Control of Fusarium Crown and Root Rot of Tomato, Caused by Fusarium oxysporum f. sp. radicis-lycopersici, by Grafting onto Resistant Rootstocks

MATERIALS AND METHODS

Plant material: Two tomato (Lycopersicon esculentum Mill. ‘Priscas’) cultivars, Durintha F₁ and Bochra F₁, were used as scions and two commercial hybrids (Lycopersicon hirsutumxL. esculentum) Beaufort F₁ (De Ruiter) and He-Man F₁ (Syngenta) were used as rootstocks.

These two rootstocks were chosen because already known as resistant to Fusarium (Table 1). Tomato cultivars (Table 2) were used as scion because they represent 70% (10 ha) of the total covered area reserved to tomato culture in “5ème saison” exploitation located in Hammet Gabes, in South Tunisia, where Fusarium crown and root rot causes serious problems to tomato.

Table 1:	Characteristics of the two tomato rootstocks used in this study (Carrier, 2003)
C3: Cladosporium fulvum races A, B and C, C5: Cladosporium fulvum races A, B, C, D and E, F2: Fusarium oxysporum f.sp. lycopercisi races 1 and 2, Fr: Fusarium oxysporum f. sp. radicis-lycopersici (crown rot), K: Pyrenochaeta lycopersici (Corky Root), N: Nematodes most common species, Tm: Tomato Mosaic Virus, V: Verticillium spp.

Table 2:	Characteristics of the two tomato cultivars used as scions
F2: Fusarium oxysporum f.sp. lycopercisi races 1 and 2, Tm: Tomato Mosaic Virus, V: Verticillium spp.

Testing rootstock resistance to Fusarium crown and root rot of tomato under growth chamber conditions: The resistance to Fusarium of rootstock and scion cultivars utilised in this research were evaluated in growth chamber experiment on organic substrate (peat) artificially inoculated with FORL. The experiment was conduced in the glace house of the Horticultural High School of Chott-Mariem. Seeds were sown in seedling trays filled with previously sterilized peat. Young plants at the three-to four-true-leaf stage were removed from the peat; their roots were washed with sterilized water and then plants were transplanted into inoculated peat. The substrate (peat) was artificially inoculated with a 10⁷ mL^-1 spore suspension of one of four single-spore isolates (Fo2.01, Fo4.02, Fo1.03 or Fo1.04) of FORL 1 h before planting (Vakalounakis et Fragkiadakis, 1999). Plants were kept in growth chamber for 30 days at 23°C with a 12 h daylength (Woo et al., 1996), using a completely randomised design with 10 replicates per treatment. Tomato plants of susceptible cvs Durintha F₁ and Bochra F₁ treated similarly and transplanted into sterilized un-inoculated substrate served as controls. Plants were watered daily and no fertilizers were applied. The experiment was conducted twice.

Plants were classified either as resistant when they were healthy (no disease symptoms) or susceptible when they were dead or almost dead (Pavlou et al., 2002).

Rootstock evaluations for grafting tomatoes under greenhouse conditions: To investigate grafting compatibility (expressed as tomato plant growth, fruit yield and fruit quality) between cvs Bochra F₁ and Durintha F₁ and the two rootstocks Beaufort F₁ and He-Man F_1, one greenhouse experiment carried out in 2002-03 was repeated in 2003-04 crop season at the “5ème Saison” exploitation, situated in Hammet Gabes in South Tunisia where we have detected FORL for the first time in Tunisia (Hajlaoui et al., 2001; Hibar, 2002). The scion/rootstock combinations Durintha F₁/Beaufort F_1, Bochra F₁/Beaufort F₁, Durintha F₁/He-Man F₁, Bochra F₁/He-Man F₁ were evaluated with respect to plant growth, fruit production and fruit quality. Seeds of the four cultivars were sown in seedling trays filled with disinfected peat and kept in the growth chamber heated with geothermal water. Temperature fluctuated between 20 and 30°C and relative humidity between 80 and 90%. Seedlings of rootstock cultivars with 3-4 true leaves were cut over the cotyledons and immediately grafted with shoots of scion cultivars. Grafting clips were used to adhere the graft union (Santa-Cruz et al., 2002). Un-grafted plants of Bochra F₁ and Durintha F₁ were included as controls. Grafted and un-grafted seedlings were covered with a transparent plastic film to maintain high humidity level and to avoid leaf dehydration (Fernandez-Garcia et al., 2002).

Grafted plants were transferred from growth chamber to polyethylene covered greenhouses, heated with geothermal water. Soilless culture was adopted in sausage bags filled with perlite infested with FORL and using a drip irrigation system. The (scion/rootstock) combinations Durintha F₁/Beaufort, Bochra F₁/Beaufort F₁, Durintha F₁/He-Man F₁ and Bochra F₁/He-Man F₁ were planted in four greenhouses (one greenhouse for each combination) with a density of 2.5 stem m^-2 (2 stems per plant), while un-grafted plants of Bochra F₁ and Durintha F₁ used as control were transplanted into sterilized perlite and grown in two other greenhouses. Plants in all greenhouses were treated similarly and grown according to local horticultural practices.

In 2003-04 crop season seeds of rootstocks, scions and un-grafted cultivars were sown on 10 August, 20 August and 30 August 2003, respectively, plants were transplanted to the greenhouses on 20 September 2003. In the following crop season sowing and plantation occurred 20 days earlier. From ten randomly selected plants per row (four rows per treatment) of each greenhouse, the following data were recorded: stem diameter (mm) and growth speed (cm j^-1) of plant, pH of fruit juice, soluble solids (°Brix), electric conductivity (EC) (ms), fruit weight (g/fruit), fruit caliber (mm) and total yield (kg/plant). All data were taken after seven months from the plantation date.

Variance analysis of the treatment effect on measured data was performed by using the general linear model procedure of SPSS (SPSS 10.0) with trials and replications treated as random effects and grafting combinations as fixed effects.

When F values were significant at p>0.05, differences among the treatments were determined by SNK (Student Newman Keuls) test.

RESULTS

Evaluation of rootstock resistance to Fusarium crown and root rot of tomato: Under growth chamber conditions, the two commercial rootstocks Beaufort F₁ and He-Man F₁ were found to be resistant to FORL as no disease symptoms were observed. In contrast, tomato cvs Bochra F₁ and Durintha F₁ were considered susceptible because all plants were dead (Table 3).

Rootstock resistance and effect on plant growth, fruit yield and quality of the used tomato cultivars: In both crop seasons, stem diameter and growth speed of plants; soluble solids concentration, EC, mean weight and size of fruit; total yield of cv Bochra F₁ grafted onto He-Man F₁ were higher respect to plants of cv Bochra F₁ grafted onto the rootstock Beaufort F₁, as well as of the un-grafted (self-rooted) plants cv Bochra F₁ (control) (Table 4a). The pH of fruit was lower for the same combination (Bochra F₁/He-Man F₁) and this was an additional criterion of fruit quality.

For cv Durintha F₁ the best values of all measured parameters, were obtained when grafted onto the rootstock Beaufort F₁. However; stem diameter, growth speed, soluble solids, EC, fruit weight, fruit size and yield of cv Durintha F₁ grafted onto the rootstock Beaufort F₁ were higher than those of cv Durintha F₁ grafted onto the rootstock He-man F₁, as well as of the un-grafted (self-rooted) plants cv Durintha F₁ (control) (Table 4b). The pH of fruit was lower for the same combination (DurinthaF₁/Beaufort F₁) which was also an additional criterion of fruit quality. These results were obtained in 2003-04 crop season and confirmed in the next crop season.

Table 3:	Evaluation of the two rootstocks for resistance to four isolates of Fusarium oxysporum f. sp. radicis-lycopersici under growth chamber conditions
yR = Resistant reaction, healthy plants; S = Susceptible reaction, dead plants. The growth chamber experiment was conduced twice with 10 plants per tomato species and per Fusarium isolates

Table 4a:	Growth parameters, fruit quality and total yield of tomato plants of the cv. Bochra F1 grafted onto two rootstocks (Beaufort F1 and He-Man F1) in two greenhouse experiments carried out during 2003-04 and 2004-05 crop season at the ‘5ème saison” exploitation in Hammet Gabesy in south Tunisia
yIn the experiments of both crop seasons there were Four replicates (four rows per greenhouse) of ten plants per each treatment, zWithin lines, means followed by the same letters are not significantly different (p = 0.05) according to SNK test

Table 4b:	Growth parameters, fruit quality and total yield of tomato plants of the cv. Durintha F1 grafted onto two rootstocks (Beaufort F1 and He-Man F1) in two greenhouse experiments carried out during 2003-04 and 2004-05 crop season at the ‘5ème saison” exploitation in Hammet Gabesy in south Tunisia
yIn the experiments of both crop seasons there were four replicates (four rows per greenhouse) of ten plants per each treatment, zWithin lines, means followed by the same letters are not significantly different (p = 0.05) according to SNK test

Basing on these results, the rootstock He-Man F₁ seemed to be more appropriate for tomato plants cv Bochra F₁; however, for tomato plants cv Durintha F₁, the rootstock Beaufort F₁ seems to be the best one.

Also, in both greenhouse experiments, none of the grafted plants were infected by FORL confirming the resistance of these rootstocks to this pathogen.

DISCUSSION

In the past, grafting tomato 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 tomato soil borne pathogens and particularly of F. oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicis-lycopersici, Pyrenochaeta lycopersici and Meloidogyne spp. In addition, tomato grafting gave others advantages such as plant growth promotion, yield increase, extension of yield period and improvement of fruit quality (Rivero et al., 2003).

Since Fusarium crown and root rot (F. oxysporum f. sp. radicis-lycopersici) is a new disease of tomato (Hajlaoui et al., 2001; Hibar, 2002), the possible use of grafting tomato onto resistant rootstocks to protect against this disease has not yet examined. In our growth chamber and greenhouse experiments, it was found that the two rootstocks, Beaufort F₁ and He-Man F₁ were resistant to FORL; therefore they could be used as rootstocks for grafting tomatoes to protect against Fusarium crown and root rot.

Data obtained from this study suggest that grafting the susceptible tomatoes cv Bochra F₁ and cv Durintha is an effective control measure against Fusarium crown and root rot. 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 also effective in controlling some other pathogens such as sudden wilt in melons caused by Monosporascus cannonballus (Edelstein et al., 1999) and sclerotinia stem rot of soybean caused by Sclerotinia sclerotiorum (Vuong and Hartman, 2003).

Present results have shown that grafting susceptible tomatoes cvs Bochra F₁ and Durintha F₁ onto the rootstocks Beaufort F₁ and He-Man F₁ increase tomato growth, tomato yield and improve fruit quality. This increase in tomato yield through the use of grafted plants can be attributed mainly to disease control and secondly to better plant growth. Increased plant growth responses are a well-known phenomenon in grafted plants (Ibrahim et al., 2001; Santa-Cruz et al., 2002). In grafted plants, the rootstock’s vigorous root system is often capable of absorbing water and nutrients more efficiently compared to the un-grafted plant and may serve as a good supplier of endogenous plant hormones (Fernandez-Garcia et al., 2002; Estan et al., 2005). However, the rootstock effect varies greatly with scion cultivar and growing season (Lee, 1994). An increased growth effect have observed by Pavlou et al. (2002) by grafting commercial Dutch type cucumber hybrids onto various resistant Cucurbita rootstocks; indeed, total fruit yield of cucumber plants cv Brunex F₁ grafted onto all rootstocks tested (A27, Cucurbita fucifolia, Patron, Peto 42.91, TS-1358 and TZ-148) was greater than that of the un-grafted (self-rooted) plants cv Brunex F₁ (control).

However the results shown by Trionfetti et al. (2002), by evaluating the potential of grafting for resistance to F. oxysporum f. sp. melonis on 13 commercial melon rootstocks and various Cucurbitaceae spp. and determining productivity and fruit quality characteristics of grafting on resistant rootstocks, suggested that yield and quality attributes of scion cultivars (Supermarket and Proteo) grafted on P360 and PGM 96-05 rootstocks were not improved relative to un-grafted controls.

CONCLUSIONS

On the basis of the results obtained in these experiments on tomato, grafting effectiveness seems to be determined not only by disease resistance of the rootstocks but also by their influence on both production and fruit quality. The rootstock Beaufort F₁, resistant to FORL, was also the best genotype capable of significantly improving the productivity and fruit quality of tomatoes cv. Durinth F₁; whereas, the rootstock He-Man F₁, also resistant to FORL, seemed to be more suitable for tomatoes cv Bochra F₁. Moreover and regardless of the used tomato cultivars, grafted plants have procreate the best results, concerning plant growth, fruit yield and fruit quality, compared to un-grafted (self-rooted) plants cv. Durintha F₁ and cv Bochra F₁ (controls).

However, grafting tomato 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 best alternative effective control methods found up to now for Fusarium.