Watermelon Fruit Blotch Infection Rates in Diploids and Triploids

Cucurbit Genetics Cooperative Report 19:70-72 (article 26) 1996

B.B. Rhodes, X.P. Zhang, J.T. Garrett and C. Fang
Horticulture Department, E 142 Poole Center, Clemson University, Clemson, SC 29634

Watermelon fruit blotch (WFB) caused by the bacterium now known as Acidovorax avenae subsp. citrulli was first noted in south Carolina in 1989. the pathogen from an infected ‘Prince Charles’ fruit was used to inoculate fruit of Sc-7, an inbred diploid line. Infested SC-7 seed were grown out at the Pee Dee Research and Education Center (PDREC) in 1994 at Florence, SC. In a triploid trial where SC-7 was the pollenizer (3), these plants exhibited a fruit infection frequency of 95% with severe symptoms. The 20 triploid varieties exhibited a range of resistance to secondary infection by the pathogen from SC-7, but all triploids were more resistant than the diploids.

In 1990, we made selections from genotypes identified by Sowell and Schaad (8) as having resistance in the seedling stage to the bacterium that Schaad et al. (7) identified as Pseudomonas pseudoalcaligenes subsp. citrulli. A selection of PI299378 identified by Sowell et al. (8) as resistant to P. pseudoalcaligines subspecies citrulli, but exhibiting large lesions in our seedling trial (5), was also saved. Seed were saved from fruit selfed in the greenhouse after inoculation with an isolate of Acidovorax avenae subsp. citrulli provided by Hopkins (University of FL, Leesburg). We reported possible resistance to WFB in certain seed lots of ‘Congo’ and in PI 295843 (5), but field observations by Hopkins et al. (3) did not confirm this resistance.

We planted these selections to test to hypothesis that WFB resistance existed in these germplasms. We also sought to evaluate more triploids for WFB resistance.

Materials and Methods: In 1995 we set up a trial similar to the 1994 triploid trial, including the selections saved from the 1990 experiment. We also included a seed lot of ‘TRITEN’ triploid that had been artificially inoculated by soaking in a 108 cfu WFB solution for ten minutes (4).

Selections from ‘Congo’, PI 295843 and PI 299378, all inoculated with WFB in 1990, infested SC-7 seed from 1989 and ‘TRITEN’ seed artificially infested with WFB in 1994, and clean triploid seed were selected into flats in April in the greenhouse at PDREC. These seeds were grown until May in a greenhouse with overhead watering before they were transplanted to the field. The SC-7 from 1989 which became 95% infected with severe symptoms in the same field in 1994 was transplanted in every other plot. After transplanting of 5 hills or less of each genotype in three randomized complete blocks, the blocks were watered with overhead irrigation weekly. Ammonium nitrate was used to sidedress the plants in early July near the time of anthesis, and leaf burn was evident on all the plots, in some cases enough to kill the plant. Almost all of the plants recovered completely from fertilizer burn. A month later, all the fruit in the field were rated for WFB. Percent infection (percentage of fruit infected in each plot) and severity of symptoms (rating of each fruit by size of lesions, rating scale in Table 1) were scored. Chi-square analysis was used to test the effect of genotype and ploidy level(1). Computation was performed using FREQ on SAS (6).

Results and Discussion: The artificially infested ‘TRITEN’ seed germinated poorly, and most seedlings died during germination from the inoculation with WFB. Although all but one of the ‘TRITEN’ seed infested with WFB did not survive at the seedling stage, the single plant that survived produced fruit free of WFB. In contrast to the trials of 1994, where infection frequency of SC-7 fruit at maturity was 95% and infection was in the most severe category, infection of SC-7 in this test was only 22.5%, with less severity. However, a bulked lot of seed from the pollenizers of the 1994 experiment, SC-7 and ‘Crimson Sweet’ were infected at a rate of 47.6%. Infection levels varied significantly among replications. The sparse infection on SC-7 and the significant difference among replications suggest that the pathogen did not spread quickly and uniformly across the field. The high levels of infection on all three replications of bulk 1994 PD indicate that the disease was expressed when it was present on susceptible fruit.

Infection frequency was significantly higher in the diploids, originating from infested seed, than in the triploids (P<0.0004). With the exception of the single ‘TRITEN’ plant, the triploids were secondarily infected in the field.

In 1994 and 19i95, we observed resistance of triploids to secondary infection during fruit development. Even so, in both tests there was significant variation in infection levels among triploid varieties.

Apparent differences existed among and within ‘Congo’ seed lots (Table 1). For example, one selfed ‘Congo’ selection within lot 109991(H-2) was free of WFB. The”resistant” selection had a dark green / darker green striped fruit. The “susceptible” selfed selection (H-1) of lot 109991 was segregating for light green / dark green striped fruit and dark green/darker green striped fruit. This association of infection with light green background was 90% in all the plots of the ‘Congo’ selections and is consistent with the observations of Hopkins et al. (2) that susceptibility is associated with light green fruit color.

There is now agreement that the WFB pathogen, Acidovorax avenae subsp. citrulli and the pathogen Pseudomonas pseudoalcaligenes subsp, citrulli that Sowell and Schaad (8) used to identify seedling resistance in ‘Congo’ and PI 295843 and 299378 are essentially the same (3). Thus, it is reasonable to expect that WFB resistance may exist among the genotypes where Sowell and Schaad (8) found seedling resistance to Pseudomonas pseudocalignes subsp. citruli.

Seed were saved from lesion-free fruit of ‘Congo’ adjacent to contaminated plots of SC-7. Seed were also saved from infected SC-7 and from the two PIs.

Table 1. Percentage and severity of fruit blotch in diploid and triploid watermelons¹

Primary Infection (seed infestation)
Genotype2	Ploidy	Total # Fruit	% Infection + std. dev.	Severity + std. dev.	Rind Color Pattern
Bulk ’94 PD	2N	19	47.6 + 4.1	3.0 + 0.5	mix
SC-7	2N	392	22.5 + 22.4	2.0 + 1.0	light green/green stripe
C-1 Congo	2N	22	63.9 + 12.7	4.4 + 0.7	two types3
C-2 Congo	2N	25	28.2 + 8.2	2.3 + 0.4	two types3
H-1 Congo	2N	14	17.5 + 10.6	1.7 + 0.4	two types3
H-2 Congo	2N	25	0	–	dark green/darker stripe
S Congo	2N	22	9.5 + 16.5	1.4 + 0.7	two types3
WR Congo	2N	13	6.3 + 8.8	1.4 + 0.5	two types3
M Congo	2N	33	3.3 + 8.8	1.1 + 0.2	dark green/darker stripe
PI 295843	2N	9	0	–	cream
PI 299378	2N	21	0	–	cream
TRITEN	3N	3	0	–	light green/green stripe
Secondary Infection (acquired from diploids and TRITEN)
AC 2532	3N	34	13.9 + 17.3	1.6 + 0.7	light green/green stripe
AC 3731	3N	22	9.5 + 16.5	1.6 + 1.0	light green/green stripe
AC 5244	3N	26	4.2 + 7.2	1.1 + 0.1	light green/green stripe
ACR 94W001	3N	12	4.2 + 7.2	1.1 + 0.1	light green/green stripe
AC 3521Y	3N	31	2.8 + 4.8	1.1 + 0	light green/green stripe
AC 5032	3N	22	0	–	light green/green stripe
AC 5444	3N	35	0	–	light green/green stripe
ACR 94W003	3N	19	0	–	light green/green stripe
Factor	X 2 Value	P Value
Genotype	98.00	<0.0001
Ploidy	24.25	<0.0001

¹ Three replications of 5-hill plots were planted. ‘SC-7’ contaminated with WFB was transplanted in every other plot. Per cent infection was calculated from the number of fruit infected and total amount of the fruit in the plot. Severity ratings on individual fruit were as follows: No lesions = 1; lesions < 6.45 cm2 = 3; lesions > 6.45 cm2 but < 100 cm2 = 5; lesions > 100 cm2 and open wounds = 7
² Seed sources of Congo: H – Holler, Inc. Rocky Ford, CO; C = coffey Seed Co., Plainview, TX; WR = US Vegetable Lab, Charleston, SC; M = Musser Seed Company, Twin Falls, ID; S = Shumway Seed Co., Graniteville, SC. Seed sources of PIs: Southern REgional PI Station, Griffin, GA. Seed sources of triploids: TRITEN – Xinjiang Western China Seed Company, Changhi, Xinjiang. AC Triploids – Abbott & Cobb, Inc., Feasterville, PA.
³ These lots were segregating for rind color: dark green background and darker stripes vs. light green background with green stripes.

Literature Cited

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