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Cucurbit Genetics Cooperative Report 19:59-60 (article 21) 1996

Temperature-Conditioned Response to Sphaerotheca fuliginea Race 1 in the Spanish Melon Cultivar ANC-57

J.A. Tores, M.L. Gomez and I. Canovas.

Estacion Experimental "La Mayora", Algarrobo Costa Malaga 29750, Spain

The response of a Spanish melon cultivar, ANC-57, to Sphaerotheca fuliginea race 1 is conditioned at 26 C and susceptible at 21 C (2). The resistance of this cultivar is due to one dominant gene different than Pm-1 (Gomez-Guillamon et al., unpublished data). The behavior of this cultivar against S. fuliginea race 1 was observed under artificial conditions at different temperature regimes.

The following temperature regimes were studied: 1) 19 C constant temperature, 2) 26 C constant temperature, 3) 19 C from sowing until inoculation and 26 C after inoculation; 4) 26 C from sowing until inoculation, and 19 C after inoculation; and 5) alternating periods of 12 hours at 19 C and 12 hours at 26 C. Relative humidity was 70% and the photoperiod was 16h light/18h darkness in all cases. The inoculations were carried out by spraying the second leaf of seedlings at the third true-leaf stage with a suspension of spores in an aqueous solution of Triton X-100 (10-3 ml/l) (1). The suspension was made from a monosporic isolation of S. fulginea race 1 taken from melon (SF26A). Three genotypes were inoculated: 'Bola de Oro', known to be susceptible to the fungus, 'PMR-45', resistant to race 1 of S. funginea, and the Spanish cultivar ANC-57.

Data were collected 10 days after inoculation. The percentage of surface area of the leaf covered with powdery mildew and the number of spores per unit of surface were recorded. Two discs were cut out of each inoculated leaf and were placed in a known volume of Triton X-100 solution (with the same characteristics as that used for spores solution) and the spores were counted with a haemacytometer. Two readings were made of the samples from each disc.

All plants of 'Bola de Oro' showed symptoms of powdery mildew in all temperature regimes. 'PMR-45' was always resistant and its resistance was independent of experimental temperatures (Table 1).

Plants of 'ANC-57' cultivated at a constant 19 C showed slight symptoms of powdery mildew, and none of the plants maintained at 26 C constantly showed symptoms of infection (Table 1). When plants of 'ANC-57' were cultivated at 19 C after inoculation, they were resistant to race 1 of S. fulginea; however, when infected plants were cultivated at 26 C and maintained at 19 C after inoculation, they were susceptible. Plants cultivated under the alternating temperature regime 12h 19 C/12h 26 C showed some powdery mildew tolerance; only 4% of the lea surface was infected and the number of conidia per mm2 was very small (Table 1).

Temperatures of 26 C or more were needed for the resistance gene to be expressed; nevertheless, when inoculated plants were exposed to the alternating temperature regime 12 h 19 C/12h 26 C, they showed only slight symptoms of infection. The resistance gene in 'ANC-57' is allelic to the one in other Spanish melon cultivars which do not show differential temperature response to S. fulginea race 1 (Gomez-Guillamon et al., unpublished data). Therefore, modifier genes that affect the expression of the resistance gene could be present in 'ANC-57', as suggested earlier (2). These genes would have a temperature-conditioned response which could stimulate or inhibit the resistance gene action.

Table 1. Response of three melon genotypes to S. fuliginea race 1 under five different temperature regimes.

 
19 ˚ Cz
 
19 ˚ Cy
26 ˚ Cy
Cultivar
p/px
Leaf area infected (%)
Spore no. w
p/px
Leaf area infected (%)
Spore no.w
Bola de Oro
5/5
60
99
5/5
46
129
PMR 45
0/7
0
0
0/5
0
0
ANC-57
4/4
35
44
0/5
0
0
26 ˚ Cz
 
19 ˚ Cy
26 ˚ Cy
Bola de Oro
4/4
100
292
4/4
95
295
PMR 45
0/9
0
0
0/3
0
0
ANC-57
6/6
53
108
0/4
0
0
12 h 26 ˚ C / 12h 19 ˚ Cz
12 h 26 ˚ C / 12 h 19 ˚ Cy
Bola de Oro
5/5
52
193
PMR 45
0/8
0
0
ANC-57
5/5
4
18

z Temperature before inoculation.
y Temperature after inoculation.
z p/p: plants infected / plants inoculated.
w per mm2 .

Literature Cited

  1. Floris, E. 1993. Oidopsis en melon. Doctoral Thesis. Universidad de Zaragoza, Spain.
  2. Gomez-Guillamon, M.L., and J.A. Tores. 1992. Influence of temperature on the resistance of Sphaerotheca fuliginea in Spanish melon cultivars. Proceedings of the Fifth EUCARPIC Cucurbitaceae Symposium, Skierniewice (Poland):155-159.
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