Introducing Grafted Cucurbits to Modern Agriculture

Grafted vegetables have been cultivated in eastern Asia for decades, but their adoption in the western world has only begun since the banning of the fumigant methyl bromide in 2005 by the Montreal Protocol. The primary motive for using grafted plants is to avoid damage caused by soilborne pests and pathogens in situations in which genetic or chemical approaches for disease management are not available. Grafting a susceptible scion onto a resistant rootstock provides a resistant plant without the need for a prolonged breeding program. Furthermore, grafting plants allows for a more rapid response to the appearance of new races of a pathogen, and provides a less expensive and more flexible solution for controlling soilborne diseases than breeding new resistant cultivars. In addition, grafted plants may contribute to enhanced tolerance to abiotic stresses, more efficient water and nutrient use, and improved fruit yield and quality.

Methyl bromide is still in use in certain areas and countries under special permits. The feasibility of grafting compared with soil disinfestations with methyl bromide is thus different for each country, affected by conditions such as the price of the chemical, the price of seeds and grafted transplants, and the growers’ income. For example, in Israel, grafted cantaloupe is more expensive than soil disinfestation by $3,000 per hectare, whereas grafted watermelons are economically feasible.

In Japan, farmers generally have very small land areas, and they often encounter increasing populations of pathogens that threaten their crops due to continuous cropping. With the establishment of largescale greenhouse cultivation, grafting became an essential technique for production of vegetable crops.

The ban on the use of methyl bromide for soil fumigation stimulated the introduction of grafting technology to the Mediterranean area and to European countries.  Currently, researchers in the United States are also becoming more interested in this technology. Studies with grafted melons for the suppression of Monosporascus sudden wilt are in progress in Texas as part of a joint project with Israel. Grafting is also being investigated in Oklahoma, where researchers are using grafted watermelons for controlling soilborne disease and improving fruit quality. In addition, they are trying to improve the strength of grafted watermelons in order to reduce damage due to windy weather. In the United States, however, plant cultivation is generally carried out on large acreages and crop rotation prevents catastrophic crop losses. There is also very little greenhouse cultivation. Thus, to the best of our knowledge, vegetable grafting is not yet commercialized. Nevertheless, in fields with high disease risk, some growers in Texas use grafting for the early watermelon season (Kevin Crosby, personal communication). Melons grafted onto various Cucurbita rootstocks have been tested in Honduras and Guatemala, an activity being promoted by UNIDO (United Nations Industrial Development Organization) as part of the worldwide effort to phase out methyl bromide. Subsequently, almost 20% of the commercial melon fields of Honduras are now grafted (Reuben Ausher, personal communication).

Grafting cucurbits was first introduced to Israeli agriculture in 1995. Until that time, methyl bromide was extensively used for managing soilborne diseases of annual crops. With the phasing out of methyl bromide and the lack of available farmland for appropriate crop rotation, growers began to look for alternative pest management tools. When we first began studying the grafting issue, we assumed that the performance of grafted plants, including suppression of soilborne diseases, depended only on the contribution of the rootstock. We were sure that finding a resistant and vigorous rootstock would be all that was required and that everything else needed for use in Israel could be adapted from practices used in the Far East. Reality, however, turned out to be different: as with the introduction of new cultivars, adaptation of the grafted plants to the specific combination of crop and environment is necessary. Not only does the response of the grafted plants depend on the rootstock, but the total performance also depends, to a great extent, on the rootstock–scion physiological interaction, and on the response of the scion to biotic and abiotic factors prevailing in the field.

The use of grafted vegetables in Israel is increasing rapidly and is estimated at five million transplants per year. Grafted watermelons now account for 60 to 70% of the total cultivated area of this crop. All early watermelons grown in the Arava Valley of southern Israel (transplanted in mid-winter and harvested in April) are grafted. Grafting muskmelons and cucumbers is less common due to the high price of the grafted transplants. With watermelons, growers commonly reduce the stand of grafted transplants in the field as compared with nongrafted transplants. This reduction is often accompanied by a compensatory increase in plant productivity such that total yields are maintained and the crop is profitable despite the higher cost of grafted transplants. Moreover, grafted plants are protected from the Melon Necrotic Spotted Virus (MNSV), which is not controlled by methyl bromide fumigation. In muskmelons and cucumbers, however, yield compensation is less pronounced or nonexistent and the higher price for grafted transplants threatens the profitability of the crop; nevertheless, the acreage of grafted melons increased in the last two years, especially for early melons, which bring high prices in the local and export markets. Comprehensive descriptions of various aspects (grafting methods, diseases targeted, acreage, etc.) related to grafted vegetables, especially in the Far East, has been reviewed by Lee and Oda. In this article, we will concentrate on sharing the experience we acquired in Israel with grafted cucurbits, which is the main plant family grafted around the world. 

Intra- and Interspecific

Grafting Suppression of soilborne pathogens can be achieved by grafting susceptible scions onto resistant rootstocks of the same species (intraspecific grafting) or on a close member of the same botanical family (interspecific grafting). In tomatoes, for example, a large collection of tomato rootstocks is available. These stocks contain different genes for resistance to fungal pathogens, viruses, and nematodes; thus intraspecific grafting is very common. In melons, however, resistant muskmelons or Cucurbita rootstocks are used for different purposes. Intraspecific grafting is mainly used to avoid damage caused by wilt pathogens such as Fusarium oxysporum f. sp. melonis for which resistance genes exist in certain melon varieties. This approach provides complete protection from the disease, with no reduction in fruit quality and quantity. The possibility of using melon rootstocks for reducing Monosporascus wilt damage was studied in order to avoid undesirable effects of the Cucurbita rootstock on fruit quality. Although there are muskmelon accessions with differing levels of quantitative resistance to the disease that may serve as rootstocks, it has not been possible to obtain grafted plants with reliable resistance under a variety of environmental conditions (R. Cohen and M. Edelstein, unpublished data). In fact, for suppression of root and stem rot diseases for which dependable resistance is not available in melons, only Cucurbita rootstocks provide the required nonspecific resistance. Intra- and interspecific grafting have their own beneficial and detrimental characteristics as to phytopathological and horticultural behavior. Cucurbita rootstocks provide nonspecific but efficient protection against a wide range of soilborne pathogens and against some abiotic stresses, but such rootstocks may in certain cases affect fruit size and quality. On the other hand, susceptible muskmelons grafted onto resistant muskmelons have fewer horticultural problems related to scion–rootstock compatibility, but their resistance is often limited to one or a few pathogens, or even to a specific race of one pathogen. 

Muskmelon Diseases 

Fusarium wilt of muskmelons is an important disease worldwide. In Israel, the disease is a limiting factor for growing Ananas-type muskmelons in the summer under dry-land farming conditions. Fusarium-resistant Ananas types do exist, but their fruit quality is lower than that of the susceptible ‘Ofir’, the most desirable commercial cultivar grown in Israel. Grafting a susceptible cultivar onto a resistant rootstock is one of the approaches that enable the cultivation of such varieties in Fusarium-infested soils in the Mediterranean area, including Israel. We evaluated the effects of grafting on the horticultural and pathological performance of grafted Fusarium-susceptible muskmelons in Fusarium-infested and -noninfested soils. Muskmelon/muskmelon combinations performed better than the muskmelon/Cucurbita combinations with respect to yield. In infested soil, Fusarium wilt symptoms were observed only in the nongrafted susceptible muskmelons, and there was no wilting with any of the grafted combinations. However, some yellowing of old vines of the muskmelons grafted onto Cucurbita was observed, and these plants were also less vigorous than those grafted onto muskmelon rootstocks, suggesting that the pathogen may have caused some damage to the grafted plants. Interestingly, a difference in rootstock– scion compatibility, as expressed in yield, was evident in muskmelons grafted on two different muskmelon rootstocks. The Ananas-type muskmelon Ofir had a significantly higher yield when grafted onto the Ananas-type muskmelon ‘Adir’ than when grafted onto the Charentais-type muskmelon ‘Orca’, indicating differences in rootstock–scion compatibility even among melon rootstocks. The vigor of the muskmelon/Cucurbita combinations was lower than that of the muskmelon/muskmelon combinations. We also found that the contribution of grafting to disease reduction was influenced by the susceptibility of the scion. Disease incidence in nongrafted ‘Ananas En Dor’ muskmelons (highly susceptible) and in those grafted onto ‘Brava’ (Cucurbita rootstock), was 82 and 20%, respectively, compared with only 36 and 0% in nongrafted and grafted Ofir muskmelons (moderately susceptible), respectively. Susceptible muskmelon scions grafted onto resistant muskmelon rootstocks were colonized less by F. oxysporum f. sp. melonis than the same muskmelons grafted onto Cucurbita rootstocks. In Fusarium-free soil, the grafting process itself did not affect yield. The yield was similar in all grafted plant combinations and in the nongrafted control. Monosporascus sudden wilt. Pathological aspects. Sudden wilt of muskmelons caused by Monosporascus cannonballus is a worldwide problem mainly in arid and semiarid regions , including southern Israel. Soil disinfestation with methyl bromide is a common and effective tactic for disease control but tation by the pathogen is relatively rapid due to sandstorms carrying the pathogens’ ascospores.

Conclusions

Grafting is an old technology that has been, and still is being investigated in fruit trees. The use of grafted vegetables will likely become more common as a replacement for soil fumigation as a means for managing soilborne diseases. Integrating this technology with other environment-friendly technologies could ease the transition into the post–methyl bromide era. There is, however, much to learn about how to optimize this technology. It seems that combating soilborne pests is the easier part of the whole grafting issue, while rootstock–scion compatibility is one of the major obstacles to increased usage of the grafting technology. Advanced molecular technologies are offering new tools for understanding rootstock–scion compatibility. Time will tell if nurseries will find ways to lower the grafted plants’ prices and if research will lead to enhancement of grafted plant performance. If not, we will have to learn to live with the limitations of this technology and adapt it to our needs wherever possible.