Melon Dieback: Effect of Thermic Stress and Inoculum

Cucurbit Genetics Cooperative Report 20:35-36 (article 16) 1997

A. Iglesias and F. Nuez
Deparagemto de Biotecnologia, Universidad Politecnica de Valecia , 46022 Valencia , Spain

Melon dieback is a serious disease affecting melon, and has been responsible for severe economic losses in many areas of Spain since the early 1980’s (1).

With the aim of developing a method that allows the screening of materials with tolerance to melon dieback, we conducted a trial using a field resistant and highly susceptible cultivar. We also tested the hypothesis observed by several farmers that the appearance of melon dieback is linked to low temperatures just after transplant. This is supposed to be independent of the temperature during the other development stages.

The cultivar with field resistance (‘Pat 81’) and the susceptible control (‘BG 13.819’) were cultivated in 12 L pots. The pots were filled with soil obtained from plots with high incidence of melon dieback and in which Acremonium cucurbitacearum was found in former years. This soil was subjected to the following four treatments before filling the pots:

• Sterile control: autoclaved soil.
• Naturally infested soil: no treatment was given to the soil.
• Artificially infested soil: An isolate of A. cucurbitacearum (100,000 propagules per g of soil) were added to the soil sterilizing by autoclaving (2).
• Naturally plus artificially infested soil: An isolate of A. cucurbitacearum (100,000 propagules per g of soil) were added to the naturally infested soil.

Each variety x inoculum level was tested under two temperature regimes. One of them consisted of optimum thermic growing conditions. In the second regime, plants were grown during 6 days under a low temperature stress (7-12 C minimum temperature), 6 days after transplanting.

Starting 110 days after transplanting, aerial biomass (g), vine length (cm), and disease severity in the roots (RI) were recorded. RI was scored on a 0 (healthy) to 5 (very lesioned or necrotic) scale. In this scoring system root development, discoloration, corking, rootlet losses and necrosis were considered. A similar system has previously been used by other researchers (3). In some plants of each treatment, fungi associated with the affected areas were studied after isolation.

Results obtained did not show significant differences between the temperature regimes (Table 1), therefore results from the temperature regimes were combined (Table 2). Disease severity in roots is the best criterion for distinguishing among accessions and inoculation treatments. The naturally infested soil treatment caused 34% reduction in biomass in ‘Pat 81’ and 46% in ‘BG 13.819’. Decreases in length of the vine were 5 and 28%, respectively. The resistant cultivar showed less damage than the susceptible one, although these differences were not significant with the experimental design used. However, RI scores were significantly different.

The effect of inoculation with 100,000 propagules of A. cucurbitacearum per g of soil on plant development and root system was slight in both cultivars. The effect of natural infestation was more severe. The treatment with both artificial and natural infested soil was not significantly different than the natural infestation only treatment.

Acknowledgments: Authors are grateful to Department of Pathology for the A. cucurbitacearum isolate used in this study. A. Iglesias acknowledges a fellowship from Generalitat Valenciana.

Table 1. AVOVA for disease severity in roots (RI), aerial biomass production and vine length.

Source of variation	df	RI z		Biomass (g)		Vine length (cm)
		Mean square	F ratio	Mean square	F ratio	Mean square	F ratio
Inoculum (I)	3	8058	765.1**	169077	24.1**	149	6.6**
Cultivar (C)	1	2813	267.1**	111	0	10	4
Temperature (T)	1	1	1	17293	25	10	5
I x C	3	573	54.4**	9531	14	83	37
I x T	3	21	19	6525	9	58	26
C x T	1	8	7	8825	13	16	7
I x C x T	3	45	4.2**	11358	16	34	15

^z RI=root disease index, 0 (healthy) to 5 (very lesioned or necrotic).
*.** Significant at P 0.05 or 0.01, respectively.

Table 2. RI, aerial biomass production (g) and vine length (cm) for each treatment and cultivar.

	RI z		Biomass (g)		Vine length (cm)
	BG 13.819	Pat 81	BG 13.819	Pat 81	BG 13.819	Pat 81
Control y	0.11aAx	0.03aA	373.58 a. A	368.58 aA	277.1 aA	233.2 aB
Artificial I.	1.27bA	0.89bB	348.23 aA	305.14 abA	260.9 aA	242.0 aA
Natural I.	3.98cA	2.34cB	200.44 bA	242.58 bA	200.2 bA	221.4 aA
Nat. + Art. I.	3.98cA	2.32cB	223.43 bA	242.00 bA	215.6 bA	237.4 aA

^z RI = root disease index, 0 (healthy) to 5 (very lesioned or necrotic).
^y Control = sterile soil, Artificial I. = artificial inoculum (100,000 propagules/g), Natural I. = naturally infested field soil, Nat. + Art.I. = naturally infested soil with 100,000 propagules/g of A. cucurbitacearum added.
^x Numbers in the same column/row followed by the same small/capital letter are not significantly different at the P=0.05 level.

Literature Cited

1. Garcia-Jimenez, J., M.T. Velazquez, C. Jorda and A. Alfaro-Garcia. 1994. Acremonium species as the causal agent of muskmelon collapse in Spain. Plant Disease 78:416-419.
2. Iglesias, A. and F. Nuez. 1996. Respuesta a la inoculacion controlada de dos genotipos de melon con susceptibilidad y resistencia de campo al colapso. Actas de Horticultura 14:249-256.
3. Mertely, J.C., R.D. Martyn, M.E. Miller, and B.D. Bruton. 1993. Quantification of Monosporascus cannonballus ascospores in three commercial muskmelons fields in South Texas. Plant Disease 77:766-771.