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Cucurbit Genetics Cooperative Report 14:98-101 (article 34) 1991

Screening of Domestic and Wild Citrullus Germplasm for Resistance to the Yellow-Stunting Disorder in the United Arab Emirates

A.A. Hassan, N.E. Quronfilah, U.A. Obaji, M.A. Al-Rays and M.S.Wafi

Faculty of Agricultural Sciences, U.A.E. University, Al-Ain, U.A.E. (first authors) and Department of Agriculture and Animal Production, Al-Ain, U.A.E. (other authors).

Watermelon, Citrullus lanatus (Thunb.) Matsum & Nakai, is one of the most important vegetables in the U.A.E. However, it's production has been seriously curtailed in recent years due to a yellow-stunting disorder (YSD) caused by a new virus of the closterovirus-like group similar to the lettuce infectious yellows virus (LIYV) (2). It was observed in most samples examined by electron microscopy as long filamentous particles (3). It infects all cucurbits commercially grown in the U.A.E. and is thought to be transmitted by the tobacco whitefly, Benisia tabaci Genn.

All attempts made localy to control the disorder through the control of whiteflies has failed. The use of spunponded polysters and polypropylene plant covers for about 35 days (until flowering) was promising in controlling the disorder in muskmelon (4). However, this practice has not gained acceptance among U.A.E. melon farmers, probably due to its high cost. At present, none of the tested commercial watermelon cultivars were found resistant to this disorder (1, 2). The use of resistant cultivars would be the simplest and most efficient control method. Therefore, the objective of this study was to search for sources of resistance to the YSD in C. colocynthis and in a large collection of domestic and wild C. lanatus accessions.

Four hundred fifty-eight wild and domestic Citrullus accessions were evaluated for resistance to the YSD under field conditions in Al-Ain, U.A.E. in the spring and autumn plantings of 1989. The spring trial included 441 and 16 USDA plant introductions (PIs) of C. lanatus and C. colocynthis, respectively. The experiments: (a) 15 C. lanatus PIs in comparison with cv. Crimson Sweet (Table 1) and (b) C. colocynthis PIs in comparison with a local (U.A.E.) strain o f the same species, and the watermelon cv. Charleston Gray (Table 2). Seeds of PIs evaluated were kindly provided by the USDA through Dr. G. Lovell (Plant Introduction Station, Experiment, Georgia).

Seeds were sowed on March 10 and September 3 in the spring and autumn trials, respectively. After thinning, individual plants were spaced 50 cm apart in rows 2.0 m-wide. Each accession was assigned to one 8.0 m row in the spring trial and to 2 to 6 similar rows in a completely random design in each of the autumn experiments. The number of plants evaluated of each accession were as follows: 3 - 16 C. lanatus (mean = 9.5, sd = 2.74, mode = 10) and 3-11 C. colocynthis in the spring trial; and 2-6 per replicate of either species in the autumn trial.

Natural infection to produce the YSD was enhanced by placing heavily whitefly-infested melon plants, showing severe symptoms of the disorder, nearby the evaluated plants one week after germination. No pesticides were applied to maintain a high population of viruliferous whiteflies which were continuously observed on the plants in both trials. Plants were examined for symptomsof the YSD 10 and 12 weeks after sowing. Each plant was given a disease score according to the following scale: 1 = no symptoms; 2 = slight; 3 = moderate; and 4 = severe symptoms. Individual plant ratings for each accession were added, mean disease score (MDS). Then, accessions were classified as either resistant (R), slightly susceptible (SS), moderately susceptible (MS), or highly susceptible (HS) when their MDSs were 1,0-1,5, 1.6-2.5, 2.6-3.5, and 3.6-4.0, respectively. Only the second reading was assigned to accessions which were classified in the first reading of the spring trial as R or SS. These accessions were signaled out in the delayed symptoms development category (Table 1).

C. lanatus accessions: Out of 441 C. lanatis PIs evaluated in the 1989 spring trial, only 2 (0.45%) were rated R, 4 (0.91%) and 30 (6.80%) were found SS and MS, respectively; while 405 (91.84%) PIs were found HS. All R and SS PIs, 9 of the MS PIs and 4 HS PIs showed delayed symptom development. In the autumn trial, PIs 186490, 189318, 346082, 482258, 482264 and 482269 had significantly lower mean disease scores (MDSs) than 'Crimson Sweet' at 10 weeks after sowing. However, these significant differences disappeared two weeks later, though some variations in MDSs remained among accessions. PIs exhibiting the lowest MDSs at 12 weeks after sowing were 189316, 189318 and 482258. Delayed symptom development, regardless of the final MDS assigned to an accession, was believed to be a dependable criterion for differentiating genotypes in their response to the YSD.

C. colocynthis accessions: Out of the 16 C. colocynthis PIs evaluated in the spring trial, one (PI 235118) was highly susceptible, 2 were MS, 6 were SS, while 7 9PIs 386015, 386016, 386024, 386026, 494528, 494529 and 494530) were R (Table 2). Elevin out of 13 accessions evaluated in the autumn trial remained significantly lower (P = 0.01) than 'Charleston Gray' in MDS 12 weeks after sowing. The highest level of resistance (MDS - 1.0-2.0) was found in PIs 386015, 386016 386018. 386025, 494528, 494530 and the U.A. E. strain.

Table 1. Response of selected C. lanatus accessions re-evaluated in the 1989/1990 autumn planting to the yellow stunting disorder.

Accessions (PI)
Response in spring, 1989a
No. of replicates

Mean disease score

(weeks after sowing)

     
10 weeksb
12 weeksc
186490
MS
6
3.26*
3.52
189316
HS-d
6
3.62 n.s.
3.27
189318
HS-d
6
2.93*
3.12
294008
MS-d
6
3.89 n.s.
4.00
346082
SS=d
3
3.11*
3.57
457916
ss-d
2
3.38 n.s.
3.65
481871
R-d
3
3.44 n.s.
3.90
482247
MS-d
4
4.00 n.s.
4.00
482258
MS-d
6
2.93*
3.32
482264
MS-d
4
2.71*
4.00
482269
MS-d
5
3.27*
3.60
482275
SS-d
3
3.44 n.s.
3.50
500349
R-d
4
3.63 n.s.
3.75
500350
SS-d
3
3.78 n.s.
4.00
500352
MS
4
3.25 n.s.
4.00

'Crimson Sweet"

(Control)

5
4.00
4.00

 aR = resistant' SS = slightly susceptible' MS = moderately susceptible; HS = highly susceptible. The letter 'd' refers to delayed symptom development at 12 weeks after sowing.

b LSD at P = 0.05 between accessions with 2, 3, 4, 5 and 6 replications and the control (cv. 'Crimson Sweet') were, respectively, 0.95, 0.82, 0.76, 0.72 and 0.69. Accessions were either not significant (n.s.) or significant (*) from the control.

cAccessions were not significantly different from the control at P = 0.05.

Table 2. Response of C. colocynthis accessions to the yellow-stunting disorder.

 
Spring 1989
Autumn 1989/1990
Accession
Number evaluated
MDSa
No. of replicates
Mean disease score (weeks after sowing)
10 weeksb
12 weeksb
235118
7
4.0
-
-
-
386015
11
1.5
2
1.30**
1.80**
386016
6
1.0
5
1.32**
1.72**
386018
9
1.9
4
1.44**
1.50**
386019
11
2.4
-
-
-
386024
6
1.2
4
1.51**
2.38**
386025
6
2.3
5
1.28**
1.64**
386026
9
1.2
5
1.51**
2.26**
388770
8
3.0
-
-
-
432337
3
2.0
2
2.43**
2.15**
494527
6
2.5
6
2.81**
3.15 n.s.
494528
9
1.3
6
1.51**
1.72**
494529
9
1.0
5
2.52**
2.44**
494530
10
1.0
5
1.60**
1.88**
494531
7
3.0
-
-
-
494532
10
3.4
2
2.92 n.s.
3.25 n.s.
U.A.E. (local)
-
-
2
1.25**
2.00**

'Charleston Gray'

(Control)

-
-
6
3.76
4.00

aMDS = mean disease score. Readings were recorded 12 weeks after sowing.
bLSD between sccessions with 2, 4, 5 and 6 replications and the control (cv. Charleston Gray) were respectively, as follows:
P = 0.05 at 10 weeks: 0.99, 0.78, 0.73 and 0.70.
P = 0.01 at 10 weeks: 1,32, 1.04, 0.98 and 0.93.
P = 0.05 at 12 weeks: 1.21, 0.99, 0.94 and 0.90.
P - 0.01 at 12 weeks: 1.61, 1.32, 1.25 and 1.20
Accessions were either not significant (n.s.) or highly significant (**) from the control.

Literature Cited

  1. Hassan, A.A. and H.H. Al-Masri. 1990. Preliminary evaluation of cucurbits for resistance to the yellow stunting disorder. Emirates J. Agr. Sci. 2.
  2. Hassan, A.A. and J.E. Duffus. 1990. A review of a yellowing and stunting disorder of cucurbits in the United Arab Emirates. Emirates J. Agr. Sci. 2.
  3. Lecoq, H. 1986. Report of a visit to the United Arab Emirates to study a yellowing and stunting disorder of cucurbit crops INRA, Monfavet, France. (Available at the Dept. of Agr. & Animal Prod. Box 1004, Al-Ain, USE).
  4. Ministry of Agriculture and Fishery, UAE. 1987. A report on the decline of melon and watermelon crops. Part I (in Arabic). Northern Agr. Region, (Available at the Ministry of Agriculture and Fishery, Box 1509, Dubai, UAE).
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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 14 December, 2009