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Crop Protection 73 (2015) 93e99

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Crop Protection

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Review

Fusarium wilt of watermelon: Towards sustainable management of are-emerging plant disease

Kathryne L. Everts a, b, *, Jennifer C. Himmelstein c

a University of Maryland College Park, Plant Science and Landscape Architecture, Lower Eastern Shore Research and Education Center, 21801 Salisbury, MD,United Statesb University of Delaware, Newark, DE, United Statesc University of Maryland, Plant Science and Landscape Architecture, College Park, United States

a r t i c l e i n f o

Article history:Received 4 October 2014Received in revised form27 December 2014Accepted 20 February 2015Available online 12 March 2015

Keywords:Fusarium wiltWatermelonControlFusarium oxysporum f. sp. niveumFungicidesGraftingSuppressive soils

* Corresponding author. University of Maryland, PArchitecture, 27664 Nanticoke Rd., Salisbury, MD 218

E-mail address: keverts@umd.edu (K.L. Everts).

http://dx.doi.org/10.1016/j.cropro.2015.02.0190261-2194/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Changes in watermelon production practices throughout the world such as the reduction in fumigationwith methyl bromide, increasing human population density, which reduces rotational land availability,and the spread of more virulent forms of Fusarium oxysporum f. sp. niveum, have led to a resurgence inFusarium wilt. Public research on Fusarium wilt management practices has focused on grafting, theidentification of new sources for resistant rootstock germplasm, chemical control, improved under-standing of suppressive soils, cover crop-induced suppression, impact of plant secondary metabolites onFON, and the role of biological control microbes in disease suppression. Additional research on rapididentification techniques for FON and economical methods for field level identification of FON races willimprove management of this reemerging disease.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Fusarium wilt of watermelon caused by Fusarium oxysporum f.sp. niveum (FON) is found in watermelon production regionsthroughout the world (Egel and Martyn, 2013). Long established inNorth America, Asia, and theMiddle East, FON has been reported onall six continents wherewatermelon is cultivated (Egel andMartyn,2013). Watermelon yield losses due to Fusarium wilt have recentlyincreased, and Fusariumwilt is common where land is limited andwatermelon are produced on the same land in successive years orin short rotations, such as in China (Hao et al., 2010; Wu et al.,2013).

Recently, outbreaks have occurred in locations where water-melon Fusarium wilt has not been reported, or has not previouslybeen severe. A Fusarium wilt outbreak on three triploid water-melon cultivars grown in the Northern Territories of Australia in2011 resulted in reductions in watermelon yield and unusual

lant Science and Landscape01, United States.

symptomology of watermelon seedlings (Tran-Nguyen et al., 2012).The disease has also increased in areas where the highly virulentFON race 2 has become prevalent. FON race 1 originally predomi-nated in the United States. However, FON race 2 is now consideredan emerging problem and is reported in eight of the U.S. states, inthe Mediterranean region of Turkey and other countries, includingSpain, Tunisia, Korea, Greece and Israel (Boughalleb and Mahjoub,2006; Bruton et al., 1988, 2008; Egel et al., 2005; Gonz�alez-Torreset al., 1993; Keinath and DuBose, 2009; Kurt et al., 2008; Kwonand Om, 1998; Martyn and Bruton, 1989; Zhou and Everts, 2001).FON race 2 isolates make up approx. one-fourth of the overall FONpopulation in Maryland and Delaware (Zhou and Everts, 2003) and14% in the Mediterranean region of Turkey (Kurt et al., 2008). Inaddition to the spread of FON race 2, Zhou et al. (2010) reported anew, more virulent race, FON race 3, in Maryland.

Besides this changing geographic distribution of FON races, theimportance of Fusarium wilt of watermelon is increasingthroughout theworld because production practices for watermelonhave changed dramatically over the past two decades. In the UnitedStates, one major change has been the Montreal Protocols'mandated reduction of the soil fumigant methyl bromide (MeBr),which was widely utilized for management of Fusarium wilt,

Table 1Disease reaction of watermelon genotypes used to differentiate races of Fusariumoxysporum f. sp. niveum.

Genotype Race

0 1 2 3

Sugar Baby or Black Diamond S S S SCharleston Gray R S S SCalhoun Gray R R S SPI-296341-FR R R R S

K.L. Everts, J.C. Himmelstein / Crop Protection 73 (2015) 93e9994

weeds, and other pests of vegetable crops including watermelon(King et al., 2008). Alternative soil fumigants generally aren't aseffective as MeBr at controlling FON or weeds, are expensive, andalso negatively impact the environment (Sande et al., 2011).

Another major change has been in the production of triploidwatermelon cultivars. Consumer demand for triploid (seedless)watermelons has increased since the 1990's and triploid cultivarsnow dominate the U.S. watermelon acreage (Lucier and Biing-Hwan, 2001). Fusarium wilt in diploid (seeded) watermelon culti-vars was previously managed through genetic resistance to FONrace 1 (Martyn, 2014). Unlike diploid watermelons, the majority oftriploid watermelon cultivars have little or no resistance to Fusa-rium wilt and additional methods of control are necessary (Evertsand Hochmuth, 2011; Everts and Zhou, 2010).

As watermelon yield losses due to Fusarium wilt are risingthroughout the world, there is an increased need for alternativecontrol practices. Potential alternatives include cultural methods,improved host resistance, grafting watermelon scions onto resis-tant rootstocks, and biological and chemical controls. This review isnot meant to be a comprehensive assessment of the biology ofFusarium wilt of watermelon nor a comprehensive historical re-view of previously used management practices. A recent historicalreview of watermelon Fusarium wilt focused on early resistancebreeding, beginning in the 1900's and continuing through thecurrent genomics research (Martyn, 2014). Therefore our reviewfocuses on recent changes in management, the status of currentmanagement practices, and recent research on management. Wecover management practices that are commercially available andcurrent research on potential management practices.

2. Diagnosis of watermelon Fusarium wilt

The first step in management of Fusarium wilt is accuratediagnosis. Management recommendations are usually donefollowing putative diagnosis. Putative diagnosis of Fusariumwilt ofwatermelon at the field level is performed by examining thevascular system for discoloration and isolation of F. oxysporum(Kleczewski and Egel, 2011). Confirmation that an isolate from asymptomatic host is FON requires a process of plating roots ontoselective media, obtaining a single spore culture and conducting ahost pathogenicity assay. This confirmation process, which isnecessary for use of isolates for research, requires a minimum ofonemonth to complete, a period too long for use inmaking a timelymanagement decision.

Recently Zhang et al. (2005) developed a rapid diagnosticmethod. They developed a primer set, Fn1/Fn2, to differentiate FONfromDidymella bryoniae and a broad group of other fungi, includingthree other F. oxysporum formae speciales (FO). Their techniquewasrapid and reliable for their isolates. This primer set, however, wasunable to differentiate Taiwanese FON isolates from other FO. Linet al. (2010) developed another primer set FON1/FON2 that wasmore suitable for differentiating Taiwanese FON from FO. The setFON1/FON2 was also able to detect FON in diseased watermelontissue at early stages of wilt (Lin et al., 2010). These studies repre-sent a tremendous improvement in rapid diagnosis, however, dif-ferentiation of FON races remains problematic.

Identification of the races of FON present in the soil or infectingwatermelon hosts would aid in cultivar selection for diseasemanagement. However, the identification of the four known FONraces is difficult. Races of FON are identified based on the level ofspecific resistance of watermelon differential cultivars (Brutonet al., 2010). Watermelon cultivars ‘Black Diamond’ and ‘SugarBaby’ are susceptible to all FON races (Table 1). ‘Charleston Gray’ isresistant to race 0 but susceptible to races 1, 2, and 3. FON race 0 isof limited economic importance because most commercially grown

cultivars are resistant to FON race 0 and because race 0 is notwidespread. ‘Calhoun Gray’ is resistant to races 0 and 1 and sus-ceptible to races 2 and 3. The plant introductions (PI line) ‘PI-296341-FR’ is resistant to races 0, 1, and 2 but susceptible to race 3.Bruton et al. (2010) described some of the weaknesses of this dif-ferential system, which does not account for variations in FONpathogen aggressiveness. Environmentally dependent symptomexpression includes wilting, yellowing and stunting. Disease re-actions are influenced by inoculation methodology (for example,preparation and concentration of inoculum, and plant growthstage). Even more problematic is obtaining the cultivars thatcompose the differential set. ‘Calhoun Gray’ and ‘PI-296341-FR’ arenot commercially available. Therefore, researchers must increaseseed themselves. An alternative suggested by Egel and Martyn(2013) is to use watermelon cultivars ‘Super Pollinizer-5’ (SP-5) or‘Super Pollinizer-6’ (SP-6), developed by Syngenta Seeds, Inc.,instead of ‘PI-296341-FR’. These cultivars, which are more readilyavailable than ‘PI-296341-FR’, derive their resistance to FON race 2from ‘PI-296341-FR’, however they have not been tested for use asdifferential lines.

Another problem with the differential series is that ‘PI-296341-FR’ lacks a uniform resistance reaction to FON. In fact, this becamean issue in the initial identification of FON race 3. Zhou et al. (2010)used two sources of ‘PI-296341-FR’ in initial evaluations of FONisolates. When they observed segregation, they grew seedlings,inoculated with races 1 and 2, selected fruit of disease free plants,and harvested seed for additional testing. The subsequent testswere more uniform; however, this inoculation and selection step islaborious and there are no standardized guidelines for race evalu-ation (Bruton et al., 2010). Ideally molecular methods will bedeveloped that will differentiate FON races. Development of thesemethods will be heavily dependent on the accuracy of previousdifferential race testing and other methods such as VCG testing(Zhou and Everts, 2003).

3. Crop rotation

Rotation of land out of watermelon cultivation is the oldest ofFusarium wilt management practices. Early anecdotal reports onsurvival in soil indicated that FON persisted for 16 years (Martyn,2014). Survival of FON occurs primarily as thick-walled chlamydo-spores. FO can be stimulated to germinate in soil in the proximity ofsome non-host roots. Closely related formae specialis are able tocolonize roots of non-host plants (Nonumura et al., 2003).Nonumura et al. (2003) labeled F. oxysporum f. sp.melonis, which isa pathogen of muskmelon, with green fluorescent protein andvisualized hyphal growth, elongation, and penetration of themuskmelon root surface. Nonpathogenic F. oxysporum, which wassimilarly labeled, attached, germinated, and grew vegetatively butwas unable to penetrate the muskmelon root (Nonumura et al.,2003). The role that the pathogens' ability to grow on a non-hostplays in FON survival in soil is unknown. In addition, root exu-dates of watermelon stimulate spore germination and sporulation

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of FON (Hao et al., 2010). Rotation to non-host crops, whose rootexudates are not stimulatory to germination or sporulation, mayinhibit FON.

Long term monoculture of watermelon leads to high diseaselevels. There are few recent studies on crop rotations; however,previous research showed that in plots where the watermeloncultivars planted had resistance to FON race 1, the proportion ofFON race 2 in the population increased (Hopkins et al., 1987). Anotable exception to the observation that Fusarium wilt occurswhen watermelon is repeatedly cropped in successive years is thedevelopment of suppressive soils following a long term mono-culture of cultivar ‘Crimson Sweet’ (Hopkins et al., 1987). The natureof this suppressiveness is not entirely understood. However, adiverse group of non-pathogenic F. oxysporum strains isolated fromwatermelon roots in the suppressive soil were able to reduce dis-ease (Larkin and Fravel, 1998). One isolate from the cultivar-induced suppressive soil, CS-20, has been evaluated as a biocon-trol against several isolates of F. oxysporum f. sp. lycopersici (Fravelet al., 2005).

Wu et al. (2013) were interested in microbial soil populations inlong term watermelon monoculture and subsequent remediationof soil by fallowing land. They examined soil with a continuous five-year history of watermelon production under a subsequent three-year fallow period. When they examined culturable fungi recov-ered from soil, FONwas reduced by approx. 20, 40, and 50% in yearsone, two, and three of the fallow. Interestingly, other Fusarium spp.,Fusarium merismorides, and Fusarium fusariodes, also declined. Incontrast, culturable bacteria increased as a proportion of the pop-ulation as the fallow time increased.

Research on crop rotation is difficult and, by its nature, timeconsuming. However, further advancements in our understandingof the impact of crop rotation on pathogens such as FON wouldgreatly benefit disease management.

4. Host resistance

Second only to crop rotation, the employment of cultivars withhost resistance is the most widely used Fusariumwilt managementpractice for watermelon. The first resistant diploid cultivar wasintroduced in the early 1900's (Martyn, 2014) and diploid cultivarsresistant to FON race 1, such as ‘Conquerer’ and several others werewidely deployed in the United States prior to the shift to productionof triploid cultivars (Gunar and Wehner, 2004). Resistance to FONrace 1 in early cultivars such as ‘Calhoun Grey’ and ‘Summit’ wasconferred by a dominant gene (Netzer and Weintall, 1980). A highlevel of resistance to race 1 and 2 has not yet been deployed intriploid cultivars (Everts and Hochmuth, 2011; Everts and Zhou,2010). However, there is some variation in the relative level ofresistance among triploid cultivars (Egel and Hoke, 2010; Egel et al.,2011; Everts and Zhou, 2010). In two greenhouse trials conducted inIndiana, where FON race 1 was inoculated, ‘AC 4674’ (Abbott andCobb), and ‘RWT 8228’ (Syngenta) performed well (Egel and Hoke,2010; Egel et al., 2011). Additional cultivars from Syngenta (‘Fasci-nation,’ ‘Summer King,’ and ‘Distinction,’), Seminis (‘Majestic,’ and‘Cronos’), and Seedway (‘Indiana’) also performed well. However, inthe presence of a mixed population of FON race 1 and 2 in aninfested field, wilt on ‘AC 4674’ and ‘RWT 8228’ was intermediate(Everts and Hochmuth, 2011).

Although the development of triploid cultivars that have highlevels of resistance to FON race 1 and 2 has not yet been achieved,there is some value in the deployment of cultivars resistant to FONrace 1 in fields where race 2 also is present. This was demonstratedin trials in the eastern U.S. in fields with a mixed population of FONconsisting of races 1 and 2. Cultivars that were resistant to FON race1 sustained less wilt than cultivars without resistance, possibly due

to the resulting reduction in effective inoculum (Keinath et al.,2010; Zhou and Everts, 2006, 2007). Identification and deploy-ment of new sources of resistance are urgently needed. However,challenges remain. For example, in a study evaluating geneticrelatedness and diversity among 46 American cultivars, Levi et al.(2001a and 2001b) found that there was a high level of geneticsimilarity. They concluded that it was necessary to broaden thegenetic base of watermelon cultivars to reduce vulnerability todisease losses.

Breeding of commercial cultivars and pollenizers used in trip-loid watermelon production is conducted at several privatebreeding companies. However, cultivar evaluation and germplasmdiscovery and development still occurs at public institutions in theU.S (Gunter and Egel, 2012). In a search for additional sources ofhost resistance, Wechter et al. (2012a, 2012b) screened 110 U.S. PIaccessions, for potential sources of resistance, ultimately identi-fying fifteen accessions, including ‘PI-271769’, which exhibited aresistant reaction in previous research (Dane et al., 1998). Theseaccessions were not uniform in their resistances and the authorsreported that the accessions were segregating for resistance to FONrace 2. Subsequently (Wechter et al. 2012a, 2012b) improved threeof the accessionswith selection and resistance screenings followingeach of three selfings. The improved lines, designated ‘USVL246-FR2,’ ‘USVL252-FR2’ and ‘USVL335-FR2,’ were extensively testedand found to be uniform in their reaction to FON race 2. Theimproved lines' levels of resistance to race 2 were equal to, orhigher than that of ‘PI-296341-FR’ or ‘PI-271769’.

There are several recent studies which have shed some light onone potential mechanism that may contribute to host resistance.During watermelon growth, An et al. (2011) evaluated the effects oftwo watermelon cultivars with different levels of resistance to FONrace 1 on rhizosphere microbial community structure. Theyobserved significant differences of culturable bacteria and actino-myces, which were more abundant during growth of a resistantcultivar (‘Sweet Girl’) than the susceptible cultivar (‘Little Angel’).However there were significantly fewer fungi in the soil rhizo-sphere of the resistant cultivar. Polymerase chain reaction e

denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting,in addition to plate culture showed differences in the abundance ofculturable bacteria in the rhizophere of the resistant vs. susceptiblecultivar (An et al., 2011). This demonstrates that the influence of thehost genotype on soil microbial community structure maycontribute to the host defense response.

5. Grafting

Grafting was first used commercially on watermelon in Japanand Korea and in the last 20 years, has increased throughout manyof the world's watermelon producing regions (Davis et al., 2008;King et al., 2008). Grafting was originally adopted in China, Japanand Korea, where land use was intensive and crop rotation wasdifficult. Subsequently, due to multiple derived benefits, graftinghas been adopted in additional countries such as Australia (Tran-Nguyen et al., 2012).

Watermelon, like most cucurbits, are primarily grafted ontoCucurbita rootstocks (ie. interspecifically grafted). These rootstockare non-hosts to FON. In the Mediterranean region, the primaryrootstock is interspecific hybrid squash, Cucurbita moschata xC. maxima, and in the United States both interspecific hybrid squashand bottle gourd (Lagenaria siceraria) are used (Cohen et al., 2014;Keinath and Hassell, 2014a). Grafting onto a well-selected non-hostroot stock has many benefits, including increased fruit size andnumber, and the extension of harvest duration (Cohen et al., 2007).Grafting can almost entirely eliminate Fusarium wilt (Keinath andHassell, 2014b). Fusarium wilt incidence on grafted watermelon

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plants was reduced 100% in Turkey (Yetisir et al., 2003), and 88% insoutheastern United States (Keinath and Hassell, 2014b), in com-parison to nongrafted transplants. Despite both production anddisease management benefits, few watermelon growers in the U.S.have adopted the practice as a consequence of high labor and seedcosts (Davis et al., 2008), as well as other expenses. Researchers andprivate industry are currently working on techniques and toolswhich would make watermelon grafting economically feasible forU.S. growers and costs have been reduced in recent years. Keinathand Hassell (2014b) reported that grafted watermelon plants costbetween 0.66 and 1.32 U.S. dollar (USD). Even grafted plants thatcost 1.00 USDwould result in a net return for growers in the state ofSouth Carolina. If the cost rose to 1.30 USD, then not all cultivarsbring in a net return (Keinath and Hassell, 2014b).

Interspecific grafting confers many benefits to watermelonproduction, such as improved yield and reductions in Fusariumwilt. Cohen et al. (2014) evaluated pest resistance traits of exoticwatermelon accessions for potential use as rootstock in intraspe-cific grafting. Of the twentyetwo watermelon cultivars and wildaccessions (PI lines), tested for resistance to FON race 2, only ‘PI-296341’ did not wilt. A few other plant introductions (‘PI-457916,’‘PI-273481’) also had low wilt incidence in the two experiments. Inaddition the cultivars ‘Congo’, ‘Cream of Saskatchewan’ and ‘Malali’had lower wilt incidences that most of the tested germplasm(Cohen et al., 2014).

Grafting is thought to be effective in reducing Fusarium wiltprimarily because the rootstock are non-hosts of FON. Newresearch has indicated that root exudates also play a role in diseasereduction (Ling et al., 2013). Exudates of non-grafted watermelon,non-grafted bottle gourd (Lagenaria siceraria) and watermelongrafted onto a bottle gourd rootstock were tested. All root exudatesstimulated FON germination in comparison to no exudate (Linget al., 2013). Also, the exudates of grafted watermelon weresignificantly less stimulatory to FON germination than exudatesfrom non-grafted watermelon.

Examination of root exudates by HPLC also demonstrated thatchlorogenic acid and caffeic acid, which are both inhibitory to FONgermination, were not present in nongrafted watermelon root ex-udates, but were present where watermelon was grafted ontobottle gourd (Ling et al., 2013). Although other mechanisms play arole in disease reduction, these experiments have implications thatthe deployment of resistant rootstock, in addition to reducingdisease, may decrease levels of FON in soil.

6. Cover crops for disease suppression

Cover crops provide several benefits to soil health such asreducing the need for synthetic chemicals by decreasing weedbiomass, increasing soil organic matter, contributing nutrients tothe soil, retaining soil moisture and decreasing soil erosion(Drinkwater et al., 1995; Cavigelli and Thien, 2003; Steinmaus et al.,2008). Adding organic matter or nutrients to the soil or the plantitself impacts disease development in many ways. Organicamendments, such as cover crops that are incorporated as a greenmanure, can improve crop health and increase soil microbial ac-tivity either directly or indirectly by releasing nutrients to the soil(Drinkwater et al., 1995). Cover crops are regarded as componentsof an integrated pest management system because of their poten-tial to reduce pesticide use and encourage beneficial insects.Several cover crops have been evaluated for their impact on Fusa-rium wilt of watermelon, differentially providing reductions indisease in some environments and not in others.

Many Brassica species have been studied for their ability tosuppress diseases, including Fusariumwilt. Brassica juncea containshigh levels of glucosinulates, even when compared to most other

Brassica species. The glucosinulates in B. juncea induce high levelsof biological activity (mostly antimicrobial) (Smolinska andHorbowicz, 1999). Though brassica cover crops may suppressmany diseases (Smolinska et al., 2003), there also are reports of anincrease in Fusarium disease severity following brassica incorpo-ration, or that incorporation of these crops resulted in little to noeffect. Zhou et al. (2013), evaluated nine brassica spp. for sup-pression of watermelon Fusarium wilt in a one-year trial. Whilesome mustard cultivars (‘Green Wave’ and ‘Florida Broadleaf’)reduced wilt, some (‘Sheali Hong’, ‘Pacific Gold,’ ‘Brand 199’) didnot. No cultivars improved plant vigor. More recently, Himmelsteinet al., 2014 used ‘Pacific Gold’ as a fall-planted winter cover cropand it did not reduce Fusarium wilt in comparison to a rye wintercover crop.

In an in vitro experiment done with isothiocyanates derivedfrom brassicas, Fusarium exhibited only moderate inhibition(Sarwar et al., 1998). Broccoli amendments provided inconsistentFusarium spp. suppression in a study done by Zasada et al. (2003).Mazzola et al. (2001) saw an increase in Fusarium spp. populationsin the soil following brassica seed meal amendments. Njoroge et al.(2008) recorded elevations of F. oxysporum populations in awatermelon field following the incorporation of B. juncea andBrassica napus.

All Brassica crops are nonmycorrhizal (Roberts and Anderson,2001). Using Brassica spp. as a cover crop can cause a decrease inmycorrhizal colonization of succeeding cash crops such as in thecase of tomato roots (Lycopersicon esculentum) planted after a garlicmustard cover crop (Allaria petiolat) (Roberts and Anderson, 2001).It is possible this could contribute to an increase in plant disease,where mycorrhizal associations contribute to pathogen defense(Kaya et al., 2003).

Additional cover crops have also been evaluated for their abilityto suppress Fusarium wilt. Zhou and Everts (2004) conducted astudy on the impact of thirteen different soil amendments onFusarium wilt of watermelon and found that a Vicia villosa (hairyvetch) cover crop decreased Fusarium wilt incidence as much as63% in conjunction with highly resistant cultivars, 53% withmoderately resistant cultivars, and 22% with susceptible water-melon cultivars (Zhou and Everts, 2004). The crop is also a legume,fixes nitrogen and releases nutrients when amended to soil,commonly forms mycorrhizal root associations and may increasesoil mycorrhizial populations (Galvez et al., 1995, Galvez et al.,2001). Crimson clover, Trifolium incarnatum, is also a nitrogenfixing legume that commonly forms mycorrhizal associations,functioning as a mycorrhizal inoculant for succeeding crops (Roviraand Harris, 1961). This widely used cover crop was evaluated overthree field seasons and in three locations in the mid-Atlantic regionof the U.S. Suppression of Fusariumwilt occurred in two of the fourfields where it was tested (Himmelstein et al., 2014). The magni-tude of the suppression was less than that of V. villosa, which hadbeen extensively tested in the region.

Although the mechanism of suppression is likely complex,several studies have indicated that root exudates, including sec-ondary metabolites, play a role in cover crop-induced suppression.Plant secondary metabolites have demonstrated disease suppres-sive properties. Gallic acid and ferulic acid, derived from water-melon root exudates have inhibited FON growth, but these factorsalso caused increased production of pathogen mycotoxins andvirulence factors (hydrolytic enzymes, proteinase, pectinase,amylase and cellulose) (Wu et al., 2008, 2009a, 2010). Essential oilsfrom a bush native to northern Africa (Lippia rehmannii) and lemongrass (Cymbopogon citratus) inhibited in vitro growth of F. oxy-sporum (Linde et al., 2010). The essential oils and various com-pounds extracted from the flowers of Cestrum nocturnum L. andother plants such as pecan (Carya Illinoensis) shells, pomegranate

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(Punica granatum) husks and other organic materials directlyinhibited pathogenicity and growth of organisms, including F.oxysporum and Fusarium solani (Al-Reza et al., 2010; Osorio et al.,2010).

Intercropping of rice (Oryza sativa) with other cash crops is awidespread practice used in China (Hao et al., 2010). Ren et al., 2008demonstrated that intercropping watermelon with aerobicallygrown rice, reduced Fusarium wilt. The authors hypothesized thatthe differences among specific compounds, between exudates ofrice or watermelon roots, could help explain the Fusarium wiltsuppression in the aerobic rice production system. For example, inaddition to quantitative differences in sugars and amino acids inrice vs. watermelon root exudates, p-coumeric acid was detectedonly in rice root extracts. When applied exogenously p-coumericacid reduced spore germination and sporulation as concentrationsincreased in comparison to nontreated spores (Hao et al., 2010).

Further research is needed to identify additional Fusarium wiltsuppressive cover crops and to understand the mechanisms ofsuppression. Successful deployment of suppressive cover crops willrequire knowledge of the mechanism of suppression, including theinfluence of soil biotic and abiotic factors on the ability of a covercrop to suppress Fusarium wilt.

6.1. Biocontrol products for watermelon Fusarium wilt

Biological controls for managing Fusarium wilt diseases areparticularly useful for organic farmers who have few conventionalfungicide options. Several organisms such as Penicillium oxalicum,Paenibacillus polymyxa, Streptomyces lydicus, nonpathogenicF. oxysporum strain ‘CS-20,’ have been evaluated for Fusarium wiltsuppression. De Cal et al. (2009) found that the application of aconidial suspension of the fungus P. oxalicum to seeds and seedlingsof watermelon decreased disease incidence of Fusariumwilt causedby FON in both growth chamber and field experiments.

Wu et al. (2009b)made a bio-organic fertilizer that incorporatedthe organisms Paenibacillus polymyxa and T. harzianum for use onFusarium wilt of watermelon. The fertilizer reduced Fusarium wilt,which they attributed to elevations in the activities of defenserelated enzymes (catalase, superoxide dismutase, peroxidase andb-1,3-glucanase) in watermelon leaves, indicative of systematicacquired resistance response (Wu et al., 2009b). Ling et al. (2010)found that the application of a bioorganic fertilizer product, BIO,reduced Fusarium wilt of watermelon by 59e73% in the field and60e100% for pot experiments. The mode of action of the activeingredient of BIO, Paenibacillus polymyxa SQR-21, was associatedwith alterations in plant root exudation of phenolics, resulting indecreased FON conidial germination (Ling et al., 2011).

Actinovate is a commercial formulation of S. lydicus strainWYEC108, a saprophytic soil bacterium which has been shown to reduceFusarium wilt of banana (Getha et al., 2005). Natural Industries(Houston, TX), the producers of Actinovate, have labeled it formanagement of Fusarium wilt. However, in three trials whereActinovatewas evaluated for efficacy onwatermelon Fusariumwilt,it only nominally reduced wilt by 2 and 7% and increased wilt by 2%for one trial (Himmelstein et al., 2014). However, the soils wherethe trials were conducted had low pH and low organic matter and,therefore, root colonization may not have been achieved.

Despite the evidence that some disease reductions can be ach-ieved with biological products, commercial use remains very low(Fravel et al., 2005; Zhou and Everts, 2006). Improving stability ofthe biological organisms in the environment and understandingthe occasional failure in effective disease management are areas forfuture research.

7. Chemical control

Previous use of chemicals for management of Fusariumwilt hadbeen chiefly restricted to the fumigant, MeBr, and to a lesser extent1, 3-dichloropropene and metam sodium. These fumigants werehighly effective when used as broadcast applications. However,some failures occurred when these fumigants were applied asbanded applications and pathogenic Fusarium reinfested thefumigated strips from inter-row areas that were not fumigated.Following the Montreal Protocol, which mandated the reduction ofMeBr use in many countries, initial research to identify alternativesto MeBr focused on alternative fumigants such as fumigant com-binations, and use of virtually impermeable (VIF) or total imper-meable films (TIF) to reduce dosage, improve pest control efficacy,and limit non-target consequences. Additional restrictions tofumigant use have limited these options both in the U.S. and Europe(Colla et al., 2012). Colla et al., 2012 point out that dependence on asingle management method in Europe will leave growers “vulner-able to changes in regulatory policies.” This is also true in manyother countries throughout the world. More recent efforts havesought nonfumigant alternatives to MeBr use (Antoniou et al.,2014).

Although fumigants have been the primary chemical manage-ment practice for Fusarium wilt of watermelon, there has been along standing interest in fungicides. No conventional soil-appliedfungicides have been available in the U.S. for management ofFON. However, some related formae speciales have been evaluatedfor sensitivity to fungicides. In-vitro evaluations of fungicidesindicated that prochloraz, bromuconazol, benomyl, and carbenda-zim significantly reduced mycelial growth of F. oxysporum f. sp.lycopersici (Amini and Sidovich, 2010). Chung et al. (2009) docu-mented efficacy of benomyl, thiophanate-methyl, carbendazim andthiabendazole on F. o. gladioli and F. o. lillii.

More recently, soil applied fungicides were evaluated onwatermelon in the U.S., first in the greenhouse, and then under fieldconditions. Three fungicides, prothioconazole, acibenzolar-S-methyl, and thiophanate-methyl, resulted in reductions of Fusa-rium wilt of field-grown watermelon (Everts et al., 2014). Thechemicals were initially applied one time as a drench in the field,but wilt reductions were only observed at the seedling stage andthere were no season-long effects. In subsequent trials thesechemicals were applied through the drip irrigation line alone and incombination, 0, 2 and 4 weeks after planting, and significantlyreduced wilt. The soil-applied fungicides prothioconazole andthiophanate-methyl may serve as an additional field managementoption for Fusariumwilt of watermelon. In 2013, Bayer CropScienceLP, Research Triangle Park, NC received a Supplemental Label forthe application of Proline (prothioconazole) onwatermelon via dripirrigation for specific states within the U.S. The label restricts Pro-line to one drip irrigation system application, but allows for addi-tional foliar applications. Further research is needed to determine ifthese label restrictions will allow for season-long efficacy.

8. Conclusions

As watermelon Fusarium wilt has increased in prevalence andseverity over the past decade public research has concentrated onseveral specific areas on interest. These include grafting and iden-tification of resistant rootstock; evaluation of cover crops andimproved understanding of the mechanisms of Fusarium wilt dis-ease suppression; and the development of biocontrol systems.Although substantial progress has occurred, notably in our under-standing of the impact of root exudates on FON in the rhizosphereand in grower adoption of grafting technology, watermelongrowers continue to suffer economic losses from Fusarium wilt.

K.L. Everts, J.C. Himmelstein / Crop Protection 73 (2015) 93e9998

Therefore continued research in these areas will improve ourknowledge and our ability to successfully manage Fusariumwilt. Inaddition, progress in rapid diagnosis of FON and FON races wouldgreatly enhance our ability to apply Fusarium wilt management atthe farm level.

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