A Strategy for Selection Cucumis melo L. Resistance Sources to Melon Vine Decline in Field Assays

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Cucurbit Genetics Cooperative Report 23:24-26 (article 7) 2000

A. Iglesias, B. Pico, and F. Nuez
Department of Biotechnology (Genetics), Polytechnic University of Valencia, Camino de Vera, 14 46022, Valencia, Spain

Vine decline is a major root-rot disease of melon crops (cucumis melo L.) around the world (3). Plants affected by this disease suffer root damage that leads to a gradual vine following and decay as the plant approaches fruit maturity Several soil-borne fungi, virus and even bacteria have been related with this complex disease. In Eastern Spain Acremonium cucurbitacearum Alfaro Garcia, W. Gams, Garcia-Jiminez and Monosporascus cannonballus Pollack & Uecker seem to be the main causal agents (3). The development of melon varieties resistant to vine decline is difficult. Most screening assays have been performed in the field, due to the lack of artificial inoculation procedures (2,7). In these conditions, it is necessary to characterize fungal isolates from each screening field to obtain comparable results. In field assays, the rate of collapsed plants is highly dependent on certain environmental conditions such as high temperatures, warm winds, and horticultural characteristics of each accession (growth cycles, fruit load, etc) (5,6). Evaluation of the conditions of the roots could allow for a better evaluation of the resistance level of each variety.

The pathogenicity of 2 isolates of A. cucurbitacearum and 2 isolates of M. cannonballus from our screening field were compared to other Spanish and American isolates (Table 1). Fungi were grown in artificial medium as previously reported (1), and 105 colony forming units of A. cucurbitacearum per g of soil, and 40 colony forming units of M. cannonballus per g of soil were used to inoculate 1.5 L pots, containing sterilized substrate, in which plants (2nd true leaf stage) of the susceptible C. melo cultivar VC-185 were transplanted. Forty-eight days after transplanting the roots were inspected for symptoms, and a root severity index (RSI) similar to that used by Mertely et al. (4) was recorded.

Significant differences in aggressiveness were found among isolates (Table 1), AC-1 and MI-2 caused the most severe root damage among A. cucurbitacearum and M. cannonballus isolates, respectively. The former was more aggressive than A-419. the reference isolate for this fungus. The latter was significantly more severe than isolates from Texas.

After characterizing the fungal isolates, the field resistance of different melon accessions was tested. Incidence of vine decay was variable with assay conditions. The first year all accessions completed their reproductive cycle without collapse. However, on observation of the roots, lesions caused by fungal pathogens were found (Table 2) All muskmelon cultivars displayed high RSI (3-4). However, RSI was significantly lower in C. melo var. agrestis ‘Pat-81’ (2.1), indicating a partial resistance to melon vine decline. In the absence of vine collapse, the scoring of root damage allowed for the selection of this resistance source.

The resistance of ‘Pat 81’ was assayed three more consecutive years. These years vine decline was more severe and most plants of susceptible cultivars died before completing their growth cycle, whereas the percentage of symptomless plants in ‘Pat 81’ ranged from 45 to 85%. Despite the high severity of aerial symptoms, all plants were inspected for root damage the last year (Table 2). Results were consistent with those observed the first year. Indeed, when stresses appeared the root severity index was highly correlated with the above ground disease symptoms, so RSI provided a measure of the potential risk of each genotype of suffering vine decline. due to the high severity of field attack and o the aggressiveness of A. cucurbitacearum and M. cannonballus isolates from our screening field, ‘Pat 81’ may be a useful resistance source against melon vine decline not only in Spain but in other affected areas.

Table 1. Aggressiveness of different isolates of A. cucurbitacearum and M. cannonballus on melon cv. VC-185.

Isolate (Origin)
N
RSIz
Control
17
0.00 + 0.00y
A. cucurbitacearum

A-499 (Central Spain)x
20
1.20 + 0.22

A-419 (Eastern Spain)x
41
2.30 + 0.19

AC-1 (Screening field)
31
3.00 + 0.15

AC-2 (Screening field)
6
2.08 + 0.40
M. cannonballus

MI-2 (Screening field)
10
3.50 + 0.52

C-31 (Eastern Spain)x
10
1.05 + 012

TX-970059 (Texas)v
10
1.38 + 0.14

MI-1 (Screening field)
10
2.05 + 0.39

TX-970064 (Texas)v
10
2.60 + 0.30

z Root severity index evaluated as ) (healthy) to 5 (severely affected).
y Mean values + standard errors.
x Provided by Dr. Garcia-Jiminex (Department of Vegetable Pathology of the UPV, Valencia, Spain).
v Provided by Dr. Bruton (USDA-ARS, Oklahoma, USA).

Table 2. Root severity index of different C. melo accessions tested against vine decline under field conditions in two years.

Accessions
RSIz
First year

C. melo var. agrestis Pat 81
2.18 + 0.11y
UPV-5079 (Amarillo type)
2.92 +.25
Cantaloupe
3.13 + 0.26
VC-21 (Piel de sapo type)
3.21 + 0.31
VS-120 (Amarillo type)
3.32 + 0.20
Acc-6 (Cantaloupe
3.53 + 0.15
Mu-C-32 (Amarillo type)
3.57 + 0.20
Kaffer (Egyptian cultivar)
4.20 + 0.11
Fourth year

C. melo var. agrestis Pat 81
2.66 + 0.01
VC-185 (Amarillo type)
3.87 + 0.09
VC-187 (Tendral type)
4.16 + 0.14

z Root severity index evaluated as 0 (healthy) to 5 (severely affected)
y The mean values + standard errors

Literature Cited

  1. Armengol, J., R. Sales and J. Garcia-Jimenez, 1999. Evolucion de los danos causados por Acremonium cucurbitacearum en raiz de melon en sus primeros estados de desarrollo. Boil, San. Veg. Plagas, 25:265-277.
  2. Cohen, R., Y. Elkind, Y. Burger, R. Offenbach and H. Nerson, 1996. Variation in the response of melon genotypes to sudden wilt. Euyphytica 87:91-95.
  3. Martyn, R.D. and M.E. Miller, 1996. Monosporus root rot/vine decline: An emerging disease of melon worldwide. Plant Disease 80:716-725.
  4. Mertely, J.C., R.D. Miller and B.D. Bruton, 1993. An expanded host range for the muskmelon pathogen Monosporascus cannonballus Plant Disease 77:667-673.
  5. Pivonia, S., R. Cohen, U. Kafkafi, I.S. Ben Ze’ev, J. Katan, 1997. Sudden wilt of melon in Southern Israel: Fungal agents and relationship with plant development. Plant Disease 81:1264-1268.
  6. Wolff, D.W. 1996. Genotype, fruit load, and temperature affect Monosporascus root rot/vine decline symptom expression in melon. In: Cucurbits Towards 2000. Proceeding of the VIth Eucarpia Meeting on Cucurbits Genetics and Breeding, Malaga, Spain, 280-284.
  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.