time. Inoculations were initiated when seedlings were at the cotyledon stage of growth. The LIYV inoculum source was either infected melon or Chenopodium murale (chenopodium).

Symptoms were monitored beginning 2 weeks postinoculation (WPI) through 8 or 9 WPI. Symptoms were recorded as present (+), absent (­), or uncertain (?). In instances where uncertain symptoms were judged to be more indicative of a symptomatic reaction they were rated "+?", and where they were judged to be more indicative of an asymptomatic reaction, they were rated "­?".

Confirmation of virus infection. LIYV infection was verified in field and greenhouse tests by enzyme-linked immunosorbent assays (ELISA) using the double sandwich method (Clark and Adams, 1977) except that the coating globulin was used at 1 µg·mL­1 and enzyme-conjugated globulin was 1:400 or 1:800. Randomly selected leaf samples collected at the time of evaluation of the field test were assayed for presence of the virus. All plants in the greenhouse tests were tested with ELISA. Symptoms in the greenhouse tests were also confirmed by symptom expression and ELISA of chenopodium plants that were inoculated using selected melon test plants as virus source plants (serial transfer). ELISA values greater than 3x or 2x the mean control are presented.

Results and discussion

Field test, 1990. Results of the field test were in marked contrast to those of 1988 and 1989 where the mean LIYV symptom ratings of the test lines were scattered across the 1 to 9 rating scale (McCreight, 1991, 1992). In this test, PI 313970 had mild yellowing of the crown leaves (mean rating = 2.0) with no visible yellowing of the middle or tip leaves (Table 1). In the 1989 test, PI 313970 was completely symptomless (mean rating = 1.0). Most surprising in the 1990 test was the fact that all the other entries were rated 7.0 or higher for symptom expression on the crown leaves (Table 1). MR-1 was asymptomatic in 1989 (McCreight, 1992), but was rated 7.0 in 1990. These results clearly indicate that PI 313970 is quite different from the other cultigens tested for reaction to inoculation with LIYV. Subsequent data from greenhouse tests and field tests indicate that PI 313970 is not resistant to SLW (Simmons and McCreight, 1996). Resistance to SPW does not appear to be involved in the apparent resistance to infection by LIYV.

PI 313970 was the only entry rated 9.0 for plant condition (Table 1). It was not statistically different from Aly-4 and Aly-15 that had mean plant ratings of 8.0 and 7.3, respectively. The remaining 17 entries had mean plant condition ratings that ranged from 2.7 to 7.0. Seven entries had mean plant size ratings equal to or greater than PI 313970.

Mean numbers of fruit per plot ranged from zero (PI 313970) to 17.0 (V001); PI 313970 was the only entry that did not have any fruit. The absence of fruit may have contributed to the excellent plant condition of PI 313970. Though it was profuse with staminate flowers at the time the data were taken, PI 313970 had virtually no pistillate flowers that had opened and set fruit, or that were waiting to open.

Greenhouse tests. The difficulty in inoculating melons with LIYV in the greenhouse is immediately apparent from the infection rate of 'Top Mark' and 'PMR 5' in three tests (Table 2). In test 95-1, a high rate of infection of 'Top Mark' was achieved. In test 96-1, neither 'Top Mark' nor 'PMR 5' had obvious symptoms 8 WPI, but three 'PMR 5' plants had uncertain symptoms. In test 96-2, two 'Top Mark' and four 'PMR 5' plants had obvious symptoms 8 WPI, and they had two and

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Table 2. Numbers of melon plants with LIYV symptoms at different weeks postinoculation (WPI), numbers of melon plants with positive ELISA values 8 weeks WPI, and numbers of melon plants from which virus was recovered based on symptoms and positive ELISA values of Chenopodium plants following serial inoculation from their respective inoculated melon plants.

Melon (WPI) Chenopodiumz

Cultigen 4y 5 6 7 8 ELISAx Sw ELISAx

Test 95-1

Top Mark ---v --- --- 6/-/6 4/-/6u 2/6/6 5/6 0/0/6

PI 313970 --- --- --- 0/-/12 0/-/12u 0/0/12 --- ---

Test 96-1

Top Mark 0/3/7 0/3/7 0/5/7 1/3/7 0/0/7 1/2/7 2/3 3/3/3

PMR 5 1/3/7 3/2/7 5/0/6 3/2/6 0/3/6 3/4/6 4/6 4/4/6

C-87 0/2/7 0/1/7 0/1/7 0/1/7 0/0/7 1/1/7 1/2 1/1/2

PI 313970 0/0/7 0/1/7 0/0/7 0/0/7 0/0/7 0/0/7 --- ---

Test 96-2

Top Mark 0/1/7 1/1/7 2/1/7 0/3/7 2/2/7 1/1/7 3/7 4/4/7

PMR 5 4/0/7 5/1/7 5/1/7 5/0/7 4/1/7 2/5/7 4/5 5/5/5

C-105 0/0/6 0/1/6 0/1/6 0/0/6 0/2/6 0/0/6t --- ---

PI 313970 0/0/7 0/1/7 0/1/7 0/0/7 0/1/7 0/0/7 --- ---

zTests 95-1 and 96-2, three plants inoculated/melon plant; 96-1, one plant inoculated/melon plant.

ySymptomatic plants/uncertain symptoms/plants inoculated.

xAbsorbance at 405 nm; plants 3x control value/plants 2x control value/plants inoculated.

wSymptomatic plants/no. of plants inoculated.

vData not recorded at these wpi, or not tested on Chenopodium.

uRecorded 9 WPI.

tOne plant 2x control value at 14 weeks WPI.

 

one plants with uncertain symptoms, respectively, at that time. ELISA data from 'Top Mark' and 'PMR 5' in these three tests indicated higher frequencies of infection than indicated by symptoms. Symptom and ELISA data from the chenopodium plants that were serially inoculated from 'Top Mark' and 'PMR 5' showed higher frequencies of infection of these two lines than indicated by either their symptom or ELISA data in tests 96-1 and 96-2 (Table 2). The chenopodium

data from test 95-1 appear to be problematic: symptoms on chenopodium confirm the 'Top Mark' ELISA data, but the chenopodium ELISA data indicate a zero rate of infection. This is likely explained by the longer (2¥) than usual interval between the time these chenopodium plants were inoculated and assayed for the virus. Virus titer in these older, LIYV-affected plants had dropped to levels too low to be detectable.

 

Table 3. LIYV symptoms and ELISA values of three melon cultigens and their respective serially inoculated Chenopodium plant, test 96-1.

Melon Chenopodium

ELISAz ELISA

Cultigens Plant Symptomy 3¥ 2¥ 3 ¥ Cx 2 ¥ C Symptom 3¥ 2¥ 3 ¥ C 2 ¥ C

Top Mark 2-4 +? 0 1 1 1 + 1 1 1 1

3-2 +? 1 1 1 1 ­ 1 1 1 1

9-1 +? 0 0 0 1 + 1 1 1 1

PMR 5 2-3 + 0 0 0 1 + 1 1 1 1

3-3 + 0 1 1 1 ­ 0 0 0 0

4-3 + 1 1 1 1 ­ 0 0 0 1

6-2 + 1 1 1 1 + 1 1 1 1

7-2 + 1 1 1 1 + 1 1 1 1

10-1 ­? 0 0 1 1 + 1 1 1 1

C-87 6-1 ­? 1 1 1 1 + 1 1 1 1

3-4 ­? 0 0 0 0 ­ 0 0 0 0

zAbsorbance at 405 nm; 3¥ and 2¥ times control value.

yEvaluated at weekly intervals 4 through 8 weeks postinoculation: (+)symptomatic; (+?) and (-?)uncertain; (­)symptomatic. All nonnegative scores were observed at least once.

xCorrected for buffer.

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Table 3 presents individual plant data from three of the cultigens in test 96-1. Symptom and ELISA data are shown for each melon plant along with the same data from their respective serially inoculated chenopodium plant. The ELISA data are presented as 3x and 2x the control value with or without correction for the buffer. The three 'Top Mark' plants had uncertain symptoms at one or more of the dates when symptoms were recorded. One of them had a positive ELISA at 3¥ the uncorrected control value; two were positive at 2x the control value (with or without correction); and all three were positive at 2x the mean control value when corrected for the buffer. Interestingly, 'Top Mark' plant 9-1, which did not have a positive ELISA until after correction for the buffer at 2¥ the control value, produced symptoms on its respective chenopodium plant. In contrast, the one plant that had a positive ELISA without correction for the buffer at 3¥ the control value did not produce symptoms on its respective chenopodium plant. All three 'Top Mark' plants produced positive ELISA data on their respective chenopodium plants at 2¥ or the 3¥ the control value with or without correction for the buffer. Similar results were seen for 'PMR 5'. Two 'PMR 5' plants (3-3 and 4-3) with LIYV symptoms produced asymptomatic chenopodium plants one of which had a positive ELISA only at 2¥ the control value after correction for the buffer. This would suggest that chenopodium can provide negative results where melon symptom and ELISA data were positive for LIYV infection. This could have been due to failure of the SPW to acquire the virus from the source plant. Such failure could have resulted from placing SPW on a leaf that had a low titer of the virus as suggested by differences in ELISA assays of symptomatic and asymptomatic leaves from on of the 'PMR 5' plants (Figure 1). The melon ELISA data at 2x after correction for the buffer matches the chenopodium ELISA data at 3x and 2x with or without correction for the buffer more closely than the data at 3x with correction and the 3x or 2x without correction for the buffer (Table 3). In addition, using the 2x with correction threshold confirmed symptom expression in 'PMR 5' plant 3-3.

 

Despite the difficulties of infecting 'Top Mark' and 'PMR 5' in controlled greenhouse tests, PI 313970 appeared to be resistant to infection by LIYV in these three greenhouse tests. In 95-1, none of the 12 plants inoculated were symptomatic and none had a positive ELISA value (2x the control value) (Table 2). In test 96-1, one plant had what appeared to be uncertain symptoms 5 WPI. Four plants in test 96-2 had uncertain symptoms 5 WPI (one plant) and 6 WPI (two plants). In each case, the plant was asymptomatic one week later. None of the five plants in test 96-1 had a positive ELISA (Table 2). PI 313970 was not included in the serial transfers to chenopodium.

Four plants of C-87 had uncertain symptoms in test 96-1; one of these had uncertain symptoms at 6 and 7 WPI (Table 2). The single plant with uncertain symptoms at 5 WPI had a positive ELISA at 8 WPI, and produced a symptomatic chenopodium plant that had a positive ELISA at 3x and 2x the control value (Tables 2 and 3).

Three plants of C-105 had uncertain symptoms; one of these was scored uncertain at 6 and 8 WPI (Table 2). None had a positive ELISA value at 8 WPI, but at 14 WPI one of the C-105 plants was positive at 2x the control value. C-105 was not included in the serial transfers to chenopodium.

These field and greenhouse results indicate that PI 313970 is a potential source of high level resistance to LIYV. Subsequent greenhouse tests of PI 313970 that included serial transfers to chenopodium confirm these data (data not shown). C-105 remains a potential source of resistance to this virus.

 

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Figure 1. Changes in absorbance (%) at 405 nm with incubation time for ELISAs of a leaf from a healthy control plant (H), and of three leaves from a LIYV-infected 'PMR 5' plant 8 WPI: A, an asymptomatic leaf; M, leaf with mild symptoms; S, leaf with strong symptoms.

 

 

A backcrossing program should be used for transferring resistance from PI 313970 to western U.S. shipping type orange flesh and honeydew melons. The inheritance of LIYV resistance in PI 313970 will determine the details of such a backcrossing program (Scully and Federer, 1991). Preliminary analysis suggests that resistance may be conditioned by a single dominant gene (McCreight, 1998). Any backcross melon breeding program for the desert southwest United States will also entail selection for resistances to one or more foliar and soilborne fungal and viral diseases (McCreight, 1993).

Literature cited

Brown, J.K. and H.S. Costa. 1992. First report of whitefly-associated squash silverleaf disorder of Cucurbita in Arizona and of white streaking disorder of Brassica species in Arizona and California. Plant Dis. 76:426.

Brown, J.K. and M.R. Nelson. 1989. Characterization of watermelon curly mottle virus, a geminivirus distinct from squash leaf curl virus. Ann. Appl. Biol. 115:243­252.

Clark, T.M. and A.M. Adams. 1977. Characteristics of the microplate method of enzyme-linked immunosorbent assay for the detection of plant viruses. J. Gen. Virol. 34:475­483.

Cohen, S., J.E. Duffus, R.C. Larsen, H.Y. Liu, and R.A. Flock. 1987. Purification, serology, and vector relationship of squash leaf curl virus, a whitefly-transmitted geminivirus. Phytopathology 73:1169­1673.

Dickson, R.C., M.McD. Johnson, and E.F. Laird, Jr. 1954. Leaf crumple, a virus disease of cotton. Phytopathology 44:479­480.

Duffus, J.E. 1965. Beet pseudo-yellows virus, transmitted by the greenhouse whitefly (Trialeurodes vaporariorum). Phytopathology 55:450­453.

Duffus, J.E. 1995. Whitefly transmitted yellowing viruses of the Cucurbitaceae, p. 12­16. In: G.E. Lester and J.R.

Breeding for resistance to LIYV in melon will be a slow process in a greenhouse-based program. The relatively high incidence of false negatives for susceptible plants will reduce efficiency of selection. ELISA and serial transfer to chenopodium will eliminate false negatives where there is infection but no visible symptoms. Numbers of plants with positive ELISA were presented at two different thresholds (3x and 2x the control value) above which a plant is considered to be infected. For melon, the 2x threshold after correction for the buffer appears to be appropriate for minimizing the number of false negatives. The ELISA data may be useful for correcting test data used for testing genetic hypotheses in inheritance studies.

'Top Mark' is no longer included in greenhouse tests as a result of its high level of susceptibility to powdery mildew. The level of resistance to powdery mildew races 1 and 2 that is in 'PMR 5' is sufficient in Salinas to eliminate confounding of LIYV symptom expression by powdery mildew. A change from inoculation of individual plants to a mass-infestation procedure in subsequent tests has not consistently improved the efficiency of inoculation, but has greatly reduced the amount of labor required to carry out the inoculations while permitting inoculation of larger numbers of plants (data not shown).

PI 313970 has few desirable horticultural characters except perhaps for monoecious sex expression. It is originally from India and is the same as PI 315410 (U.S. Department of Agriculture, 1969). It was received from the All Union Institute of Plant Industry, Leningrad in 1996, but there is no information about exactly when or the locale from where it was collected. It is a typical C. melo ssp. melo Momordica Group accession except that the fruit do not appear to split at maturity (Robinson and Decker-Walters, 1997). Mature fruit from greenhouse plants are 15 cm long ¥ 8 cm in diameter, have a smooth surface, soft and yellow skin, white flesh, and a large seed cavity, i.e., they have thin flesh. As noted in the field test results, PI 313970 appears to be day-length sensitive for pis tillate flowers: staminate flowers were produced under long days; pistillate flowers were produced late in the fall under short days. PI 313070 had a rating of 1 on a 1 (low susceptibility) to 9 (high ) scale in a controlled greenhouse test for reaction susceptible to downy mildew, incited by Pseudoperonospora cubensis (USDA­ARS, National Genetic Resources Program. Germplasm Resources Information Network (GRIN). [Online Database] National Germplasm Resources Laboratory, Beltsville, Maryland. Available: www.ars-grin.gov/cgi-bin/npgs/html/coop.pl?65492 (30 September 1998)).

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Dunlap (eds.). Proc. Cucurbitaceae '94: Evaluation and enhancement of cucurbit germplasm. South Padre Island, Texas, 1­4 Nov. 1994. Gateway Printing and Office Supply, Edinburg, Texas.

Duffus, J.E. and R.A. Flock. 1982. Whitefly-transmitted disease complex of the desert southwest. Calif. Agr. 36(11­12):4­6.

Duffus, J.E., R.C. Larsen, and H.Y. Liu. 1986. Lettuce infectious yellows virusA new type of whitefly-transmitted virus. Phytopathology 76:97­100.

Duffus, J.E., H.Y. Liu, and M.R. Johns. 1985. Melon leaf curl virusA new gemini virus with host and serological variations from squash leaf curl virus. Phytopathology 75:1312 (abstr.).

Flock, R.A. and D.E. Mayhew. 1981. Squash leaf curl, a new disease of cucurbits in California. Plant Dis. 65:75­76.

Henneberry, T.J., N.C. Toscano, and S.J. Castle. 1998. Bemisia spp. (Homoptera: Aleyrodidae) in the United States: History, test status, and management. Res. Trends Agr. (in press).

Mayberry, K.S. and H. Johnson, Jr. 1982. Cantaloupe whitefly trial results. Imperial Agricultural Briefs, Coop. Ext. Univ. Calif. , Imperial County (December):6­7.

McCreight, J.D. 1991. Potential sources of resistance to lettuce infectious yellows in melon. Cucurbit Genet. Coop. Rpt. 14:51­52.

McCreight, J.D. 1992. Screening for lettuce infectious yellows virus resistance in melon, p. 160­162. In: R.W. Doruchowski, E. Kozik, and K. Niemirowicz-Szczytt (eds.). 5th EUCARPIA Cucurbitaceae Symp., 27­31 July 1992, Res. Inst. Veg. Crops, Skierniewice and Warsaw Univ. of Agriculture, Warsaw, Poland.

McCreight, J.D. 1993. Melon, p. 267­294. In: G. Kalloo and B.O. Bergh (eds.). Genetic improvement of vegetable crops. Pergamon Press, New York.

McCreight, J.D. 1998. Resistance to lettuce infectious yellows virus in melon. HortScience 33:533.

Robinson, R.W. and D.S. Decker-Walters. 1997. Cucurbits. CAB International, New York. p. 226

Scully, B.T. and W.T. Federer. 1991. Application of genetic theory in breeding for multiple virus resistance. In: M. Kyle (ed.). Proc. of Genet. and Breeding for Resistance to Viral diseases of Veg., the Inaugural Symp. of the Veg. Breeding Inst., Cornell Univ., Ithaca, N.Y.

Simmons, A.M. and J.D. McCreight. 1996. Evaluation of melon for resistance to Bemisia argentifolii (Homoptera: Aleyrodidae). J. Econ. Entomol. 86:1663­1668.

U.S. Department of Agriculture. 1969. Plant inventory no. 174. Plant material introduced January 1 to December 31, 1966 (nos. 310336 to 317903). USDA, Washington, D.C. p. 322.

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