A comprehensive review on management of soil borne diseases through vegetables grafting

CONTENTS

1. Introduction

2. History of Grafting in Vegetables

3. Current Status of Vegetable Grafting

4. Diseases controlled by Grafting in Vegetable production

5. Approaches to combat biotic stress in vegetable grafting

6. Basic Prerequisites of Grafting

7. Grafting methods in Vegetables

8. Defense mechanisms in disease resistance of grafted vegetables

9. Success Stories of Grafting as an effective approach against soil borne pathogens in vegetable production

10. Dangers of Grafting

11. Future Prospects

12. Conclusions

13. References

Abstract

Grafting is a method of asexual propagation where two living plant parts (the rootstock and scion) are united together to grow as a single plant. Although grafting in fruits and nuts is an ancient technique, vegetable grafting is relatively a recent innovation and has emerged as one of the most important integrated pest management strategy to manage soil borne pathogens and insect pests of solanaceous and cucurbitaceous vegetable crops at the turn of 20th century. The earliest reports from Japan included the use of Cucurbita moschata to confer Fusarium wilt resistance in watermelon production (Oda 2002; Sakata et al 2007, 2008). Grafting as a disease management tactic has rapidly expanded to solanaceous and cucurbitaceous vegetables due to the factors viz. increased pathogen inoculum densities due to intensification of production practices, reliance on susceptible cultivars to meet specific market demands , global movement and local invasion of novel pathogens, increased use of organic practices, the rapid adoption of high tunnel production systems, use of appropriate technologies for resource-limited farmers and the ban on methyl bromide via Montreal Protocol (Sakata et al 2007). The recent use of semi-automated grafting machines and grafting robots ensures higher grafting speed, higher survival rate of grafted plants, reduces the higher price of grafted seedlings, and thus encourages the cultivation of grafted plants among small-scale farmers. In addition, grafting provides advantages to manage abiotic stress, to reduce reliance on chemical and fertilizer inputs, and to enhance fruit quality and yield (Colla et al 2010a, b; Proietti et al 2008; Rouphael et al 2008a, b).

Important soil borne diseases managed by grafting in vegetables are Fusarium wilt of tomato ( F. oxysporum f. sp. lycopersici ), Fusarium wilt of eggplant (F. oxysporum f. sp. melongenae), Fusarium wilt of melon (F. oxysporum f. sp. melonis) , Fusarium wilt of watermelon (F. oxysporum f. sp. niveum) , Fusarium wilt of cucumber (F. oxysporum f. sp. cucumerinum) and Fusarium wilt of bottle gourd (F. oxysporum f. sp. lagenariae ), Phytophthora blight of cucurbits ( P. capsici), Verticillium wilt of tomato, eggplant and cucurbits ( V. dahliae ), Monosporascus root rot and vine decline of melon (M. cannonballus), Corky root rot of tomato and eggplant (Pyrenochaeta lycopersici), Southern stem blight of tomato and eggplant (Sclerotium rolfsii) ,Bacterial wilt of tomato and eggplant (Ralstonia solanacearum), Root knot of vegetables (Meloidogyne incognita, M. arenaria, M. javanica and M. hapla) and several soil borne virus pathogens (Melon necrotic spot Virus , Pepino mosaic Virus ) ( Louws et al 2010).

Rootstocks can include intraspecific selections that utilize specific major resistance genes and interspecific and intergeneric selections that exploit non-host resistance mechanisms or multigenic resistance. The most widely exploited selections of rootstocks with varying degree of resistance against various soil borne diseases in cucurbit production are interspecific hybrid squash (Cucurbita maxima x Cucurbita moschata), C. ficifolia (figleaf gourd), Lagenaria siceraria (Bottle gourd) and in solanaceous crop production, for both tomato and eggplant, are interspecific hybrids Solanum lycopersicum x Solanum species, Solanum torvum, Solanum melongena and S. lycopersicum . One advantage and associated challenge of grafting is that rootstock selection for disease management is site specific depending on the presence, population structure and dynamics of the pathogen, as well as edaphic, environmental and anthropogenic factors (Louws et al 2010). Besides, increasing evidence indicated that improved plant nutrient uptake by the virtue of vigorous root system, systemic defense mechanisms and shift of rhizosphere microbial diversity also may play an important role in plant defense as a result of grafting (Zhao et al 2012).

Besides, through the use of grafting, the breeding programs can become more efficient and effective by focusing on above- and below- ground traits separately and combining disease resistance and other horticultural traits into a so called "graft hybrid'' (Mudge et al 2009). However, over-reliance on specific rootstocks in production systems has led to the emergence of new pathogens or shifts in the host specificity of the pathogen population, emphasizing the need for multi-tactic approaches to manage soil borne pathogens (Louws et al 2010). The use of grafting as an integrated pest management tool to manage biotic stress will be most successful when carried out with increasing knowledge about the biology, diversity, and population dynamics of the pathogen or other pests and when complemented with sustainable farming system practices.

1. Introduction

Soilborne plant pathogens can significantly reduce the yield and quality in vegetable crops. These pathogens are particularly challenging because they often survive in soil for many years and each vegetable crop may be susceptible to several species. Many disease caused by the soilborne pathogens are difficult to predict, detect and diagnose. In addition, the soil environment is extremely complex, making it a challenge to understand all the aspects of the diseases caused by soil borne pathogens. Soilborne pathogens can be defined as pathogens that cause plant diseases via inoculums that come to the plant by the way of soil. The most familiar diseases caused by soilborne pathogens are probably rots that affect belowground tissues (including seed decay, damping off of seedlings, root and crown rots and vascular wilts initiated through root infection. They perrenate in the soil as resting structure sclerotia, oospores or nematode cysts or as mycelium, bacterial ooze in the infected plant debris. About 68 % of the yield losses in vegetables are reported due to the soil borne diseases under continuous cropping. Recently, with emphasis on multi tactic approaches to manage soilborne pathogens vegetable grafting has emerged as an important integrated pest management to manage soil borne diseases of cucurbitaceous and solanaceous vegetable crops. In addition, grafting provides advantages to manage abiotic stress, to reduce reliance on chemical and fertilizer inputs, and to enhance fruit quality (Colla et al 2010a,b, Proietti et al 2008 & Rouphael et al 2008a,b)

1.1 What is Grafting?

Grafting is a method of asexual propagation where two living plant parts (the rootstock and scion) are united together to grow as a single plant. Grafting is a fusion of plant parts so that vascular continuity is established between them and the resulting genetically composite organism functions as a single plant (Mudge et al 2009). In grafting, one plant is selected for its roots which is generally meant for disease resistance against soilborne pathogens and plant vigour benefits and this is called the rootstock. The other plant is selected for its stems, leaves, flowers, or fruits which is commonly meant for qualitative and quantitative horticultural traits and is called the scion.

The graft union is initially formed by rapidly dividing callus cells, originating from the scion and rootstock, which later differentiate to form the vascular cambium (a lateral meristem) and the associated vascular system. The development of a compatible graft is typically comprised of several major events: adhesion of the rootstock and scion, proliferation of callus cells at the graft interface or callus bridge, and vascular differentiation across the graft interface.

The principle stages to graft union formation are

1. Lining up the vascular cambium.
2. A wound response.
3. Callus bridge formation.
4. Xylem and phloem differentiation into a new vascular cambium.
5. Secondary xylem and phloem development across the graft.

Abbildung in dieser Leseprobe nicht enthalten

1.2 Purposes of Grafting in Vegetables

a. Tolerance to soilborne diseases

The vigorous roots of the rootstock exhibit excellent tolerance to serious soil-borne diseases, such as those caused by Fusarium, Verticillium, Phytophthora, Pseudomonas, and viruses, even though the degree of tolerance varies considerably with the rootstocks. The mechanism of disease resistance, however, has not been intensively investigated. These characteristics are crucial for the plants grown under protected environments, where extended harvesting and higher crop yield are expected. Resistant rootstocks can also effectively counteract the rapid disease spread when the plants are grown in hydroponics system.

b. Plant vigor promotion (Reduced fertilizer and agrochemical applications)

Since the root systems of selected rootstocks are usually much larger and more vigorous, they can absorb water and nutrients much more efficiently as compared to non- grafted plants. For example, in grafted watermelons, it is routinely recommended to reduce the amount of chemical fertilizers application to about one-half to two-third as compared to the standard recommendation for the non-grafted plants (Lee and Oda 2003 & Salehi et al 2009). This is especially true for nitrogen fertilizers during early seedling growth for the safe setting of fruits at the desired node positions for early fruit set. Early fruit set is crucial for the early harvesting in greenhouses to secure good market prices. Otherwise the fruit set as well as the fruit quality at harvest will not be high enough to secure highest market grading.

Cytokinin composition in bleeding xylem sap from decapitated plants, grafted or own-rooted, is much different in various cucurbits and, more interestingly, the scion portion is capable of converting the composition of cytokinins in the ascending xylem sap in relative short period, thus clearly indicating the contribution of higher cytokinin concentration in the ascending xylem sap for the growth promotion of grafted scion. The frequency of agrochemical application also can be significantly reduced by using vigorous rootstocks.

Spray of fungicides may also be greatly reduced or totally excluded depending upon the diseases, thus greatly enhancing the successful production of organically-grown fruits. It has been shown that the incidence of various diseases in tomatoes can be easily minimized by using disease tolerant rootstocks rather than using pesticides. Even the scion infection of certain virus diseases (TMV races) could be markedly influenced. Expression of deficiency symptoms may be minimized with proper rootstocks. Wise selection of rootstocks can also effectively replace methyl bromide. In cucumber, vigorous root system of the rootstock can effectively absorb water so that less frequent irrigation may be practiced.

c. Yield increase

Grafting is associated with noticeable increases in fruit yield in many fruiting vegetables regardless of infection with certain soil-borne diseases. In oriental melons, fresh fruit weight increases of 25~55% have been reported as compared to own-rooted plants. These yield increases were closely correlated with the maintenance of good plant vigor until late in the growing season in addition to disease resistance. Virtually no marketable yield was obtained from plants heavily infected with Fusarium. Similar results were obtained with tomato. Up 136 to 54% increase in marketable yield was obtained with "Kagemusia" and 51% with "Helper" rootstocks (Chung and Lee, 2007). There were also significant decreases in abnormal fruits in plant grafted onto most rootstocks as compared with the self-rooted "Seokwang" tomato. Similar yield increases have been reported by other researchers on watermelon, cucumber (Lee and Oda 2003), melon, pepper, and eggplant.

d. Tolerances to adverse soil temperature and moisture conditions

Tolerance to extreme temperature is crucial for the production of fruiting vegetables under the winter greenhouse conditions. In cucurbits, cropping area under protected structure is substantially larger than field cultivation for watermelon, cucumber, and melon in Korea. The transplanting of seedlings for protected cultivation is usually done in early to mid-winter and fruit harvesting is usually finished by spring to early summer. Even though many growers heat their greenhouse during the winter, there are more growers who do not have electric or gas­generated heating systems and depend solely on preservation of solar energy capture during the daytime. These growers find it difficult to maintain optimum temperatures in winter greenhouses, especially soil temperatures which are far below the optimum thus causing transplanted plants to suffer during the early stages of cultivation. This is especially true with crops that require high temperatures for optimum performance such as watermelon and oriental melon. Grafting watermelon, melon, cucumber, even summer squash onto low- temperature tolerant rootstocks such as interspecific hybrid between Cucurbita maxima x C. moschata or figleaf gourd can greatly reduce the risk of severe growth inhibition caused by low soil temperatures in winter greenhouses. Cucumber grafted onto figleaf gourd (Cucurbita ficifolia), an excellent grower even at low soil temperature, grows much faster than own-rooted cucumber or even summer squash because of the rootstock's ability to absorb water and

nutrient more efficiently at low temperatures (Tachibana,1982). Many physiological disorders can be effectively minimized by using grafted plants. Since the resistance to temperature stresses varies with the rootstocks, different rootstocks should be used during the hot summer season.

e. Effect of fruit quality

The fruit size of watermelons grafted to rootstock having vigorous root systems is often significantly increased compared to the fruit from intact plants, and many growers practice grafting mainly for this reason. It is also known that other quality characteristics, such as fruit shape and skin color, rind thickness, and soluble solids concentrations are influenced by rootstock. In cucumbers, especially those for export, external color and bloom development are important quality factors. Even though these are usually regarded as cultivar-specific hereditary characteristics, they can be greatly influenced by the rootstock. However, the effects of rootstocks on some fruit quality are often detrimental, except for increasing fruit size, shape, and bloomless-fruit production in cucumber. Therefore, most newly-devised growing recommendations are aimed at minimizing the detrimental effects of rootstock on fruit quality (Cushman and Huan 2008 and Ko 2008)

f. Flood tolerance

Flooding and submergence are major abiotic stresses and are serious problems for the growth and yield of flood sensitive crops. Flooding conditions cause oxygen starvation, which arises from the slow diffusion of gases in water and from oxygen consumption by microorganisms and plant roots. Problems caused by flooding may be solved by growing flood-tolerant crops or grafting intolerant plants onto tolerant ones. For instance, grafting improved flooding tolerance of bitter melon (Momordia charanthia cv. New Known You ) when grafted onto luffa (Luffa cylindria cv. Cylinder) (Liao and Lin 1996). A milder depression of photosynthetic rate, stomatal conductance, transpiration, soluble proteins, and/or activity of RuBisCO was possibly related to this difference in flooding tolerance. In contrast, the reduction of the chlorophyll content in cucumber leaves induced by waterlogging was enhanced by grafting onto squash rootstocks (Kato et al 2001). A chemical signal present in xylem sap stimulates ethylene biosynthesis in the shoot and thus may be responsible for the decreases. When grafting watermelon [Citrullus lanatus (Thunb.) Matsum and

Nakai cv. 'Crimson Tide'] onto Lagenaria siceraria SKP (Landrace) the decrease in chlorophyll content was less pronounced compared with non-grafted water melons (Yetisir et al 2006 and Liao and Lin 1996). Moreover, adventitious roots and aerenchyma formation were observed in grafted but not in ungrafted watermelon under flooding. Flooding occurs also during the heat period in the lowland tropics. Here, the AVRDC recommends growing tomatoes on eggplants 'EG195' or 'EG203' and pepper on chili accessions 'PP0237-7502', 'PP0242-62' and 'Lee B' (AVRDC 2003, 2009). Mode of action described for watermelons are also valid for the eggplant and pepper rootstocks. Moreover, cultivation manuals mention that these rootstocks are tolerant against bacterial or Fusarium wilt or Phytophthora blight or root-knot nematodes.

g. Miscellaneous

Grafting can be demonstrated for various other reasons. For example, tomatoes, eggplants, pepinos can be grafted on potatoes so that four or more different kind of vegetables could be harvested from a plant. Chinese cabbages and cabbages may be grafted on top of radish with radish roots. Grafting can be made for some physiological studies such as flower induction and early flowering. Grafting is also commonly used for bioassays of virus infection. Use of grafted plants is highly recommended for hydroponics to avoid rapid spread of root disease within the system (Lee and Oda 2003 and Davis et al 2008).

The purposes of grafting are summarized in the table below:

Abbildung in dieser Leseprobe nicht enthalten

(Heo 2003, Lee 1994, Lee et al 1998 and Lee and Oda 2003)

2. History of Grafting in Vegetables

Grafting, with selected resistant rootstocks, for the purpose of controlling diseases and pests is an ancient practice widely used in cultivating a variety of fruits and nuts. Some of the well known examples include controlling citrus tristeza, fireblight and collar rot on apples, and nematodes on peaches and walnuts (Mudge et al 2009). However, Commercial use of Vegetable Grafting is relatively recent innovation. The production of grafted vegetable plants was first reported in Japan and Korea in the late 1920s with watermelon (Citrullus lanatus) grafted onto pumpkin (Cucurbita moschata) rootstock to confer resistance to Fusarium wilt in water melon production (Lee 1994).Soon after, watermelons (Citrullus lanatus) were grafted onto bottle gourd (Lagenaria siceraria) rootstocks for managing Fusarium wilt . Eggplant (Solanum melongena) was grafted onto scarlet eggplant (Solanum integrifolium Poir.) in the 1950s for combating Bacterial wilt. The invention of plastic films and active uses for the production of vegetables in the late 1950s provided the momentum for generalized production and use of grafted vegetables. The early use of grafted vegetables was associated with protected cultivation which involved successive cropping. Commercial vegetable grafting, originated in Japan and Korea and practiced for about 30 years until 1990, was introduced to the Western countries from the early 1990s and is currently being globally practiced using local scion cultivars and introduced rootstocks with the primary intent being mitigation of soilborne pathogens.

3. Current Status of Vegetable Grafting

In Korea and Japan, among cucurbitaceous crops, over 90% of watermelon & 75% cucumbers seedlings are grafted onto various rootstocks in Korea and Japan while in Solanaceous vegetables, 20-40% of tomatoes are grafted, 20-40% of eggplants, and 5-10% of peppers (Lee et al 2010). Since grafting is mostly practiced in cucurbits and Solanaceous vegetables, the percentages of grafting in all vegetables was only about 5% in 2007. More than 700 million grafted seedlings were estimated to be produced in 2008 in Korea & Japan. 40-45 million grafted seedlings were estimated to be transplanted in North America in 2005 (Kubota et al 2008).

Among European countries, Spain is by far the leading country in using grafted vegetable transplants (129.8 million in 2009) (Hoyos Echeverria 2010), followed by Italy (47.1 million) (Morra and Bilotto 2009) and France (about 28 million). It was estimated that about 20% of China's watermelon and cucumber are grafted. Lam Dong Province of Vietnam, which is the country's major tomato production area, 100% of commercial farmers are now using grafted seedlings to counter hostile soil borne diseases (Keatinge et al 2014)

Table: Current status of the estimated use of grafted vegetables in some Asian and other countries and regions.

Abbildung in dieser Leseprobe nicht enthalten

a Cultivation area was obtained from FAO statistics 2008 except Taiwan.
b Percentage of cultivation area with grafted plants.
c Data not available (NA)
d Greenhouse hydroponic tomato cultivation area only. Little or no graftmg had been reported for field tomatoes of 162,580 ha m the USA.

Source: Lee et al. (2010)

Table: Current status of the estimated use of grafted vegetables in some European and other countries

Abbildung in dieser Leseprobe nicht enthalten

Source: Lee etal. (2010)

However, Grafting is rare in India due to lack of expertise, high initial cost and lack of facilities, but with alarming threat due to various hostile soil borne diseases to vegetable production, vegetable grafting is an attractive option.

Factors that have led to increased expansion of grafting include: increased pathogen inoculum densities due to intensification of production practices, reliance on susceptible cultivars to meet specific market demands (Sakata et al 2007), global movement and/or local invasion of novel pathogens, increased use of organic practices, the rapid adoption of high tunnel production systems, use of appropriate technologies for resource-limited farmers, and the loss of methyl bromide via Montreal Protocol. Methyl bromide has been used as a broad spectrum soil fumigant to manage soilborne pathogens and pests and efforts to find alternatives to Methyl Bromide for vegetable production included tactic diversification, focused on non-fumigant and integrated pest management based strategies and longer term alternative strategies with an emphasis on filling scientific and biological information gaps complemented with farming systems research (Louws 2010). Grafting represents a viable tactic diversification strategy with long-term potential to mitigate biotic stress.

The Montreal Protocol is a treaty signed by over 160 countries to protect the stratospheric ozone layer, which protects the earth from harmful solar radiation. The Protocol controls global production and trade of ozone depleting substances. The Parties to the Protocol classified methyl bromide as an ozone-depleting substance in 1992 and agreed to the current phase-out schedule in 1997.Under the Protocol's provisions, developed countries were scheduled to reduce methyl bromide consumption (production plus imports minus exports) from a 1991 baseline by 25 percent in 1999, 50 percent in 2001, 70 percent in 2003, and 100 percent in 2005 and developing countries were scheduled to reduce consumption n by 20 percent in 2005 and 100 percent in 2015.

4. Diseases Controlled by Grafting in Vegetable Production

Improved resistances to many soilborne fungal, oomycete, bacterial, and nematode pathogens have been reported in grafted solanaceous and cucurbitaceous crops. Moreover, certain foliar fungal and viral diseases were suppressed when susceptible scions were grafted onto specific rootstocks (Louws et al 2010). Diseases controlled by grafting in different vegetable crops are listed in table in the next page.

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