Abstract: From 3 watermelon cultivars Sentinel, Charleston Gray and Sugar Pack tested against some Tunisian isolates of Fusarium solani f. sp. cucurbitae and Fusarium oxysporum f. sp. niveum, Sugar Pack revealed to be the most susceptible cultivar and was used for grafting to evaluate the resistance of 9 cucurbit rootstocks. Strong toza, TZ148, Emphasis, Polifemo and Ercole were resistant to highly resistant to five Fusarium isolates tested. Rootstocks of Ferro RZ, Macis and S.Camel showed a partial resistance to some isolates of Fsc and Fon. Achille revealed to be highly resistant to Fsc, but susceptible to some isolates of Fon. Seedlings grafted Strong toza x Sugar Pack, TZ 148 x Sugar Pack, Emphasis x Sugar Pack, Achille x Sugar Pack and Ercole x Sugar Pack were resistant to Fsc and Fon isolates. These rootstocks could be used in grafting of watermelon to resolve the problem of two Fusarium species affecting watermelon.
Introduction
Fusarium wilt and Fusarium crown and root rot of watermelon induced by Fusarium oxysporum Schlechtend.:Fr. f. sp. niveum (E.F.Sm.) W.C. Snyder and H.N. Hans. (Fon) and Fusarium solani (Mart.) Sacc. f. sp. cucurbitae W.C. Snyder and H.N. Hans. (Fsc) race 1, respectively, were the most damaging soilborne diseases of this cucurbit causing heavy economic losses (Bruton et al., 1998; Martyn and Bruton, 1989). These diseases were serious in many areas of the world such as Northern of Africa, Italy, United States and Israel (Messiaen et al., 1991). In Tunisia, the problem becomes more and more important and the races of these fungi were frequently identified in the majority of watermelon cropping areas (Boughalleb, 2003; Boughalleb and El Mahjoub, 2005; Boughalleb et al., 2005).
The watermelon grafting onto cucurbit rootstocks is a best alternative to control soilborne diseases and an agronomic interest for plant vigour and production (Blancard et al., 1991; Messiaen et al., 1991; Gignoux, 1993; Jebari, 1994; Aounallah and Jebari, 1999; Aounallah et al., 2002; Tarchoun et al., 2005). Benincasa cerifera, Cucurbita maxima, Lagenaria vulgaris and Lagenaria leucantha were reported by many authors as good watermelon rootstocks (Messiaen et al., 1991; Gignoux, 1993). In Tunisia, the hybrid RS 841 has often been used for melon grafting. This rootstock is known as resistant to Fusarium wilt and to nematodes (Blancard et al., 1991). Since several years, many rootstocks such RS841, Emphasis, Strong Toza, originated from the hybrid Cucurbita moschata x Cucurbita maxima were used for cucurbit grafting (Blancard et al., 1991). Currently, several rootstocks were subscribed and recommended for watermelon grafting. Aounallah et al. (2002) showed that, among the three watermelon rootstocks tested (Lagenaria siceraria, local of Mahdia and RS 841), L. siceraria is resistant to Fon isolated but susceptible to Fsc. However, local of Mahdia is resistant to Fsc and RS 841 proved to be resistant to Fon and moderately resistant to Fsc. In the same way, the variety Sugar Baby used like control revealed to be susceptible to Fon and Fsc.
The objective of this study is to evaluate resistance level of some new cucurbit rootstocks against Fon and Fsc isolates under controlled conditions as potential sources for grafting of commercial watermelon varieties.
Materials and Methods
Plant Material
Nine rootstocks, hybrids of Cucurbita maxima x Cucurbita moschata,
were tested in this work: Strong toza, TZ 148, Ferro RZ, Polifemo, S. Camel,
Macis, Achille, Ercole and Emphasis. Three watermelon cultivars (Sentinel, Charleston
Gray and Sugar Pack) were also tested in the same conditions (Table
1).
Growth Conditions and Testing Rootstocks Resistance to Fusarium Wilt and
Crown and Root Rot of Watermelon
Seeds of rootstocks were sown in seedling trays filled with sterilised peat
five days before those of scion variety. Young plants, at one-two true leaf
or one true leaf stage, for rootstocks and scion, respectively, were transplanted
into pots containing previously sterilized peat. Seedlings of rootstock cultivars
were cut over the cotyledons and immediately grafted with shoots of scion cultivars.
We adopted the method described by Taussing et al. (1996). The scion/rootstock
combinations Sugar Pack x Strong toza, Sugar Pack x TZ 148, Sugar Pack x Ferro
RZ, Sugar Pack x Polifemo, Sugar Pack x S. Camel, Sugar Pack x Macis, Sugar
Pack x Achille, Sugar Pack x Ercole and Sugar Pack x Emphasis were evaluated.
Plants were kept at growth chamber for 30 days at 23°C with a 12 h day length (Woo et al., 1966), using a completely randomised design with 10 replicates per treatment. Watermelon plants of susceptible cv. Sugar pack treated similarly and transplanted into sterilized substrate served as controls. Plants were watered daily and no fertilizers were applied. The treatment was conducted twice.
Two Fusarium species were used for plants evaluation resistance F. solani f. sp. cucurbitae with two isolates (Fsc1 and Fsc2) and F. oxysporum. f. sp. niveum (Fon1, Fon2 and Fon3) collected from different regions in Tunisia and preserved in glycerol (50%) to 4°C.
The method of inoculation used for resistance evaluation of plants was similar to that developed by Latin and Snell (1986).
| Table 1: | Cultivars characteristics used for grafting evaluation against Fusarium sp. |
Spore suspensions were prepared from cultures grown on PDB on a rotary shaker at room temperature (22°C) for 14 days and adjusted at a concentration of 1x106 conidia mL-1 with a hematocymeter. When the first true leaf was evident, the plants were uprooted and the roots washed under a stream of gently flowing water. Seedlings were root-dipped into the respective inocula for 15-20 sec, swirled several times and transplanted into 7.5 cm diameter pots (three seedlings per pot containing vermiculite) and five pots per isolate. Thus, fifteen plants per isolate were tested. Controls were prepared by root-dipping the plants into sterile distilled water. All plants were maintained in the greenhouse. The average air temperature, during the experiment, was about 27°C.
Plants were classified as very resistant, when the level of mortality is ranged between 0 to 15%, resistant, if the percent of infested plants varied from 15 and 30% and were considered as susceptible when presented more than 30% of diseased plants.
Statistical Analysis
Variance analysis of the treatment effect was made using SPSS software.
Means were compared by Duncan multiple test at 5% level.
Results and Discussion
Evaluation of Scion Resistance
The inoculation of the 3 cultivars used as scion revealed a meaningful difference
against isolates of Fusarium sp. tested in this study.
Sugar Pack inoculated by Fsc1 showed the highest percent of infected plants (73%), while the lowest percent was found with Fsc2 (33%). Reaction difference of these isolates is due to their variability aggressiveness. Charleston Gray is the most sensitive to Fsc2 (43%). Sentinel showed to be the most resistant to Fsc1 having 42% of diseased plants (Table 2).
For Fon, we concluded, that Sugar Pack is the most susceptible to the three isolates with level varying from 54 to 72%, whereas Sentinel and Charleston Gray showed a variable sensitivity according to isolates. The comparison of susceptibility of the 3 cultivars showed that Sentinel and Charleston Gray, seems to be moderately resistant to Fsc and resistant to Fon3 with rates varying from 15 to 17%, whereas they appear very susceptible to Fon1. On the other hand, Sentinel and Charleston Gray revealed to be resistant to Fon3 but susceptible to the other isolates with 67% level for Fon1, which appeared to be the most virulent. Sugar Pack (type Sugar Baby) was susceptible to all isolates of Fusarium sp., infested plants level varying from 33 to 73%. These results confirmed those of Aounallah et al. (2002) who found that Sugar Baby is susceptible to Fon and Fsc although the plant material type is different. For this reason, Sugar Pack was used as scion to evaluate the resistance of the nine rootstocks tested in this study.
Evaluation of Rootstocks Resistance
The results showed a meaningful difference between the nine rootstocks to
the five isolates of Fusarium sp. The rootstocks Polifemo, Ercole and
Emphasis seemed to be very resistant (0 to 8%) to all isolates of Fusarium
sp. tested in this study (Table 2). Strong toza was resistant
to Fsc (12%) and very resistant to Fon (4%). TZ 148 was very
resistant to Fsc (0%) and resistant to Fon (0 to 16%), while
Ferro RZ appeared to be very resistant to Fon (0%) but moderately resistant
to Fsc (24%). The rootstocks S. Camel and Macis were the most susceptible
to Fsc and relatively resistant to Fon. Besides, Achille, very
resistant to Fsc, proved to be moderately susceptible to Fon and
particularly to isolate Fon3.
It revealed that only rootstocks Strong toza, TZ148, Emphasis, Polifemo and Ercole were very resistant to the five isolates of Fusarium. Ferro RZS, Macis and S. Camel showed a partial resistance to some isolates of Fsc with percent of infested plants ranged from 0 to 38% and those of Fon (0 to 23%). Achille proved to be very resistant to Fsc (0%) but susceptible to some isolates of Fon (0 to 25%) (Table 2).
| Table 2: | Percentage of plants showing the symptoms of wilting of crown and root rot of the commercial cultivars, rootstocks and grafted plants inoculated by F. solani f. sp. cucurbitae (Fsc 1 and Fsc 2) and F. oxysporum f. sp. niveum (Fon 1, Fon 2 and Fon 3) |
| The values followed of the same letter are not meaningfully different to the doorstep 5%. The reading of the results has been done after 15 on 20 days of the inoculation | |
| Table 3: | Behaviour of different rootstocks and grafted plants inoculated by Fsc and Fon |
| VR: Very Resistant, S: Sensible, R: Resistant, MR: Moderately Résistant | |
The results of rootstocks behaviour against Fsc and Fon are presented in Table 3. The first five rootstocks were very good included to the two species of Fusarium sp. affecting watermelon and are considered like very resistant or resistant. Ferro RZ showed an intermediate behaviour and was moderately resistant to Fsc. Rootstocks S. Camel and Macis proved to be susceptible to Fsc, resistant to moderately resistant to Fon. On the other hand, Achille appeared very resistant to Fsc and moderately resistant to Fon.
Evaluation of Resistance Combinations Scion/rootstocks
The percentage of grafted success plants ranged from 40 to 88% according
to combination scion/rootstock. The best level was of 88% with the rootstocks
S. Camel and Macis, whereas the lower (40%) with Ferro RZ.
The grafting of Sugar Pack (scion) on the 9 rootstocks revealed highly meaningful differences (Table 2). It revealed that the combinations Strong toza x Sugar Pack, TZ 148 x Sugar Pack, Emphasis x Sugar Pack, Achille x Sugar Pack and Ercole x Sugar Pack were resistant to Fsc and Fon isolates. The grafting of Sugar Pack on Ferro RZ showed plants very resistant to Fon, but resistant to Fsc (22%). The grafting on Macis and S.Camel seemed susceptible to Fsc and Fon isolates (Table 2 and 3).
Discussion
In the past, grafting of watermelon plants was considered too expensive but at present it is adopted at a commercial level in Tunisia and in many countries. Resistant rootstocks provide excellent control of many watermelon soilborne pathogens and particularly F. oxysporum f. sp. niveum, F. solani f. sp. cucurbitae, Monosporascus cannoballus and nematods. In addition, watermelon grafting gave others advantages such as plant growth promotion, yield increase, extension of yield period and improvement of fruit quality (Tarchoun et al., 2005).
Data obtained from this study suggest that grafting the susceptible watermelon cv. Sugar Pack is an effective control measure against of Fusarium wilt and Fusarium Crown and root rot affecting watermelon. Similar results were reported by Trionfetti et al. (2002) and Miguel et al. (2004) on controlling Fusarium wilt by grafting two muskmelon cultivars and triploid watermelon, respectively onto commercial rootstocks. Grafting was effective in controlling some other pathogens such as melon sudden wilt caused by Monosporascus cannonballus (Edelstein et al., 1999). However, grafting watermelon cultivars onto resistant rootstocks are more expensive, since both scions and rootstocks are expensive hybrids. In addition, the development of grafted plants requires more time, materials, space, a high level of expertise, improved cultivation methods and expensive postgraft handling. But, actually in Tunisia, grafting is expected to increase significantly despite the high cost of labor and supplies, since it is one of the bet alternative effect cxdive control methods found up to now for Fusarium.
Conclusions
The watermelon grafting proved to be an effective method to attenuate the impact of Fusarium wilt and Fusarium Crown and root rot of watermelon caused by Fon and Fsc, respectively. The present study permitted to enlarge the game of rootstocks used for grafting of watermelon. In fact, five rootstocks could be kept from this work: Strong toza, TZ 148, Emphasis, Achille and Ercole.