Symptoms of bacterial blight of Pseudomonas syringae on cantaloupe - brown necroses of oily appearance on leaves and fruits - oily spots in depression on the fruits with cavities wide and deep in the flesh

Context

• France produces annually about 300,000 tons of cantaloupe (Cucumis melo var. cantalupensis Naud.) on about 15000 ha situated in the southwest, central west and southeast regions
• in 1993, cantaloupe fields in the Tarn-et-Garonne district of southwestern France were devastated by a bacterial blight that growers considered to be a new disease. That region has been the victim of the most important and re-occurring epidemics, notably in 1995, 1996 and 1997. In 1997, virtually all fields in the Lot and Tarn-et-Garonne districts were attacked and the regional governments officially recognized that the diseased fields had suffered a natural catastrophe.
• since 1993, this disease has been observed in all cantaloupe regions of France when favorable conditions for the disease prevail
• the rainy and cool spring of 2002 was conducive to the first severe epidemics observed in southeast France

Our main results

Identification of the causal agent as Pseudomonas syringae and creation of a collection of about 2000 strains of P. syringae from blight epidemics in France since 1995

Most of the strains isolated from blight epidemics are similar to the pathovar aptata 

Creation of an illustrated bulletin to aid visual diagnosis (only available in french)

Elucidation of potential inoculum sources for disease epidemics

• water retention basins used for irrigating fields harbor concentrations of P. syringae sufficient for inducing epidemics under favorable environmental conditions
• soil and seeds apparently do not favor survival of P. syringae in detectable quantities
• rain and snow, stream and lake water, wild plants and epilithic biofilms harbor populations of P. syringae

Identification of climate conditions favorable for disease epidemics. An analysis of several years of epidemiological data reveals that the risk of bacterial blight epidemics is important if, over a period of 4 days:

• the mean minimal temperature is less than 12-13 °C
• the duration of rain exceeds 7 – 9 h
• the cumulative amount of rain is greater than 11 mm.
• about 95% of bacterial blight epidemics have occurred during these conditions

Bibliography

• Mention P., Lavigne, D., Leix-Henry F., Bouchu S., Morris C.E., Prior P., Pitrat M., Mercy L., Dours O. 2004. Bactériose du melon : acquis et perspectives. PHM-Revue Horticole, 459, 22-26. https://hal.inrae.fr/hal-02682345
• Morris C.E., Glaux C., Latour X., Gardan L., Samson R., Pitrat M. 2000. The relationship of host range, physiology and genotype to virulence on cantaloupe in Pseudomonas syringae from cantaloupe blight epidemics in France. Phytopathology 90: 636-646. DOI : 10.1094/PHYTO.2000.90.6.636  https://hal.inrae.fr/hal-02698283v1
• Morris, C.E., Pitrat, M. (1998). La bactériose du melon : connaissances acquises et travaux en cours. PHM Revue Horticole, 44-47. https://hal.inrae.fr/hal-02685362v1
• Riffaud C.M.-H. 2002. La bactériose du melon: écologie et stratégies de lutte contre Pseudomonas syringae pv. aptata. INRA, Avignon, Unité de pathologie végétale, Montfavet (FRA), Université Claude Bernard – Lyon I (FRA). Thèse (Dr. d'Université) option Ecologie microbienne. 135 p. https://hal.inrae.fr/tel-02832845
• Riffaud C. M.-H., Morris C.E. 2002. Detection of Pseudomonas syringae pv. aptata in irrigation water retention basins by immunofluorescence colony staining. European Journal of Plant Pathology 108: 539-545. DOI : 10.1023/A:1019919627886ISTEX  https://hal.inrae.fr/hal-02683383v1
• Riffaud C.M-H., Glaux C., Guilbaud C, Dominguez H., Prior P., Morris C.E. 2003. Epidemiological clues for developing methods of control of bacterial blight of cantaloupe caused by Pseudomonas syringae pv. aptata. pp. 3-15 In: Iacobellis N. et al (eds). Pseudomonas syringae Biology and Genetics. Kluwer Plenum, London. https://hal.inrae.fr/hal-02833120

National working group on bacterial blight of cantaloupe

In 1997 a national working group on bacterial blight of cantaloupe was created. Our laboratory was among the founding members of this working group that also consists of representatives of regional experimental stations, extension services, inter-professional organizations, chambers of agriculture, grower's associations and INRA’s plant breeders. The group meets twice a year to plan and evaluate results from field experimentation and laboratory research to characterize the causal agent and its sources of inoculum, to identify the climatic factors favoring disease, and to evaluate different means of control.