Selection of Snake Melon Lines (Cucumis melo var. flexuosus) Resistant to Different Races of Powdery Mildew (Sphaerotheca fuliginea (Schlecht ex Fr.) Poll. in Sudan

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Cucurbit Genetics Cooperative Report 23:27-29 (article 8) 2000

E. A. Ahmed, H.S. Ibn Oaf, M.E. Suliman, A.E. El Jack, and Y.F. Mohamed
Faculty of Agricultural Sciences, University of Gezira, P.O. Box 20, Wad Medani, Sudan

Snake melon or snake cucumber is widely cultivated in the Sudan as well as in other countries from North Africa to India. It is used in fresh salad and in pickles. Only landraces that resulted from farmer selections are cultivated, and they are all susceptible to the prevailing pests and diseases. Among the most important diseases, powdery mildew is a limiting factor of melon production in all producing countries and conditions (8). In the Sudan the disease is widely spread, and chemical control, it possible, is economically and technically difficult to practice and may present some health and environmental risks. Sphaerotheca fuliginea (Schlecht ex Fr.) Poll. and Erysiphe cichoracearum DC were reported as powdery mildew causal agents in Sudan with S. fuliginea being more prevalent (4). Fusarium wilt is another serious disease of melon in sudan. viral diseases are prevalent in different cucurbits growing agro-ecosystems and now are considered as serious factors threatening cucurbit production. Among these, Zucchini Yellow Mosaic Potyvirus (ZYMV), Cucurbit Aphid-Borne Yellows Luteovirus (CABYV), Watermelon chlorotic Stunt Gemini virus (WCSV), Squash Mosaic Comovirus (SqMV) and Melon Rugose Mosaic Thymovirus (MRMV) were reported (1,2).

PI 414723 is resistant to powdery mildew, ZYMV, CABYV, Aphis gossypii Glover, fusarium wilt and Papaya Ringspot Potyvirus (PRSV) (6). Another melon accession from India, PI 124112, also was reported to have resistance to downy mildew (7, 9), powdery mildew, fusarium wilt and CABYV (5, 6), A program was initiated in 1994-95 to introduce the powdery mildew resistance of the PI 414723 line obtained from Cornell University into the susceptible cultivar Shendi. the experiment was conducted at the University Research farm. Seeds were sown on raised beds 1m wide with 50 cm spacing between plants. Recommended cultural practices were used throughout the experiment. In the winter season (Nov. – Feb.) 1998-00 the F1 (‘Shendi’ x PI 414723), F2 , F1BC1 (F1 x ‘Shendi’), F2BC1 and (BC1 X PI 124112) were planted. In the second season (Nov. – Feb.) 1999 – 2000 the same material was included with the addition of F1BC2[F2 BC1 (PMR=9) X ‘Shendi’]. In both seasons parents and differential genotypes for powdery mildew resistance were included. A rating scale of 1 to 9 (1=highly resistant) was used to evaluate powdery mildew resistance under natural inoculum conditions. One rating was done when ‘Shendi’ was completely infected (PMR=1).

In both seasons, Erysiphe cichoracearum and race 0 of Sphaerotheca fuliginea were not considered to be present since ‘Nantais oblong’ was infected (Table 2). ‘Nantais oblong’ is resistant to E. cichoracearum and race 0 of S. fuliginea (Table 1). A laboratory test of samples collected from the field confirmed S. fuliginea as the causal agent. In the first season (1998-99), S. fuliginea race 2 was predominant according to the reaction of the differential genotypes (Table 2). The differential genotype “PMR 45′, which is resistant to race 0 and 1 (Table 1), was infected together with the susceptible ‘Shendi’ at the beginning of the season (Table 2). Since race 2 arrived first it is difficult to know the presence or absence of race 1 because there is no differential genotype resistant to race 2 and susceptible to race 1. Both PI 414723 and PI 124112 were resistant (PMR=9) while ‘Shendi’ was completely infected (PMR=1). The F1 (‘Shendi’ x PI 414723) and F1BC1 (F1 x ‘Shendi’) were intermediate in resistance to powdery mildew (mean PMR-4.9 and mean PMR=4, respectively. The F2 (‘Shendi’ x PI 414723) showed a segregation with about 23% of the plants having the level of resistance of the PI 414723 whereas the F2BC1 showed a segregation with about 9% of the plants having the level of resistance of the PI 414723. The progeny of the cross (BC1 x PI 124112) were completely resistant (PMR=9).

In the second season (1999-2000), S. fuliginea race 1 was the causal agent since the differential genotype ‘PMR 45’ remained resistant (Table 2) until evaluation time. At the same time the susceptible check ‘Shendi’ was completely infected (PMR=1) and PIO 414723 and PI 124112 were completely resistant (PMR=9). Towards the end of the season (3rd week of Feb.) ‘PMR 45’ was infected suggesting the arrival of race 2. Therefore, during this season the rating was done mainly for race 1. The F1 (‘Shendi’ x PI 414723) and the F1BC2 had a resistance rating of 5.0 and 4.0, respectively. the F2 (‘Shendi’ x PI 414723) was segregating with 38% of the plants having the level of resistance of the PI 414723. The progeny of the cross (BC1 X PI 124112) were segregating this time with 37% of the plants resistant and 63% of the plants susceptible.

It is obvious from the results that resistance to race 1 and 2 conferred by the PI 414723 is incompletely dominant since the F1 is intermediate in resistance level. It is also evident from this study that race prevalence of S. fuliginea is changing from one season to another. In a previous study it was reported that race 1 prevails during the summer (June-Sept.) and race 2 in the winter (Nov.-Feb.) (4). For S. fuliginea race 2 resistance in melons, two recessive genes against the U.S.A. strain and one dominant gene against the French strain from PI 414723 were reported (3). Segregation observed in this study does not fit a single dominant gene or a two recessive gene model for resistance. This could be because S. fuliginea race 2 in Sudan is different from that of USA and the French strain. Further study of the race 2 strains in sudan is needed for the advancement of breeding for powdery mildew resistance. Artificial inoculation under controlled conditions is important to select for resistance to race 1 and 2 of S. fuliginea. Since the donor genotypes have multiple resistances, backcrossing is done on the plants that are free of other disease symptoms in an attempt to advance multiple disease resistant selections. The two loci that confer resistance to powdery mildew in PI 414723 and PI 124112 are not allelic (Pitrat and Dogimont, unpublished data); therefore, breeding powdery mildew resistant genotypes using the two donor parents might permit the recombination of the two loci in F1 hybrids.

Table 1: The reaction of differential genotypes to powdery mildew pathogens and races.

Sphaerotheca fuliginea Erysiphe cichoracearum
Genotype Race 0 Race 1 Race 2 Race 0 Race 1
Iran H Sz S S S S
Nantais oblong R S S R R
PMR 45 R R S R S
WMR 29 R R R R R
MR 1 R R R R R
PMR 5 R R R R R
PI 124112 R R R R R
PI 414723 R R R R R

z R=resistant, S=Susceptible

Table 2 Observed reaction of differential hosts to infection of powdery mildew in two field seasons.

Season Iran H Nantais oblong PMR 45 WMR 29 PMR 1 PMR 5 PI 414723 PI 124112
1998-99 Sz S S R R R R R
1999-00 S S R R R R R R

zR=resistant, S=susceptible

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