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Cucurbit Genetics Cooperative Report 17:66-68 (article 17) 1994

Races of Fusarium oxysporum f. sp. melonis Causing Wilt of Melons in Central Sudan

Yousif F. Mohamed and Gasim A. Dafalla; Sadig K. Omara

Department of Plant Pathology, Faculty of Agricultural Sciences, University of Gezira, Wad Medani, Sudan, AFRICA; Department of Horticulture, Faculty of Agricultural Sciences, University of Gezira, Wad Medani, Sudan, AFRICA

Fusarium wilt of melons is a disease of worldwide occurrence. Four races of the causal organism, Fusarium oxysporum Schlecht F. sp. melonis Snyyder & Hansen, are known to occur. These are RO, R1, R2, and R1-2. Race 1-2 is further subdivided into isolates that cause wilt and others that cause necrotic yellowing. This nomenclature, proposed by Risser et al. (1976), is the one adopted in the present study.

In Sudan, Fusarium wilt is observed to pose a grave threat to melons in the major melon producing areas in central Sudan, especially when the fall crop, which coincides with the rainy season (July-October), is grown in the central heavy clay plains. Early infections under these conditions could lead to total crop loss. The winter crop of the 'Gallia' type F1 's which is intended for export to Western Europe and the Middle East, also faces a similar threat from the disease, and crop losses of 40% or more are common. In contrast, melon land races, that still predominate in the White Nile growing areas of central Sudan, are less affected by the disease and have shown variable degrees of resistance.

The fungus attacks muskmelon at various stages of its growth and typical symptoms of the disease are often observed to occur at flowering and during early fruit-setting. In the mature infected plants, wilt is observed to progress rather slowly, with later symptoms appearing as tip-burning or leaf edges, foliar chlorosis, stunting of plants, and greatly reduced fruit sizes. When affected stems are sliced, yellow, orange or brownish staining is observed in the water-conducting tissues. Under moist conditions, a white or pink fungal growth may be visible on dead stems.

On further examination of diseased plants of 'Gallia', 'Alma', 'Ananas', and 'Regal' hybrids collected from melon fields in central Sudan and examined at INRA-Avignon at Montfavet Cedex-France, the pathogen isolated was positively identified as F. oxysporum f.sp. melonis. To identify the prevalent races of the fungal pathogen in central Sudan, two isolates of the fungus recovered from the diseased plants were inoculated into five different hosts of C. melo, vis., 'Charentais T', 'Isoblon', Isovac', 'Margot', and 'Isabelle'. Two weeks later, the inoculated differential hosts were graded for resistance and susceptibility and the results are shown in Table 1. These results clearly indicate the presence of Race 0 and Race 1 in the Sudan.

In another test, the popular muskmelon hybrids and cultivars grown in Sudan were exposed to inoculum from each of the four races of the fungus. The results shown in Table 2 indicate that 'Gallia' and 'Alma' are susceptible to all races, 'Ananas' is susceptible to races 1 and 1-2, while 'Regal' is resistant to all races except race 1-2. These results are clearly in line with our own field observations under Sudan conditions where 'Gallia' and 'Alma' are seen to suffer most, followed by 'Ananas' and then 'Regal', which was the least affected.

When field-tested during last summer and fall seasons at the University of Gezira research fields, varieties 'Isabelle', 'Margot' and 'Isovac' have proved highly resistant to the disease. Consequently, 'Isabelle' is now used as a donor parent in a backcross program to transfer wilt resistance to 'Ananas' and some other breeding lines intended for hybrid seed production. The wilt problem in the F1 hybrids grown for the export markets will probably remain with us for some time, until resistant 'Gallia' or 'Gallia'-type melons are developed in Sudan or otherwise supplied by foreign seed companies.

Table 1. Reaction of differential cultivars to known races and to Sudanese isolates.

Reaction to known races
Reaction to Sudanese races
Cultivar
Resistance genes
0
1
2
1-2
Isolate 1
Isolate 2
Charentais T
Non
S
S
S
S
S
S
Isoblon
Fom-1
R
S
R
S
S
R
Isovac
Fom-2
R
R
S
S
R
R
Margot
Fom-1 + Fom-2
R
R
R
S
R
R
Isabelle
Fom-1 + Fom-2 + Polygenic recessives
R
R
R
S
R
R

Table 2. Reaction of major cultivars grown in Sudan to the four wilt races.

Cultivar
Race 0
Race 1
Race 2
Race 1-2
Alma F1
S
S
S
S
Ananas
R
S
R
S
Gallia F1
S
S
S
S
Reagal
R
R
R
S

S = Susceptible reaction
R = Resistant reaction

Literature Cited

  1. Risser, G. Z. Banihashemi, and D.W. Davis, 1976. A proposed nomenclature of Fusariu oxysporum f. sp. melonis races and resistance genes in Cucumis melo. Phytopathology 66:1105-1106.
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