Cucurbitaceae 1996 EUCARPIA VI Cucurbits Towards 2000 (PD & DR)

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Section: Plant Diseases & Disease Resistances

Abstracts

Resistance to Physiological Races of Fusarium oxysporum f.sp. melonis in Iberian Melon Genotypes

J. Alvarez, R. Gonzalez-Torres

Servicio de Investigacion Agroalimentaria (D.G.A.) Apartado 727, 50080 Zaragoza, Spain

Fusarium wilt of muskmelon, caused by Fusarium oxysporum f.sp. melonis (Fom), has been reported as an economically important problem in many countries and also in southeastern Spain. Four known races of Fom were found attacking muskmelon, in Almeria (races 0, 2 and 1,2 wilt), Cordoba (race 0), Huelva (race 0) and Murcia provinces (race 1). The disease is only well managed by using wilt-resistant muskmelon genotypes. Forty Iberian melon genotypes were tested by growing seedlings into plastic pots. Inoculum was prepared from isolates of Fom races 0, 1 and 2, collected from wilted melon in Almeria and Murcia provinces. ‘Amarillo Cascara Pinta’, ‘Amarillo Manchado’, ‘Banda de Godoy’, ‘BG 4078’, ‘Piel de Sapo Monoico’ and ‘Tortuga’ provided the highest level of resistance to Fom race 0 and 2, similar to that of control genotype ‘Charentais Fom 1’. It is interesting to note that all the genotypes with resistance to races 0 and 2 are local cultivars from Extremadura (western Spain), where the pathogens have not been established.

Three Fom race 1,2 isolates were inoculated on differential melon lines ‘Charentais T’, ‘Charentais Fom 1’, and ‘Charentais Fom 2’. One of the isolates belonging to yellow patotipe appeared to be the less agressive one. The two others, one from France and the other from Almeria, showed different reactions on the different hosts depending on the environmental conditions.

Variation in Cucurbita spp. for Field Resistance to Powdery Mildew

A. Lebeda¹ and E. Krístková²

¹Palacký University, Faculty of Natural Sciences, Department of Botany, 772 36 Olomouc-Holice, Czech Republic, and ²Research Institute of Crop Production, Praha; Gene Bank Division, 772 36 Olomouc- Holice, Czech Republic

Abstract. The aim of this work was to evaluate the variation in Cucurbita species for field (partial) resistance to powdery mildew of cucurbits caused by Erysiphe cichoracearum and Sphaerotheca fuliginea. For this purpose, two methods were used – evaluation of field resistance and evaluation of spore quantity production. The sixty-four cultivars of four Cucurbita species (C. argyrosperma /2 accessions/, C. maxima /7 access./, C. moschata /10 access./, C. pepo /45 access./) were screened in 1993 for field resistance to powdery mildew. The fifty-eight cultivars of C. pepo (48 zucchini type, 8 vegetable marrow, 2 cocomelle) were evaluated in three-year (1991 -1993) experiments for field resistance to the same disease. During both experiments, the degree of infection was evaluated subsequently (4-5 times) during vegetation using a 0-4 scale. The level of field resistance of each accession was expressed as percentage (%) of maximum infection degree of the sum of all subsequent assessments and as area below infection curve. Nine cultivars of C. pepo representing eight morphological groups were grown in 1994 in the field under conditions of natural powdery mildew infection. Ten most infected leaves were collected from each cultivar and evaluated for the infection degree according to a 0-4 scale. Two discs were cut out from the most infected part of one leaf and the number of spores on each disc was determined using a hemocytometer. The highest level of field resistance to powdery mildew showed C. argyrosperma and C. moschata, the lowest C. pepo. There are significant differences among the seven edible groups of C. pepo morphotypes in the field resistance to powdery mildew. The most resistant cultivars occurred in acorn and scallop groups. In highly susceptible C. pepo group zucchini, there are significant differences in the expression of field resistance to powdery mildew. The highest level of field resistance occurred e.g. by cvs. Acceste F1, Albina, Ambassador F1, Elite F1, Goldfinger, Parmanta F1. Spore yield on leaf discs was correlated with the level of field resistance. There were significant differences in spore yield between genotypes of C. pepo belonging to various groups of fruit morphotypes. Peroxidase isozyme polymorphism should serve as a useful method for discrimination of specific genotypes of C. pepo with various level of field resistance.

Heritability of greenhouse and field methods for evaluating powdery mildew resistance in tropical pumpkin.

Linda Wessel-Beaver and Doré Lazo-Flores

University of Puerto Rico, Mayaguez, Puerto Rico

Abstract. Heritability of polygenic resistance to powdery mildew (Erysiphe cichoracearum) was studied in a broad-base population of tropical pumpkin (Cucurbita moschata) using various measures of resistance. Eighty random half sib families from Puerto Rico Synthetic 3 (PRSYN3) were planted in the greenhouse in a reps-in-sets design with 16 families assigned to each of 5 sets. Families in each set were arranged in a randomized complete block design with four replications of one plant each. Four drops of spore suspension were micro-pipeted onto the upper surface of the second true leaf of each plant, placing one drop in each leaf quadrant. Greenhouse resistance was measured as average severity of infection of the four inoculation sites, frequency of infection (number of infected sites per leaf) and incubation period. Twelve to 13 days after inoculation plants were set in the field in the same experimental design as in the greenhouse with 4 m² plots consisting of single plants. We determined incubation period (from transplant to symptom development) on stems and leaves and, at 2 to 5 weeks after transplant, number of infected leaves, percent foliar infection and whole plant infection severity. The various types of greenhouse readings were highly correlated. Correlations between resistance readings in the field were significantly positive but much lower than between greenhouse readings. Correlations between greenhouse and field readings ranged from nonsignificant to intermediate. In PRSYN3 greenhouse screening was not useful for predicting the level of resistance to quantitatively controlled powdery mildew resistance in the field. Both greenhouse and field evaluation methods had very low or nonsignificant heritabilities. Selection for quantitative powdery mildew resistance will be difficult in PRSYN3 and will require the use of recurrent selection with progeny testing.

Anatomical and Cytological Mechanism of Cucumber Resistance and Susceptibility to Downy Mildew (Pseudoperonospora cubensis Berk. Curt)

H. Habdas, M. Staniaszek, R.W. Durochowski and E. Lakowska-Ryk

Research Institute of Vegetable Crops, stree Konstytucji 3 Maja 1/3, 95-100 Skierniewice, Poland

Abstract. Downy mildew has been the most serious disease of cucumber in Poland since 1965. Pickling cucumber F1 hybrids Aladyn, Atlas, Parys and Cezar with high level of resistance to downy mildew were developed. The anatomical and cytological studies of field cucumber have shown the presence of thick cuticle layer on the leaf epidermis in resistance F1 hybrids to downy mildew. Intense condensation of hyphae and necrosis of leaf cells may have restricted the pathogen penetration.

Genotype, Fruit Load, and Temperature Affect Monosporascus Root Rot/vine Decline Symptom Expression in Melon (Cucumis melo L.)

David W. Wolff

Texas Agricultural Experiment Station, The Texas A & M University System, 2415 E. Highway 83, Weslaco, TX 78596 USA (d- [email protected])

Monosporascus root rot/vine decline (MRR/VD) is a soil borne disease of melons caused by the fungus Monosporascus cannonballus. Tolerance to MRR/VD has been identified in melon germplasm, and we have observed that fruit load is associated with vine decline symptom expression. The objective of this study was to determine the effect of 0, 1, and 2 fruit per vine on vine decline symptom onset and severity. The test was conducted under two temperature regimes as well (Spring season – temperatures increasing; Fall season – temperatures decreasing). Three western-shipper US cantaloupe hybrids (‘Caravelle’ [very susceptible], O’Magnum 45′ [moderately susceptible], ‘Primo’ [partial tolerance]), and one Ananas hybrid (‘Deltex’ [highest tolerance identified to date]) were transplanted in a M. cannonballus infested field. Weekly disease symptom ratings were recorded prior to fruit maturity and continued for 5 weeks. Vine decline symptoms were delayed in each genotype by removing fruit prior to maturity. Symptom expression increased in severity as the number of fruit per vine was increased from 0 to 1, and from 1 to 2. The 0 fruit treatment of ‘Deltex’ and ‘Primo’ showed a noticeably lower increase in disease symptoms than the 1 and 2 fruit treatments, and did not show complete plot death at the last rating date. When comparing the 0 treatment of each hybrid, ‘Deltex’ showed less disease symptoms late in the season than the other 3 hybrids. MRR/VD symptoms were reduced in the Fall season, as temperatures went from warm to cool as compared to Spring (cool to warm). In the Fall test the 0 fruit treatments of all genotypes showed little decline symptoms even at the last rating. Plants under low temperature and physiological stress (no fruit load) can withstand MRR/VD.

Resistance of PI 124112 and ‘Eldorado-300’ melons (Cucumis melo L.) to Papaya Ringspot Virus Watermelon Strain

James D. McCreight and Patti Fashing-Burdette

U.S. Department of Agriculture, Agricultural Research Service, U.S. Agricultural Research Station, Salinas, California, USA

Melon (Cucumis melo L.) plant introduction (PI) 124112 was reported variable in response to inoculation with to papaya ringspot virus watermelon strain, but inheritance of the reaction of PI 124112 to PRSV-W has not been reported. Differential reactions among melon lines to PRSV-W isolates prompt the need to determine inheritance and allelic relationships of all sources of PRSV-W resistance. ‘Cinco,’ W1, W3, W4, W5, W6 have PRSV-W resistance derived from PI 180280 (10,11). ‘Eldorado-300,’ a honeydew-type melon developed in Brazil, was derived from a cross of W6 x ‘Amarelo.’ There are no readily available data of its reaction to PRSV-W. PI 124112 and ‘Eldorado-300’ were found to possess single dominant genes for resistance to PRSV-W. Tests to determine allelism of resistance in PI 124112 with Prv1, Prv2 and Nm are in progress.

Double-stranded RNA as a Test for Infectivity of ‘Yellowing Syndrome in Cucurbits in Almeria

Guerra-Sanz, J.M.¹ and I.M. Cuadrado

C.I.D.H.-La Mohonera Almeria (Spain). ¹Author for correspondence: C.I.D.H. P.O. Box 91 El Ejido (Almeria.- Spain). Fax: +34 50 558055; e-mail: [email protected]

‘Yellowing’ symptoms in melon have been observed since 1990 in Almeria. In some cases, those symptoms have been attributed to viruses transmitted by Trialurodes vaporariorum, however, in other cases none Trialurodes vaporariorum has been noticed during the crop and the presence of other whitefly, namely Bemisia tabaci, has been recorded simultaneously to ‘yellowing’ in cucurbits in Almeria.

The ‘yellowing ‘symptoms in cucurbits observed in Almeria are: Internervial chlorotic spots in the basal and middle leaves of the plant at the beginning of the appearance, then the yellowing patches grow until the leaf becomes completely yellow, even the nerves.

A test based on the dsRNA profiles of crop plans with ‘yellowing’ symptoms in Almeria is proposed, which could allow us to do an accurate discrimination between ‘infected’ and ‘not-infected’ plants.

Standard dsRNA extraction procedure was applied to the plan samples, and molecular weight of toludine-blue-stained agarose gels were obtained.

In melons, two bands can be seen with a MW of 14.3 and 9.95 kb respectively, which are resistant to DNase digestion, but not to RNase. However, in healthy control of melons, these two bands can be also seen, and therefore no conclusions can be deduced from the test. On the other hand, the cucumber banding is a little more complicated because two different patterns have been found in our little survey among the crops of Campo de Dalias: there are samples with a unique band of 10.6 kb and other samples with two bands (in more concentrated samples, up to three bands can be found) of 10.6 kb and 3.4 kb respectively. There were not any bands in healthy controls.

When those MW were compared with published results in the bibliography, we tentatively think that there are at least two different viruses present in our crops: one is possibly the already described Beet Pseudo-Yellows Virus and the other has not been described yet, but is possibly another member of the Closteroviruses group, attending to the MW and the vector of transmission (Bemisia tabaci). These preliminary results have yet to be completed.

Evaluation of Cucurbita spp. Germplasm Resistance to CMV and WMV-2

A. Lebeda¹ and E. K4ístková²

¹Palacký University, Faculty of Natural Sciences, Department of Botany, 772 36 Olomouc-Holice, Czech Republic, and ²Research Institute of Crop Production, Praha; Gene Bank Division, 772 36 Olomouc- Holice, Czech Republic

Abstract. A set of about 400 C. pepo and C. maxima genotypes was screened for the resistance to the Cucumber Mosaic Virus (CMV) and Watermelon Mosaic Virus (WMV-2) after an artificial inoculation in greenhouse or growth chamber. In 1994, a total of 384 accessions were screened for the resistance to the CMV (254 C. pepo and 130 C. maxima acc.) and in 1995, practically the same set of 438 accessions was tested for the resistance to the WMV-2 (279 C. pepo and 159 C. maxima acc.). This set was formed by accessions of Cucurbita pepo genetic resources provided by PI Stn., Ames, (USA) and by Cucurbita maxima genetic resources from New York State Agr. Exp. Stn., Geneva, (USA) with addition of two commercial C. pepo cultivars. The CMV- yellow strain and WMV-2 were isolated in Czech Republic from cucumber. The seeds of accessions under study were incubated and the seedlings were cultivated in boxes with garden soil and/or sterile Perlite respectively and they were kept at 20- 22C/18-20C day/night temperatures, under 12 hr photoperiod and cca 100 W/m2 light intensity. At the cotyledon stage, i.e. before the first true leaf developed, the seedlings were mechanically inoculated by rubbing with virus inoculum. Disease severity was assessed visually 7-10 days (on cotyledons) and 14-18 days (on true leaves) after inoculation with CMV and 21-25 days (on true leaves) after WMV-2 inoculation using a 0 – 3 scale (O = symptomless, cotyledons and true leaves free of any visual symptoms). For each accession represented by 10-12 plants, the mean values for disease severity of cotyledons and separately of true leaves were expressed as a percentage of the maximum score. This value – infection degree – varies from 0 (without symptoms of infection in any plant) to 100 (most severe symptoms of infection on each plant). The whole set is significantly more susceptible to the WMV-2 than to the CMV. 54.7% of C. pepo and 89.2% of C. maxima accessions were found to be without systemic symptoms of CMV infection. On the contrary, only 1.1% of C. pepo and 3.1% of C. maxima accessions did not develop the systemic symptoms of WMV-2 infection. The C. maxima accessions are less susceptible to the CMV than C. pepo accessions. The response of both Cucurbita species to the WMV-2 was similar. The C. pepo accession PI 193501 was fully susceptible to both viruses. No accession was found to be completely free of symptoms of infection with both viruses. C. pepo acc. PI 438699 and C. maxima acc. PI 176530, PI 178891, PI 265555, PI 295342, PI 368564, PI 368572, PT 370458, PI 458685, PI 458691, PT 458693, PI 482466, PI 490352, PI 500483 did not develop visual symptoms of CMV infection on cotyledons and true leaves. The C. pepo acc. PI 512875, PI 518867 and PI 518688 and C. maxima acc. PI 184745, PI 265557, PI 368570, PI 379291 and PI 458672 were free of visual systemic symptoms of WMV-2 infection. The C. maxima acc. PI 265557, PI 295342, PI 368572, PI 458691, PI 500483 and C. pepo acc. PI 512875 could serve a potential sources of resistance to both viruses after a detailed study of this phenomenon using serological assays and special methods of molecular biology or genetics. The frequent occurrence of the WMV-2 on the field crops and the severe symptoms after its artificial inoculation indicate that the next resistance breeding programmes would be aimed at this pathogen.

Feeding Behaviour of Aphis gossypii during Transmission of Cucumber Mosaic Virus on Resistance and Susceptible Cucumis melo Lines

B. Martin¹, J.I. Alvarez² and A. Fereres¹

¹Consejo Superior de Investigaciones Cient’ficas. Centro de Ciencias Medioambientales.C/Serrano, 115 dpdo.28006 Madrid, Spain, and ²S & G Semillas S.A.Carretera de Málaga km 87,2.04700 El Ejido (Almería), Spain

Abstract. Both the aphids and the virus they transmit, cause every year severe damage on melon crops world-wide. The melon aphid, Aphis gossypii Glover, is a serious direct pest of melon and an important vector of non-persistent viruses such as cucumber mosaic virus (CMV), watermelon mosaic virus 2 (WMV-2) and zucchini yellow mosaic virus (ZYMV). The melon plants bearing the Vat gene (virus aphid transmission resistance) are resistant to A. gossypii by means of antixenosis and antibiosis mechanisms, linked to resistance to virus inoculation by this aphid. This resistance is vector- specific but not virus-specific.

It was presumed that the behavioural differences induced by the Vat gene in the melon aphid could be related to virus transmission resistance. So as to elucidate this point, the feeding behaviour of two aphid species, Aphis gossypii and Myzus persicae were studied, with the help of the electrical recording techniques, during CMV transmission on two closely related melon lines, one resistant and the other susceptible to A. gossypii. The electronic monitoring system gave us information about the aphid activities and showed the position of the stylet tips in the plant tissues, allowing a precise study of the aphid feeding process.

Although no correlation among virus transmission and probing behaviour has been found yet, significant differences were observed in the behaviour of the aphids, between the resistant and susceptible melon genotypes. These differences were mainly related to the duration of the intracellular penetrations of the stylet of the aphids. It is important to bear in mind that non- persistent virus transmission takes place during this kind of intracellular probes. It was also observed a major preference of A. gossypii for melon plants, proving to be more efficient virus vector than M. persicae.

Diversity of Resistance Sources to Cucurbit Aphid-Borne Yellows Luteovirus in Melon and Genetics of Resistance

C. Dogimont¹, A. Bussemakers¹, S. Slama¹, J. Martin¹, H. Lecoq², and M. Pitrat¹

¹Station d’Amelioration des Plantes Maraicheres, and ²Station de Pathologie Vegetale, Institut National de la Recherche Agronomique (INRA), BP 94, F-84143 Montfavet cedex, (France)

Cucurbit aphid-borne yellows luteovirus (CABYV) has been described as the main causal agent of a damaging yellowing disease of cultivated cucurbits in France and throughout the Mediterranean area. It is a phloem-limited virus, transmitted by Aphis gossypii (Glover) and Myzus persicae (Sulzer) in a persistent manner.

Five hundred twenty-three melon accessions originating from different parts of the world were evaluated for resistance to CABYV in south-eastern France, under natural infection conditions. Several potential sources of CABYV resistance were found, mostly originating from India, but also some coming from Korea and Africa. The resistance of most promising genotypes was confirmed in a field trial in five locations and under controlled conditions using viruliferous Myzus persicae to inoculate plants. Resistant genotypes are 90625, Faizabadi Phoont, PI 124112, PI 124440, PI 164723, PI 164797, PI 255478, PI 282448, PI 414723. Other accessions were found segregating for CABYV resistance.

The genetic control of resistance in PI 124112 was studied in a F2 progeny and on recombinant inbred lines in open field, and in a back-cross progeny in artificial inoculation. In each case, segregation analysis was consistent with the hypothesis that the resistance to CABYV in PI 124112 was conferred by two complementary recessive genes.

A Survey of Tolerance to Aphis gossypii Glover in Part of the World Collection of Cucumis melo L.

G. Weston Bohn, Albert N. Kishaba and James D. McCreight

U.S. Department of Agriculture, Agricultural Research Service, U.S. Agricultural Research Station, Salinas, California, USA

Tolerance to melon aphid from PI 414723 consists of freedom from leaf curling and stunting. Though independently controlled, these traits can easily be evaluated in mass-infested, greenhouse tests. Fifty-six U.S. Plant Introductions and other accessions of Cucumis melo L. from different sources including the melon aphid susceptible ‘PMR 45’ were evaluated for to the Western biotype of Aphis gossypii Glover in controlled greenhouses at Riverside, California. Tests were mass-infested at the first true leaf stage of growth (most populations at 14 to 16 days after planting) with aphids cultured on aphid-susceptible melons. Plants were rated on arbitrary 1 to 9 scales for severity of leaf curl (1-3, little curl; 2-6, medium; 7-9, severe) and plant vigor (1-3, poor; 4-6, medium; 7-9, good) 10 days after infestation. The susceptible check ‘PMR 45’ was severely curled and stunted; some plants were killed by the aphids. That response has been typical of all USA cultivars of C. melo. Melon aphid tolerance was noted frequently in this limited survey. Some of the entries had high levels of expression for both forms of tolerance. In contrast, others had high level of expression of freedom from curling and moderate or low level of expression of freedom from stunting. None were found to have high levels of expression for freedom from stunting with low or moderate level of expression for freedom from curling. Occurrence of melon aphid tolerance in many different melon accessions should enhance its usefulness to breeders because it may easily be transferred simultaneously with other traits of interest to improved breeding lines and populations.

Resistance to Western Flower Thrips in Cucumber

A.G. Balkema-Boomstra and C. Mollema

DLO-Centre for Plant Breeding and Reproduction Research CPRO-DLO, Wageningen (The Netherlands)

Introduction. Western Flower Thrips (WFT) is a major pest throughout greenhouse crops in Europe, the United States and Canada (1). The damage caused by this insect is both direct (feeding punctures and necrosis) and indirect (fruit malformation and transmission of tomato spotted wilt virus). Chemical control is difficult, because WFT has developed resistance to insecticides, because of incompatibility with biological control and because of the inaccessible stages of WFT. Particularly in cucumber, biological control is not satisfactory. A solution to this problem can be found in host plant resistance. To study host plant resistance, methods were developed to rear and synchronize WFT (3) and quantify WFT damage (2). In this paper, the origin of sources of resistance to WFT in cucumber (Cucumis sativus L.) will be discussed.

Materials and Methods. A total of 367 cucumber accessions were obtained from the CPRO-DLO genebank. The accessions were chosen from seven regions: Western Europe (mainly The Netherlands), Eastern Europe, Turkey, the Near East (Egypt, Israel, Lebanon, Syria), Asia (Iran, India, Pakistan, Birma), the Far East (Vietnam, China, Japan) and the U.S.A. The accessions were evaluated in groups of 20 to 40 genotypes in four replicates. The 60 most interesting accessions were evaluated a second time in four experiments with a total of eleven replicates. After germination in sand, the plants were grown in a greenhouse in pots with peat soil (18-23 C). The cultivar ‘Corona’ (De Ruiter Seeds) was used as a susceptible control in all the experiments. Standard infestation with WFT was done by depositing 30 young first instars on each first true leaf of the seedlings. The damage of this leaf was evaluated 2 weeks after infestation, before the next generation of WFT had developed. WFT-damage was quantified with an image-analysis device (2). This system determines the total leaf area and the damaged area in mm2. The level of damage was expressed in relation to the damage of ‘Corona’. Three resistant accessions and the susceptible control ‘G6’ were also tested at the mature plant stage in separate compartments of a greenhouse under semi- practice conditions, i.e. a no-choice situation. In twelve compartments, three per accession, 12 cucumber plants were grown. First true leaves were inoculated with 25 first instar (L1) and 25 second instar (L2) larvae. The damage on leaves 7 to 16 was given an index value on a scale of 0 (no damage) to 5 (maximal damage) 7.5 weeks after inoculation.

Results and Discussion. The results of the screening showed significant differences in the amount of damage among the accessions tested. Damage levels varied from 35 to 113% in relation to the susceptible control ‘Corona’. The accessions from ‘the Near East’ were on the whole the least damaged: the mean damage was respectively 58% and 56% in relation to ‘Corona’ in the two experiments. The Western European accessions were the most susceptible with a mean damage of respectively 92 and 79% in relation to ‘Corona’ (Table 1).

Table 1. Number of tested cucumber accesions from different regions and the mean damage by WFT instars of these accessions, relative to the damage of the susceptible cultivar ‘Corona’.
Region
Experiment 1
Experiment 2
n
mean damage
n
mean damage
Western Europe 54 0.92 8 0.79
Eastern Europe 6 0.63 2 0.65
Turkey 71 0.76 10 0.74
Near East 38 0.58 13 0.56
Asia 89 0.86 6 0.68
Far East 96 0.80 12 0.75
U.S.A. 13 0.55 7 0.64
Total 367 0.80 58 0.69

The resistance to WFT of three cucumber accessions was confirmed in the experiment with full grown plants under continuous exposure to thrips in a no-choice situation (Table 2).

Table 2. Damage caused by WFT to four cucumber accessions. Damage in mm2 on first leaf measured two weeks after inoculation. Damage index determined 7.5 weeks after inoculation (0=no damage; 5=maximal damage). Values followed by the same letter are not significantly different (P<0.05).
Accession
mm2 damage (leaf 1)
damage index
G6 (susc.) 623a 3.25a
9104 335b 0.27b
9140 265b 0.43b
9143 224b 0.61b

In additional experiments with the same accessions, it has been shown that WFT reproduction (larvae/female/24 hours) on young full grown leaves highly correlates with the damage index determined in the above mentioned experiment (De Kogel et al, in preparation).

In life-history studies with WFT by Soria & Mollema (4) with five resistant accessions, it was shown that reproduction rates were reduced and preadult mortality was higher on all of the evaluated resistant genotypes than on the susceptible control. On certain resistant genotypes, the developmental period was also prolonged.

All these results demonstrate that the accessions selected for low damage in the initial screening have WFT resistance rather than tolerance.

Although the Near East in particular was found to be a rich source of resistance to WFT, the other areas should not be neglected. It is possible that accessions of one region are genetically related and thus have similar resistances. The chance to find genetically different sources of resistance will be greater when accessions from different origin are used.

Conclusion. Among 367 cucumber accessions, several promising sources of resistance to WFT were found. WFT resistance can be detected by selecting for low damage on first true leaves after inoculation with L1 instars. The resistant accessions are subject of further studies e.g. genetics and causal factors. Thus host plant resistance to WFT could be a characteristic of future cucumber cultivars.

Literature cited:

  1. Brdsgaard, H.F. 1989. Frankliniella occidentalis (Thysanoptera: Thripidae) a new pest in Danish glasshouses; A review. Tidsskr. Planteavl. 93:83-91.
  2. Mollema, C., Dijken, F.R. van, Reinink, K. & Jansen, R.C. 1992. An automatic and accurate evaluation of thrips-damage. Image analysis: a new tool in breeding for resistance, pp. 261-263. In S.B.J. Menken, Visser, J.H. & Harrewijn, P. (eds), Proc. 8th Int. Symp. Insect-Plant Relationships, Kluwer Acad. Publ., Dordrecht.
  3. Mollema, C., Steenhuis-Broers, M.M. & Inggamer, H. 1995. Genotypic effects of cucumber responses to infestation by Western Flower Thrips. In B.L. Parker et al.,Thrips Biology and management, Plenum Press, New York.
  4. Soria, C. & Mollema, C. 1995. Life-history parameters of western flower thrips on susceptible and resistant cucumber genotypes. Entomol. Exp. Appl. 74:177-184.

Resistance to Bemisia tabaci in Cucumis melo

A.I.L. Sesé, C. Soria & M.L. Gómez-Guillamón

Experimental Station La Mayora, #-29750 Algarrobo-Costa, Málaga, Spain

The increase that the population of the whitefly Bemisia tabaci has shown since 1992 in Southern Spain has notably affected cucumber and melon crops since B. tabaci is the vector of a new yellowing virus. In order to evaluate possible resistance mechanisms against the whitefly in several Cucumis accessions (C. melo ‘TGR 1551’, C. melo var. agrestis, C. meeusii and C. melo cv. ‘Bola de Oro’) three parameters associated with the degree of antibiosis were studied: reproduction, developmental time and adult longevity.

On the basis of our results, reproduction showed clear significant differences among the accessions. Using whitefly reproduction, two groups could be established: one including C. melo cv. ‘Bola de Oro’ and C. meeusii as susceptible accessions, and another including C. melo var. agrestis and C. melo ‘TGR 1551’ as resistant accessions against B. tabaci.