Resistance to Fungal Diseases of Foliage in Melon
M. Pitrat and C. Dogimont
INRA, Station de Génétique et d'Amélioration des Fruits et Légumes, BP 94, 84143Montfavet, France
INRA, Station de Pathologie Végétale, BP 94, 84143Montfavet, France
Additional index words. Cucumis melo, alternaria, Alternaria cucumerina, Gummy stem blight, Didymella bryoniae, downy mildew, Pseudoperonospora cubensis, powdery mildew, Sphaerotheca fuliginea, Erysiphe cichoracearum
Abstract. Breeding for resistance to fungal diseases of foliage is one of the best ways to control these diseases. Screening germplasm for resistance, study of the inheritance and release of resistant breeding lines and cultivars to alternaria leaf blight (Alternaria cucumerina), gummy stem blight (Didymella bryoniae), downy mildew (Pseudoperonospora cubensis) and powdery mildew (Sphaerotheca fuliginea and Erysiphe cichoracearum) are presented. A special emphasis is placed on powdery mildew.
Many different fungi can attack melon foliage. Most of them induce necrotic lesions on the leaves. They can kill the plants even at the adult stage (downy mildew) or decrease photosynthetic activity reducing yield and fruit quality (powdery mildew). Some of them can also develop lesions on the stems resulting in destruction of branches or the whole plant (gummy stem blight), while others develop lesions on the fruits making them unmarketable (Scab, Anthracnose).
Control can be achieved by spraying fungicides, and by the use of genetically resistant cultivars.
We will consider mainly alternaria leaf blight, gummy stem blight, downy mildew, and powdery mildew. The other diseases are not very common on melon (they are found mainly on cucumber or other cucurbits), are more geographically localized, or no sources of resistance have been published: e.g., Alternaria alternata (Fr.) Keissler f. sp. cucurbitae Vakalounakis, Colletotrichum orbiculare (Berk. & Mont.) Arx (=C. lagenarium (Pass.) Ellis & Halst.), Cercospora citrullina Cooke, Cladosporium cucumerinum Ellis & Arth., Septoria cucurbitacearum Sacc.
Alternaria leaf blight
Fungus and disease. Alternaria cucumerina (Ellis & Everh.) Elliott is the causal agent of this disease. It is found mainly in rainy areas with high
temperatures. The fungus can survive in the soil on plant parts. Lesions on the leaves are first yellow-brown and then become necrotic, leading to the death of old leaves.
Sources of resistance. There has been no general screening of germplasm in order to find sources of resistance. Resistance is common in some cultivated melon types, for instance in honeydew (Thomas and Caniglia, 1997). Resistance can also be found in other accessions like PI 164756, and AC-82-37-2 (Boyhan and Norton, 1984), and MR-1 (Thomas et al., 1990).
Inheritance of resistance. An oligogenic control of resistance has been described in AC-82-37-2 (Boyhan and Norton, 1984), while one dominant gene is involved in the resistance of MR-1 (Thomas et al., 1990).
Races. No races have been described on melon.
Breeding lines and commercial cultivars. Some commercial honeydew type cultivars are resistant, but there has been little effort to introduce resistance in other types like cantaloupes.
Gummy stem blight
Fungus and disease. Didymella bryoniae (Auersw.) Rehm (=Mycosphaerella melonis (Pass.) Chiu & Walker) induces brown spots on the leaves. Stem attacks, resulting in cankers with gummy exudates, can lead to the wilting and death of the plant. Lesions can also develop on the fruit. The
fungus can survive on plant debris in the soil. Development of the disease is more dependent on moisture and rain than on temperature.
Sources of resistance. Screening of germplasm was intensively conducted in order to find sources of resistance. Sowell et al. (1966) tested more than 1,100 accessions and found a very high level of resistance (near immunity) in PI 140471. Some Japanese lines ('Shirouri-nigo', 'Mi-tang-ting', 'Maekawanakate-shirokawa', 'Kankoku No.1', 'Sokusei-wase', 'Numame') were described as resistant (Takada et al., 1980). PI 296345, PI 266935, and PI 436533 were found among 600 other introductions to possess a good level of resistance, but not higher than that of PI 140471 (Sowell, 1981). In Australia, PI 266934 exhibited a greater level of resistance than PI 266935 (McGrath et al., 1993). Among 800 tested accessions, several promising numbers were identified: PI 157076, PI 157081, PI 157082, PI 255478, PI 432398, PI 482399 (one of the most interesting), PI 482408, PI 511890, and PI 536481 (Kyle, 1995; Zhang et al., 1995).
Inheritance of resistance. Few studies have been conducted on the genetic control of resistance to gummy stem blight. One major dominant gene (Mc) controls a high level of resistance in PI 140471, and one dominant gene (Mc-2) controls an intermediate level of resistance in breeding lines C1 and C8 (Prasad and Norton, 1967).
Races. The presence of races has not been clearly demonstrated.
Breeding lines and commercial cultivars. Breeding lines (Norton and Cosper, 1989) and commercial cultivars with an intermediate level of resistance have been released in the American cantaloupe type, e.g., 'Gulfcoast' (Norton, 1971), 'Chilton' (Norton, 1972), 'AUrora' (Norton et al., 1985). In the Japanese cantaloupe type, 'Tainan #2' (PI 321005), also exhibits a good level of resistance (Sowell and Corley, 1974a), as well as 'Ano No. 1', 'Ano No. 2' and 'Ano No. 3' (Takada, 1983).
Fungus and disease. Pseudoperonospora cubensis (Berk. et Curtis) Rostow is a devastating disease in many production areas. It needs water for its development (rain or persistently wet leaves). Very long distance transportation is probably not
very important unlike for a typical airborne fungus such as powdery mildew. The role of oospores (sexual form) in the conservation of downy mildew is probably not important, but it is not known how the fungus overwinters as it infects only Cucurbitaceae. In the laboratory, the fungus can be quite easily maintained by deep-freezing the cotyledons or leaves.
Symptoms are necrotic lesions starting on the older leaves and expanding to the whole leaf and the whole plant leading to the death of all leaves.
Sources of resistance. Partial resistance was described in accessions from India (Ivanoff, 1944). Accession PI 124112 was described as resistant (Sitterly, 1972; Thomas and Jourdain, 1992), while 'Seminole' was found as either resistant (Sitterly, 1972; Whitner, 1960) or susceptible (Thomas, 1982). Other accessions were also described with different levels of resistance: PI 124111 (and MR-1) (Thomas, 1982; Thomas, 1986), PI 414723 (Pitrat et al., 1989), PI 122487, PI 124210, PI 145594, PI 165525 (Thomas and Jourdain, 1992), Phoot (Somkuwar and More, 1993).
There seems to be general agreement that PI 124111 (MR-1) and PI 124112 have the highest level of resistance.
Inheritance of resistance. Two incompletely dominant complementary genes (Pc-1 and Pc-2) have been described in MR-1 (Cohen et al., 1985; Thomas et al., 1988). Two complementary genes (Pc-4 and Pc-1 or Pc-2 present in MR-1) control resistance in PI 124112 (Kenigsbuch and Cohen, 1989b, 1992b).
Using the line Phoot as a source of resistance, two dominant genes were observed in the crosses Phoot x Monoecious-3 and Phoot x Pusa Madhuras, but two recessive genes were observed in the cross Phoot x Lucknow Safeda (Somkuwar and More, 1993).
One gene was found for the control of partial resistance in PI 414723. Oligogenic control is present in MR-1 and PI 124112 (Epinat and Pitrat, 1994a, 1994b).
The general conclusion is that the highest level of resistance is under di- or oligogenic control and is partially dominant.
Races. Some interaction between strains and melon cultivars has been observed (Bains and Jhooty, 1976; Thomas et al., 1987).
Breeding lines and commercial cultivars. Breeding programs exist and some resistant breeding material has been obtained, for instance, starting from 'Seminole' (Angelov, 1996) or from PI 124111 (Cohen, 1996).
Resistance of intermediate level is also present in some cultivars like 'Georgia 47', 'Chilton', and 'Tainan #2' (Norton, 1972; Sowell and Corley, 1974a).
Fungus and disease. Powdery mildew is one of the first described diseases of melon. Powdery mildew can develop in all cultural systems (greenhouse or open field) in all seasons. It is almost impossible to cultivate melon without powdery mildew.
Several fungi can produce powdery mildew. Leveillula taurica can be found on cucumber, but is not a problem on melon. Both Sphaerotheca fuliginea (Schlecht. ex Fr.) Poll. and Erysiphe cichoracearum DC ex Merat can be found. They can be distinguished at the sexual stage when E. cichoracearum has numerous asci bearing two ascospores per cleistothecia, whereas S. fuliginea has one ascus with 8 ascospores. As both fungi are heterothallic, cleistothecia are seldom observed in natural conditions where only one mating type is generally present on a plant. The two species can also be recognized by observation of their asexual spores (conidia) under the microscope in a 3% KOH solution. The treated conidia of E. cichoracearum are more rectangular in shape, and are homogeneous in appearance, while those of S. fuliginea appear more ovoid in shape, and contain inclusions (fibrosin bodies).
There has been some misidentifications or lack of identification of the causal fungus in some of the old
publications. For instance, 'PMR 45' has been described as resistant to race 1 of E. cichoracearum (Jagger et al., 1938a) but, in fact, it is resistant to race 1 of S. fuliginea and susceptible to E. cichoracearum.
Both fungi have been identified in many countries (Ballantyne, 1975; Bardin, 1996; Bertrand et al., 1992; Boerema and Van Kesteren, 1964; Corbaz, 1992; Crüger and Meyer, 1976; Hirata, 1968; Jhooty, 1967; Khan, 1983; Lebeda, 1983; Mohamed et al., 1995; Nagy, 1970; Sitterly, 1978; Ulbrich and Smolka, 1994; Vakalounakis and Klironomou, 1995; Vakalounakis et al., 1994). Generally, S. fuliginea is the most prevalent. For example, in France (Table 1), S. fuliginea was isolated alone in 79% of the cases, and E. cichoracearum was isolated alone in 18% of the cases, while the remainder were a mixture of both fungi.
Sources of resistance. Breeding for powdery mildew resistance was the first modern program for disease resistance in melon. Starting from LJ 525, it resulted in the release of 'PMR 45' resistant to race 1 (Jagger and Scott, 1937), and then, starting from PI 79376, in the release of 'PMR 5' resistant to race 2 of S. fuliginea (van Haltern et al., 1943). Screening germplasm for resistance has been done with different strains (Bertrand and Pitrat, 1989; Cohen and Eyal, 1988; Floris and Alvarez, 1995b; Gómez-Guillamón and Torés, 1989; Leppik, 1968; Sowell and Corley, 1974b; Takada, 1974).
Resistance to S. fuliginea is mainly observed in accessions from India. Resistance to E. cichoracearum is more common and widespread; for instance, many Spanish accessions are resistant (Pitrat et al., 1996).
Inheritance of resistance. There are many reports that are somewhat confusing. The races have not always been clearly indicated, and very
Table 1. Fungi isolated from cucurbit leaves showing powdery mildew symptomsz in open-fields and greenhouses.
198788 199394 Total
Host Sf Sf+Ec Ec Sf Sf+Ec Ec Sf Sf+Ec Ec
Melon 45 1 13 129 0 22 174 1 35
Cucumber 35 1 23 95 1 10 130 2 33
Cucurbita spp. 20 7 9 127 4 26 147 11 35
Other cucurbit 0 0 1 4 0 0 4 0 1
Total 100 9 46 355 5 58 455 14 104
zSf = S. fuliginea; Ec = E. cichoracearum.
few allelism tests have been made. This is due to the difficulty of maintaining large collections of isolates. In most cases, monogenic dominant control has been reported (Table 2).
Races. Different races of S. fuliginea have been described starting with the discovery that the first resistant released cultivar, namely 'PMR 45', was susceptible in some locations (Jagger et al., 1938b). For a very long time, 'PMR 45' was the only differential host used to monitor the races of powdery mildew and so only two races were known; 'PMR 45' was resistant to race 1 and susceptible to race 2. A third race able to overcome the resistance of 'PMR 5' was first described in the US (Thomas, 1978), and later found in India (Kaur and Jhooty, 1986) and Israel (Cohen et al., 1996).
But during a study on the genetic control of resistance using the same segregating populations, different results were obtained with an isolate
of race 2 from California and another isolate of race 2 from France (McCreight et al., 1987). This indicates that lines other than 'PMR 45' and 'PMR 5' can be used as differential hosts to define new races (Table 3). Interaction between powdery mildew strains and melon lines is very clear in the case of S. fuliginea.
For E. cichoracearum, only two races have been described (Bertrand and Pitrat, 1989): race 0 and race 1. Iran H is susceptible to all strains while other melon lines (commonly considered as susceptible to E. cichoracearum) are resistant to race 0 and susceptible to race 1. Iran H is susceptible to race 0 of S. fuliginea and E. cichoracearum, but other melon genotypes are resistant to both. This extreme susceptibility can be compared with the susceptibility of one watermelon (Citrullus lanatus) accession to S. fuliginea (Robinson and Provvidenti, 1975), while usually watermelon cultivars
Table 2. Genetic control of resistance to races 1 and 2 of S. fuliginea or E. cichoracearum.
Resistant S. fuliginea E. cichoracearum
line race 1 race 2 race 1 References
PMR 45 Pm-1 Jagger et al., 1938a
Pm-A Epinat et al., 1993
PMR 5 Pm-1 Harwood and Markarian, 1968a;
Kenigsbuch and Cohen, 1992a
Pm-2 + modifier Bohn and Whitaker, 1964;
Kenigsbuch and Cohen, 1992a
Pm-1 + Pm-2 Harwood and Markarian, 1968a
Pm-C1 + Pm-D Pm-C1 Pm-C1 + Pm-E Epinat et al., 1993
MR-1, PI 124111 Pm-3 Harwood and Markarian, 1968a
Pm-6 Kenigsbuch and Cohen, 1989a
S1 S1 or S2 E7 + E8 Epinat, 1992
PI 124112 Pm-5 Pm-4 Kenigsbuch and Cohen, 1992a
Pm-C2 Pm-C2 Pm-F + Pm-G Epinat et al., 1993
PI 414723 S6 S2 = Pm-x E7 + E8 Epinat, 1992; Pitrat, 1991
2 recessive, US strain; McCreight et al., 1987
1 dominant, French strain
Perlita, Wescan One dominant Pm-1? Harwood and Markarian, 1968a
PI 235607z 2 dominant Harwood and Markarian, 1968a
PI 236355 2 dominant Harwood and Markarian, 1968a
Bellegarde 1 dominant (Pm-1) Harwood and Markarian, 1968a
PI 179901 2 dominant Harwood and Markarian, 1968a
WMR 29 Pm-A Pm-B = Pm-w Epinat et al., 1993; Pitrat, 1991
2 recessive, US strain; McCreight et al., 1987
Pm-B, French strain
Edisto 47 S8 S2 Epinat, 1992
Seminole Pm4 and Pm5 Harwood and Markarian, 1968b
Amarillo 1 dominant Floris and Alvarez, 1995a
Negro Pmx Floris and Alvarez, 1995a
Moscatel Grande Pmy + Pmz Floris and Alvarez, 1995a
Nantais Oblong Pm-H Epinat et al., 1993
Table 3. Differential melon lines for races of S. fuliginea.
Melon lines 0 1 U.S. France 3 4 5
Iran H Sz S nt S nt S S
Védrantais, Top Mark, Ananas R S S S S S S
PMR 45 R R S S S S S
PMR 5 R R R R S R R
WMR 29 R R H R nt S S
Edisto 47 R R S R S R S
PI 414723 R R S R R R R
MR-1, PI 124112 R R R R R R R
zS = susceptible, R = resistant, H = heterogeneous, nt = not tested.
and this is one goal that breeders must achieve. Some accessions are resistant to different diseases, not only to fungal diseases of the foliage, but also to other diseases. For instance, MR-1 is resistant to alternaria leaf blight, powdery and downy mildew, and to fusarium wilt (races 1 and 2). PI 124112 is resistant to powdery and downy mildew, fusarium wilt, and cucurbit aphid borne yellows luteovirus. Starting with one multiresistant source or several genotypes, the breeder can choose between two main methods:
1) Introducing step-by-step (disease-by-disease) resistance in breeding material. The most advanced population for horticultural characters will be resistant to one disease. Resistance to a second disease will be introduced into it by the backcross method. Only the tests for resistance to the second disease are necessary. And so on for a third disease.
2) Introducing simultaneously resistance to several diseases. This is more efficient as the number of plants to be tested is lower. But artificial inoculation techniques must be developed that allow testing a single plant in a segregating population with 2, 3 or more diseases (or strains). For some foliar diseases, it is possible to work on leaf disks.
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Breeding lines and commercial cultivars. Many resistant cultivars have been released. One problem is the crown blight symptom linked with the high-level resistance from 'PMR 5' or 'PMR 6'. Under some climatic conditions (short day length, low light intensity), resistant plants exhibit a necrotic reaction in the leaves. It is not a hypersensitive reaction, because crown blight is observed without powdery mildew. As most of the genes for resistance are dominant (particularly Pm-1 and Pm-2 in 'PMR 5'), and susceptibility to crown blight is recessive, F1 hybrids are a solution to this problem.
Club mildew. As there is a lot of confusion about powdery mildew races and resistance genes, a working group was founded in order to clarify the situation (McCreight and Pitrat, 1993). Recombinant inbred lines (RIL) have been produced using the single-seed descent method (SSD) between multiple disease and/or race/strain-resistant (multiresistant) lines ('PMR 5', MR-1, PI 124112, PI 414723) and susceptible lines. The RILs will be tested in different countries with different powdery mildew isolates. The aims of the group are
i) To have a better knowledge of the different genes for resistance in these multiresistant lines and particularly of their action on the races;
ii) To identify single-resistance-gene RILs for use as differential hosts;
iii) To classify the strains in different races using these differential hosts.
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