Citrullus lanatus – a Potential Host of Powdery Mildew in the Czech Republic

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Cucurbit Genetics Cooperative Report 23:46-48 (article 13) 2000

Eva Kristkova
Research Institute of Crop Production Praha, Division of Genetics and Plant Breeding, Department of Gene Bank,. Slechtitelu 11, 783 71 Olomouc, Czech Republic

Ales Lebeda
Palacky University, Faculty of Science, Department of Botany, Slechtitelu 11, 783 71 Olomouc, Czech Republic

Introduction .Within the cucurbits, watermelon (Citrullus lanatus / Matsum./ Thunb/ et Nakai) is generally considered the most resistant to powdery mildew (Erysiphe cichoracearum (Ec), Sphaerotheca fuliginea (Sf) (8).Its reaction to the Czech isolates of powdery mildew was evaluated as a part of our pathotype determination (3).

Material and Methods. A total of 72 isolates of powdery mildew (Ersiphe cichoracearum / Ec/, Sphaerotheca fuliginea / Sf/and mixed isolates of both species) were collected in 1997-1998 from field cultures of cucurbits (Cucurbita pepo and Cucurbita maxima) at 56 locations in the Czech Republic. they were maintained in-vitro on the cotyledons of Cucumis sativus cv. Marketer according to methods described by Bertrand (1).

Plant genotypes proposed by Bertrand (1) for pathotype determination were used. The differential set included C. melo genotypes Vedrantais and PMR 45, C. sativus cv. Marketer, C. pepo cv. Diamant F1 , C. lanatus cv. Sugar Baby and a Czech cv. Golias of C. maxima. Seeds of C. pepo andC. lanatus were kindly provided by Dr. F. Bertrand (France).

Response of differential genotypes to the powdery mildew isolates was evaluated in-vitro on leaf discs. Intensity of sporulation was assessed at 4, 7, 10 and 14 days after inoculation on a scale of 0 (no sporulation) to 4 (more than 75% of the disc surface covered by mycelium). Isolates with an intensity of sporulation 0-1 were classified as virulent and those with scores 2-4 were considered virulent. The average value of infection degree (ID) on each genotype was expressed as a % of disc surface covered by mycelium at time of the last evaluation.

Results and discussion. Sporadic mycelium development on disc margins and surface during the first two evaluations, followed by a reduction of mycelium growth, were observed on 2 Sf, 7 Ec and 8 Ec+Sf isolates. None of 8 Sf isolates were virulent to C. lanatus. Of 33 isolates of Ec and 31 mixed isolates of Ec + Sf, only eight Ec and one Ec + Sf isolates were significantly virulent to C. lanatus (Table 1). All of these virulent isolate were collected in 1998. Similar high virulence to the watermelon within isolates acquired in 1997 was not observed. Isolate 3.98 (Ec) was virulent also to the cucumis meloMR-1 and isolate 19/98 (Ec + Sf) was virulent to the C. melo PI 124112 (4).

Isolates originating from Praha (Prague) (altitude of 352 m a.s.l. and an average temperature during the vegetative growth period of 14.2 ˚C) were collected in the early September and isolates from the Bohemian-Moravian Highlands (surroundings of Trebic; altitude of 406 m a.s.l. and an average temperature during the vegetative period of 12.6 ˚C ) were collected August 24-25. These locations are not suitable for commercial cultivation of watermelon. Isolates originating from the warmest parts of the country did not express such a high virulence, in fact they were not virulent to the watermelon.

C. lanatus and C. colocynthis are mentioned as hosts of S. fusca and E. orontii in several countries of the West, Central and Eastern Europe (2). Both powdery mildew species were identified on watermelon in Slovakia in the 1980s (6) and symptoms of this disease were observed by A. L Kebeda on C. lanatus grown in a glasshouse in Moravia (Prostejov district) in the beginning of the 1990s/ At that time powdery mildew was not considered an economically important pathogen of watermelon.

A severe powdery mildew infection of S. fulginea on Citrullus lanatus was observed in Spain in the late 1990s (Dr. F. Bertrand, Avignon, France, 1999, pers. com.) and Sf caused economic losses in seedless watermelon production in California (USA) (7). Based on our investigation similar susceptibility of watermelon to the e. cichoracearum can be expected in the Czech Republic.

The geographic origin of the virulent isolates excludes their possible issue from host- pathogen interaction under natural and/or artificial ecopathosystems. On the contrary, the origin of these isolates could predict the probability of their virulence to watermelon. The host species C. pepo and C. maxima are grown throughout the Czech Republic and could serve as potential bridge species in spreading these isolates. Monitoring the virulence of pathogens is important not only under field conditions but also in-vitro. This strategy allows predicting epidemics and can be exploited by plant breeding programmes.

Table 1. Compatible response of Citrullus lanatus Sugar Baby to the Czech isolates of powdery mildew.

Isolate number Pathotypez ID (%) on C. lanatus` Host plant Location
E. cichoracearum
12/98 AB1CD 33 C. pepo ZUy Trebic
4/98 AB1B2CCmD 33 C. maxima Trebic
21/98 AB1B2CCmD 33 C. pepo VM Praha
9/98 ACCmD 50 C. pepo ZU Trebic
7/98 B1B2CCmD 50 C. pepo PU Trebic
1/98 AB1B2CCmD 50 C. maxima Trebic
3/98,11/98 AB1B2CCmD 52 C. pepo ZU Trebic
E. cichoracearum + S. fulginiea
19/98 AB1B2CCmD 50 C. maxima Praha

z C. pepo fruit shape according to (5): PU pumpkin, VM vegetable marrow, ZU zucchini.
y Comparable reaction to:

A C. sativus cv. Marketer C C. pepo cv. Diamant F1
B1 C. melo Vedrantais Cm C. maxima cv. Golias
B2 C. melo PMR 45 D C. lanatus cv. Sugar Baby

Literature Cited

  1. Bertrand, F. 1991. Les oidiums des Cucurbitacees. These Universite Paris-Sud-Orsay. Specialite “Phytopathologie”, 225 pp.
  2. Braun, U. 19915. Thepowdery mildes (Ersiphales) of Europe. Gustav Fischer Verlag, Jena, Germany 337 pp.
  3. Kristkkova, E. and A. Lebeda. 1999. Powdery mildew of cucurbits in the Czech Republic – species, pathotype and race spectra. In. The First International Powdery Mildew Conference, Programme and Abstracts, August 29 – September 2, Avignon, France, pp 14-15.
  4. Kristkova, E. and A. Lebeda. Reaction of Cucumis melo MR-1 and PI 124112 to the powdery mildew in the Czech Republic (prepared for submission to press).
  5. Paris, H,S, 1986. A proposed subspecific classification for cucurbita pepo. Phytologia 61 (3):133-138.
  6. Paulech, C. 1995. Huby mucnatkotvare (Erysiphales). Flor Solvenska X/1, Veda, Bratislava, Slovak Republic, 291 pp.
  7. Paulus, A.O., G.J. Holmes, M. Vilchez and J.L. Aguiar. 1999. Fungicides for the cotrol of cucurbit powdery miildew Sphaerotheca fuligineain California and North Carolina, USA. In: The First International Powdery Mildew Conference, Programme and Abstracts, August 29 – September 2, Avignon, France, pp. 44-45.
  8. Sitterly, W.R. 1978. Powdery mildews of cucurbits. In. Spences, D.M. (ed.) The powdery mildews. Academic Press New York, San Francisco, pp. 359-379.