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Cucurbit Genetics Cooperative Report 23:21-23 (article 6) 2000

Artificial Inoculation Methods for Screening Melons Against Melon Vine Decline

A. Iglesias, B. Pico, and F. Nuez

Department of Biotechnology (Genetics), Polytechnic University of Valencia, Camino de Vera, 14 46022, Valencia, Spain

Development of disease resistant melon cultivars is the most promising strategy to reduce the economical damage caused by melon vine decline. To date, field assays have been conducted to screen melon collections and segregating populations (2,7). However, in field conditions the lack of environmental stresses could lead to the absence of aerial symptoms (6). Then, root analysis for fungal damage is necessary to select resistant genotypes. However, root inspection in field is a very tedious task. Moreover, the variability found in aggressiveness of pathogenic fungal isolates from different geographic areas (1) requires quantification of inoculum pressure in each assay. Artificial inoculation methods to overcome all these difficulties are needed.

An assay was conducted using the resistant accessionC. melo var.agrestis 'Pat 81' (3), nd the susceptible control C. melo'VC-187' (Tendral type). Both pathogens, Acremonium cucurbitacearum Alfaro Garcia, W. Gams Garcia-Jimenez (A) and Monosporascus cannonballus Pollack & Uecker (M), reported as the two main causal agents of melon vine decline in Spain (4), were included. The assay was designed as a 2 x 2 x 2 factorial, the three factors being: accessions ('Pat 81' and 'VC-187'), soil (NS: natural soil from a commercial melon field affected by melon and vine decline, mixed with peat 2:1 and fertilized, and SS: sterilized soil, NC autoclaved twice) and treatments (T1: BS: basic substrate plus 105 colony forming units of A. cucurbitacearum (isolates A-419 and A-499)/g of soil, T3: BS + M, basic substrate plus 40 colony forming units of M. cannonballus (isolates C-29 and C-31)/g of soil, and T4: BS+A+M. The pathogenic composition of the natural soil was previously studied, confirming the presence of aggressive isolates of A. cucurbitacearum and M. cannonballus.

A significant effect of the soil employed was observed (Tables 1 and 2) with higher root severity indexes in NS inoculated roots. This may be due to the existence of other secondary agents (opportunistic and saprophytes parasites) in natural soil. A significant soil x treatment interaction was obtained, as differences among treatments were only found when SS was used as basic substrate, probably due to a threshold effect of pathogen concentration in NS (5). In SS inoculated soil the more severe symptoms were found after M or A + M addition, suggesting that M. cannonballus is a more aggressive pathogen than A. cucurbitacearum.

The high level of resistance of 'Pat 81', previously reported in field assays (3), was confirmed, and mild symptoms (RSI<1.8) were found in the roots of this accession. Highly significant genotype x soil and genotype x treatment interactions were found, as NS increased the severity of symptoms in 'VS-187', much more than did SS. Also, the artificial inoculation treatments resulted in similar effects on the susceptible cultivars. However, the severity of vine decline symptoms in 'Pat 81' did not increase significantly due to the partial resistance of this accession.

The results obtained indicated that the NS is the most rapid and simple inoculation method. However, in order to obtain comparable results we recommend using it in preliminary screening assays, confirming resistance in soil artificially inoculated with a known pathogenic composition.

Table 1. ANOVA results of root severity index in two melon accessions after inoculation with two soil types and 4 inoculation treatments (See Table 2 for soil and treatment descriptions).

Root severity index
df
MS
F ratio
Genotype
1
209.40
322.80**

Soil

1
140.00
229.70**
Treatment
3
7.35
11.30**
GxS
1
18.55
28.60**
GxT
3
4.08
6.29**
TxS
3
3.49
5.37**
GxTxS
3
3.25
5.01**

**Significant at 1% level

Table 2. root severity index in two melon accessions after inoculation with two soils and four inoculation treatments.

Soil/Inoculum treatmentz
'Pat 81'
'VC-187'
SS
0.02aAy
0.00aA
SS + A
0.20aA
2.07bB
SS + M
00.37aA
0.86bB
SS + A + M
0.26aA
3.00bB
NS
1.50bA
4.60cB
NS + A
1.33bA
4.95cB
NS + M
1.75bA
4.93cB
NS + A + M
1.80bA
5.00cB

z SS = sterilized soil (2X), NS - natural infested soil, A = A. cucurbitacearum inoculation (105 CFU/g), M = M cannonballus inoculation (40 CFU/g).
y Root severity index evaluated as 0 (healthy) to 5 (severely affected). Lower-case letters for comparison among treatments and capital letters for comparisons between accessions by a Duncan's multiple ranges test.

Literature Cited

  1. Bruton, B.D., M.E. Miller and J. Garzia-Jimenez, 1996. comparison of Acremonium sp. from the lower Rio Grande Valley of Texas with Acremonium sp. from Spain. Phytopathology 86:3.
  2. Cohen, R., Elkind, Y., Burger, R. Offenbach and H. Nerson, 1996. Variation in the response of melon genotypes to sudden wilt. Euphytica 87 91-95.
  3. Iglesias, A., B. Pico and F. Nuez. 1999. C. melo spp. agrestis. Pat 81, an interesting genetic resource highly resistant to melon dieback. Phytopathology 89:S35.
  4. Martyn, R.D. and M.E. Miller, 1996. Monosporascus root rot/ vine decline: an emerging disease of melon worldwide. Plant Disease 80:716-725.
  5. Mertely, J.C., R.D. Martyn, M.E. Miller and B.D, Bruton, 1993. quantification of Monosporacsus cannonballus ascospores in three commercial muskmelon fields in South texas. Plant Disease 77:766-771.
  6. Pivonia, S., R. Cohen, U. Kafkafi, I.S. Ben Ze'ev, J. Katan, 1997. Sudden wilt of melons in Southern Israel: Fungal agents and relationship with plant development. Plant Disease 81:1264-1268.
  7. Wolff, D. and M. Miller, 1998. tolerance to Monosporascus root rot and vine decline in melon (Cucumis melo L.) germplasm. HortScience 33:287-290.
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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 15 December, 2009