Wilt Disease of Melon (Cucumis melo L.) and Associated Organisms in Sudan

Cucurbit Genetics Cooperative Report 20:32-34 (article 15) 1997

Abbas M. Suliman, Yousif F. Mohamed, Gasim A. Daffallan and Omer H. Giha
Plant Pathology Center, Faculty of Agricultural Sciences, Gezira University, Wad Medani, P.O. Box 20, Sudan, Africa

Muskmelon in the Sudan is traditionally grown in small fields and is consumed locally. As such the crop has received very little attention. However, with the recent introduction of some high-value commercial varieties such as ‘Galia’ and ‘Ananas’ for export purposes, attention has become focused on its safety and productivity. One of the main problems of commercial production of these new varieties is a wilt disease. The present investigation focuses on the possible cause(s) of this wilt-disease complex.

A disease survey was carried out in two muskmelon fields. ‘Galia’ was sown during the first week of October and ‘Ananas’ was sown during the first week of November. Data were taken at 5-day intervals, and only completely wilted plants were counted. Wilted plants were pulled and together with their roots, examined for disease symptoms. Stems were examined for the presence or absence of discoloration of the vascular vessels.

Preliminary field observations have revealed that wilt disease of muskmelon is more severe in the fall crop (July-October) than at other times of the year. Symptoms of the disease in the field were observed mainly after flowering and during fruit-set. Some plants showed symptoms of sudden wilting without any signs of leaf yellowing, while others showed gradual wilting associated with or without leaf yellowing. Others showed yellowing on the old leaves, while the younger leaves were still green. Other symptoms observed included burned leaf margins and wilting, and wilting of individual branches, while the rest of the plant was still healthy in appearance. Foot-rot symptoms were observed on the bases of stems, appearing as soft areas, dark brown in color and sometimes associated with a brown, gummy exudates. Root rotting symptoms were also observed on some wilted plants. The rotted roots appeared brown, shredded and very much reduced in size and number. Also general stunting was observed in wilted plants. When stems of wilted plants were split open lengthwise, brownish discoloration of the water conducting vessels was observed in some plants.

The results from the ‘Galia’ field (Table 1) revealed that 24.4% of the total plant population showed wilt disease symptoms. In the last five readings of Table 1, a very high proportion (71.3%) of wilted plants had typical Fusarium wilt symptoms (vascular discoloration), while the rest have shown symptoms of foot rot (16.0%) and root-rot (4.5%).

Observations in the Ananas filed (1804 total plants, Table 2) revealed that 21.18% of these plants had wilt disease symptoms, of which 61.0% have typical Fusarium wilt symptoms. Foot-rot and root-rot were found in 16.8% and 4.0% of the total wilted plants, respectively.

Other organisms were associated with wilted plants. Larvae of the red melon beetle (Aulacophora Africana) were found in the roots and stems of wilting plants. These yellowish larvae seem to penetrate the roots and stems and feed on them. When they become mature, they enter the pupal stage in the soil and appear as whitish bodies. This presence of the beetle may not indicate a direct relationship between the beetle and wilt, but as a pest, it may indirectly help in aggravating the disease. Earlier, Schmutterer (3) reported the red melon beetle on cultivated melon in an irrigated area of central Sudan.

Towards the end of the season, termites were also observed, resulting in evacuated portions of the roots being filled with soil. Termites, in general, are often suspected to be associated with plants weakened due to some other causes. Field observations of ‘Galia’ (Table 1) and ‘Ananas’ (Table 2) showed 8.2% and 17.3%, respectively, of the wilted plants with insect damage. Examination of wilted plants also revealed white galls of varying sizes caused by the root-knot nematode, Meloidogyne incognita. The presence of root-knot nematode in association with wilted plants may suggest that it plays a role in lowering the resistance of muskmelon to Fusarium wilt.

The present results have confirmed an earlier observation by Mohamed et al. (2) that wilt is a serious disease of muskmelon in central Sudan. Wilt in the Sudan either appears suddenly, while the leaves are still green, or appears gradually, being accompanied by leaf yellowing. Mas and Riser (1) had reported that wilt may either appear suddenly, without any prior symptoms of leaf yellowing or shows slow wilting accompanied by progressive yellowing. However, in Sudan, wilt accompanied by leaf yellowing appears to be the most common form of the disease. Mohamed et al. (2) have added another type of symptom, leaf margin burning, and this again has been observed in the present investigation. Other symptoms, viz. branch wilt, foot-rot and root-rot, are rarely reported elsewhere associated with wilt disease. Mohamed et al. (2) reported that wilt of muskmelon in the Sudan represents typical Fusarium wilt caused by F. oxysporum f.sp melonis.

It appears that organisms other than Fusarium, such as red melon beetle, termites and nematodes, may play an important role in aggravating the wilt disease beside their direct effect on the root system of muskmelon in the Sudan.

Table 1. Results of the survey of wilt disease in a ‘Galia’ field (total plant population-7032).

			No. of plants showing
Rating No.	Date	Vascular discoloration	Foot-rot	Root-rot	Insect damage	Total no. of wilted plants	(%)
1	Nov. 11	–	–	–	–	46	0.65
2	Nov. 28	–	–	–	–	67	0.95
3	Dec. 4	–	–	–	–	86	1.22
4	Dec. 8	50	13	13	20	96	1.37
5	Dec. 13	59	19	11	34	123	1.75
6	Dec. 18	386	89	19	32	526	7.48
7	Dec. 23	267	69	19	19	374	5.32
8	Dec. 28	319	53	6	20	398	5.66
Total		1081	243	68	125	1716	24.40
(%) y		71.26	16.02	4.48	8.23

^z The first three readings were taken without classification of the symptoms.
^y The percentages of wilt according to the symptoms were calculated from the total of wilted plants (1517) in the last five readings.

Table 2. Results of the survey of wilt disease in the ‘Ananas’ field (total plant population-1804).

			No. of plants showing
Rating No.	Date	Vascular discoloration	Foot-rot	Root-rot	Insect damage	Total no. of wilted plants	(%) of wilted plants
1	Jan. 13	30	29	11	26	96	5.32
2	Jan. 18	27	18	4	12	61	3.38
3	Jan. 22	33	11	4	10	58	3.22
4	Jan. 28	143	6	0	18	167	9.26
Total		233	64	19	66	17.28	21.13
(%)		60.99	16.75	4.97	17.28

Literature Cited

1. Mas, P. and G. Risser. 1996. Characterization, symptoms et virulence de diverse races de Fusarium oxysporum f.sp melonis. Congres Phytopathology Mediterran, Bari-Naples: 503-509.
2. Mohamed, Y.ZF,G.A Dafalla and S.K. Omara. 1994. Races of F. oxysporum f.sp melonis causing wilt of melon in central Sudan. Cucurbit Genet. Coop. Rpt. 17:66-68.
3. Schmutterer, H. 1969.Pests of crops in North-east and central Africa. Gustav Fisher Verlag, Stuttgart, Portland, U.S.A.