Reduced severity and impact of Fusarium wilt on strawberry by manipulation of soil pH, soil organic...

Reduced severity and impact of Fusarium wilt on strawberryby manipulation of soil pH, soil organic amendmentsand crop rotation

Xiangling Fang & Ming Pei You & Martin John Barbetti

Accepted: 13 July 2012 /Published online: 1 August 2012# KNPV 2012

Abstract Strawberry (Fragaria×ananassa) is one ofthe most important berry crops worldwide. Fusariumwilt poses a serious threat to commercial strawberryproduction worldwide and causes severe economiclosses. Our previous surveys suggested that soil pH,soil amendment with organic matter and/or crop rota-tion could offer opportunities for improved manage-ment of strawberry disease. Studies were conductedfor the first time to determine the effects of soil pH,soil amendments with manure compost and crop res-idue, and crop rotation on the severity and impact ofFusarium wilt on strawberry. At soil pH 6.7, plantsshowed the least severe disease and the lowest reduc-tions in shoot and root dry weight (DW) of plants fromdisease, significantly lower than those of plants inacidic soil at pH 5.2 or 5.8. In soil amendment withmanure compost at 5.0 %, plants showed the leastsevere disease and the lowest reductions in shoot androot DW of plants from disease, significantly lower

than those of plants in the other three levels of manurecompost. In soil amendment with crop residue at2.5 % or 5.0 %, shoot and root disease of plants andreductions in shoot and root DW of plants from dis-ease were significantly lower than those of plants insoil without crop residue or excessive crop residueamendment at 10.0 %. Plants in soil rotated withtomato not only showed the least severe disease butalso showed the lowest reductions in shoot and rootDW of plants from disease, significantly lower thanthose of plants in soil continuously planted with straw-berry without rotation or rotated with capsicum. SoilpH, soil amendment with manure compost or cropresidue, and crop rotation, all significantly reducedthe severity and impact of Fusarium wilt on strawber-ry. There is great potential for manipulating soilpH, adding soil organic amendments and utilizingcrop rotation, not only to successfully manageFusarium wilt on strawberry, but to do so in asustainable way without current reliance uponchemical fumigants.

Keywords Crop residue . Fragaria × ananassa .

Management . Manure compost . Soil-borne disease

Introduction

Strawberry (Fragaria × ananassa) is one of the mosteconomically important berry crops in the world, with

Eur J Plant Pathol (2012) 134:619–629DOI 10.1007/s10658-012-0042-1

X. Fang :M. P. You :M. J. Barbetti (*)School of Plant Biology, Faculty of Natural AgriculturalSciences, The University of Western Australia,35 Stirling Highway,Crawley, WA 6009, Australiae-mail: [email protected]

M. J. BarbettiThe UWA Institute of Agriculture, Faculty of NaturalAgricultural Sciences, The University of Western Australia,35 Stirling Highway,Crawley, WA 6009, Australia

production of approximately 4.4 million metric tons in2010 (Bombarely et al. 2010; Faostat 2008). Strawberryis also a high-value export crop grown in Western Aus-tralia, constituting up to 72 % of Australia’s strawberryfruit exports (Fang et al. 2011a, b).Fusarium oxysporumis a soil-borne fungal species that occurs in diverse soiltypes across the world (Fravel et al. 2003). F. oxysporumis saprophytic/necrotrophic and grows and survives forlong periods on organic matter in soil (Fravel et al. 2003;Garrett 1970). Wilt-inducing isolates of F. oxyspo-rum are responsible for severe damage and yieldlosses on many economically important plant spe-cies and have been divided into more than 120different formae speciales based on specificity tohost species across a wide range of plant families(Fravel et al. 2003; Michielse and Rep 2009).

F. oxysporum f. sp. fragariae is a strawberry spe-cific pathogen. Fusarium wilt on strawberry, caused byF. oxysporum f. sp. fragariae, is a serious threat tocommercial strawberry production worldwide. Fusa-rium wilt on strawberry was first reported in Australiain 1965 (Winks and Williams 1965), and is a seriousdisease on strawberry in Korea (Kim et al. 1982),China (Zhao et al. 2009), Spain (Arroyo et al. 2009),the USA (Koike et al. 2009) and Australia (Fang et al.2011a, b). F. oxysporum f. sp. fragariae penetratesstrawberry plants through the roots, severely affectingthe growth and development of roots and crowns,resulting in the rapid wilting and eventual death ofplants (Fang et al. 2011b; Fang et al. 2012a, b; Koikeet al. 2009). F. oxysporum f. sp. fragariae can survivemultiple years by the formation of chlamydosporesthat persist even in the absence of a host, and asstrawberry is usually cultivated in tight rotations oreven without rotation worldwide, this creates condu-cive conditions for this pathogen to survive andmultiply.

Management worldwide of soil-borne diseases likeFusarium wilt on plants remains reliant on chemicalsoil fumigation (Fravel et al. 2003; Martin and Bull2002). However, some broad-spectrum pre-plantingfumigants, such as methyl bromide, have negativeenvironmental effects and pose risks to human healthand consequently have been phased-out in manycountries (Fravel et al. 2003; Subbarao et al. 2007).The phase-out of these previously effective fumigantshas not only forced the development of alternativechemical fumigants, but also fostered keen interest indeveloping alternative non-chemical means for

management of soil-borne diseases (Fang et al.2011a; Fang et al. 2012a, b; Subbarao et al.2007). Identifying and deploying resistant cultivarsis considered long term to be the most cost-effective and environmentally sustainable strategyto control Fusarium wilt (Fravel et al. 2003; Fanget al. 2012a, b). In Australia, strawberry cv.Camarosa is highly susceptible to F. oxysporum f.sp. fragariae while cv. Festival is highly resistant(Fang et al. 2012a, b). However, cv. Festival can-not compete with cv. Camarosa in terms of fruityield and taste, and cv. Camarosa remains themost widely grown cultivar despite its susceptibil-ity to Fusarium wilt. Several non-chemical culturalpractices, other than host resistance, have beenutilised, including the manipulation of soil pH, soilamendments with organic matter such as manurecomposts or crop residues, and crop rotation, withsuch practices often significantly reducing soil-borne diseases (Bareja et al. 2010; Bernard et al.2012; Ghorbani et al. 2008; Larkin et al. 2010;Larkin et al. 2011; Litterick et al. 2004; Millner etal. 2004; Subbarao et al. 2007; Watanabe et al.2011).

Our previous field surveys demonstrated a negativecorrelation between soil pH and disease severity instrawberry fields in Western Australia, suggesting thatincreasing soil pH may offer an opportunity for im-proved management of strawberry disease (Fang et al.2011a). Further, we observed that strawberry fieldsamended with organic matter or associated with rota-tion with other crop species between strawberry cropsshowed a degree of disease control and that this waspossible without soil chemical fumigation (Fang et al.2011a). Although several studies elsewhere have eval-uated the crop rotation effects on soil-borne diseaseson strawberry, these have focused on other pathogenssuch as Verticillium dahliae and Pythium (Subbarao etal. 2007). There have been no previous studies onmanagement of Fusarium wilt on strawberry inrelation to manipulation of soil pH, soil amend-ment with organic matter or crop rotation. Thispaper reports the first study to determine theeffects of soil pH, soil amendment with manurecompost or crop residue, and crop rotation on theseverity and impact of Fusarium wilt on strawberryso as to explore strategies to manage Fusariumwilt on strawberry, in a sustainable way withoutreliance upon chemical fumigants.

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Materials and methods

Strawberry plants

Strawberry plants (cv. Camarosa) were purchased ascertified commercial runners (Toolangi Certified Straw-berry Runner Grower’s Co-Op Ltd, Victoria, Australia),and maintained under controlled environmentalconditions at the constant temperature of 22 °C with a12-h photoperiod of 280 μE m−2 s−1. This cultivar is themost widely grown in Western Australia, and is highlysusceptible to F. oxysporum f. sp. fragariae (Fang et al.2011a, b; Fang et al. 2012a, b). Plants were removedfrom pots and washed thoroughly under running tapwater before use.

Fungal isolate

A single-spore isolate of F. oxysporum f. sp. fragariaeWUF-ST-FO51, recovered from severely affected straw-berry plants collected in 2008 from a commercial straw-berry field in Wanneroo (Western Australia, Australia;latitude/longitude: −31.8/115.8), was used. This isolatewas previously reported to be highly virulent to straw-berry plants (Fang et al. 2011b; Fang et al. 2012a, b).This isolate was stored both as lyophilized cultures inglass ampoules for long term preservation, and also asmycelial-colonised pieces of filter paper then dried atroom temperature and stored at −20 °C. When requiredfor inoculum preparation, the isolate was sub-culturedonto fresh potato dextrose agar (PDA) plates.

Inoculum preparation

Millet seed-based inoculum colonised by F. oxyspo-rum f. sp. fragariae was prepared using a modifiedprocedure of Fang et al. (2011b). Briefly, 200 g milletseed (Panicum miliaceum) was soaked in 200 mldeionized water (DI) water in a 1 litre flask for 12 h,excess water drained and subsequently autoclaved at121 °C for 20 min on three consecutive days. Six 3-mm-diam disks from margins of one-week-old colo-nies of WUF-ST-FO51 growing on PDA plates wereadded to each flask containing the sterilized milletseeds. Flasks were shaken every 2 days to ensure theuniform colonisation and incubated at constant tem-perature of 22 °C for 2 weeks. In all studies, uncolon-ized millet seed was not used as a control comparisonas it has a ‘baiting-out’ effect on any other potential

pathogens present in the soil, especially Pythium spe-cies, when introduced uncolonised into soil (Barbettiand Sivasithamparam 1987).

Effect of soil pH, soil amendment with manurecompost and crop residue on disease severity

Soil used in all experiments of this study was non-fumigated sandy soil (98.5% sand, 1.5% clay) collectedfrom the top 20 cm of a commercial strawberry field inWanneroo (Western Australia; latitude/longitude:−31.76/115.83). The collected bulk soil from this fieldwas air-dried for 2 weeks in a glasshouse, and thenpassed through a 2-mm mesh sieve to remove debrisin the soil and thoroughly mixed before using. The soilhad what was considered a low pH of 5.2 as measured ina 1:5 (w/v) suspension of soil in 0.01 M CaCl2, and alow organic matter content of 0.9 %. For the experi-ments on the effect of soil pH on disease severity, soilpH was adjusted by adding in finely-ground limestone(CaCO3) and mixing thoroughly to produce the follow-ing treatments: 1, no added lime (pH 5.2); 2, 0.3 % limeadded (pH 5.8); 3, 0.6 % lime added (pH 6.7); and, 4,1.8 % lime added (pH 7.5). Each of these soils waspasteurized by aerated steam treatment for 90 min at65 °C before use. For the experiments on the effect ofsoil amendment with manure compost on disease sever-ity, commercially available pelletized chicken manurecompost (Rooster Booster®, Bunnings, Australia), thathad been composted and pasteurized to ensure freedomfrom pathogens and weeds, was finely ground using anoster blender (AS ONE®, Osaka, Japan). Compost wasthen thoroughly mixed with the pasteurized soil at therate of 0, 2.5 %, 5.0 % and 10.0 % (v/v). For theexperiments on the effect of soil amendment with cropresidue on disease severity, commercially available cropresidue, pea straw (Johnson®, Bunnings, Australia), waschopped and finely ground using an oster blender (ASONE®, Osaka, Japan), and then passed through a 2-mmmesh sieve. Crop residue was then thoroughly mixedwith the pasteurized soil at the rate of 0, 2.5 %, 5.0 %and 10.0 % (v/v).

Four soil pH levels (viz. 5.2, 5.8, 6.7 and 7.5), fourcompost levels [viz. 0, 2.5 %, 5.0 % and 10.0 % (v/v)],and four crop residue levels [viz. 0, 2.5 %, 5.0 % and10.0 % (v/v)] were tested to determine their effect ondisease severity on strawberry. At each soil treatmentlevel of each experiment, strawberry plants wereplanted into the plastic pots (9 cm×9 cm×18 cm)

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containing soil mixed with millet seed-based inoculumof F. oxysporum f. sp. fragariae at a rate of 0.5 % (w/w),and there was one plant per pot. Control plants forcomparison were planted in pots containing uninfestedsoil for each soil treatment level, and without anyuncolonised millet seeds added as uncolonised milletcan ‘bait-out’ other non-target soil-borne pathogenspresent (Barbetti and Sivasithamparam 1987). Therewere eight replicate pots for each soil treatment levelof each experiment. All pots were kept in controlledenvironmental rooms maintained at the constanttemperature of 22 °C with a 12-h photoperiod of 280μE m−2 s−1. The experiment was arranged in a random-ized block design. Plants were watered to free drainingwith DI water daily and were harvested for assessment8 weeks later. Each experiment was repeated once moreunder the same conditions.

Effect of crop rotation on disease severity

Three crop rotation regimes were tested using the samepasteurized soil as above. The crop cycle sequenceswere (i), strawberry-strawberry-strawberry (non-rota-tion control); (ii), strawberry-capsicum-strawberry, and(iii) strawberry-tomato-strawberry. For the first cycle ofplanting, strawberry plants were planted in the plasticpots (18 cm×18 cm) containing the soil mixed withmillet seed-based inoculum of F. oxysporum f. sp. fra-gariae at a rate of 0.5 % (w/w), and there were twoplants per pot. Control plants for comparison withoutthe pathogen were planted in the pots containing thesame soil and without any uncolonised millet seedsadded. In the first cycle, there were 24 replicate potsfor each of the infested and uninfested treatments. Allpots were kept in controlled environmental rooms main-tained at the constant temperature of 22 °C with a 12-h photoperiod of 280 μE m−2 s−1. This experiment wasarranged in a randomized block design. Plants werewatered to free draining with DI water daily.

Plants were removed from pots two months later bywhich time all plants in the pots containing infested soilhad wilted and/or died. Soils from each pot within eachtreatment were pooled, dried and then thoroughlymixed.For the second cycle of planting, seeds of capsicum (cv.CalifornianWonder) and tomato (cv. Grosse Lisse) weresurface sterilized in 1.25 % sodium hypochlorite for3 min, and then sown in trays containing pasteurizedUniversity of Western Australia potting mix [finelycrushed pine bark: coco peat : sand at 2.5 : 1.0 : 1.5

(w/w)]. Two-week-old capsicum and tomato seedlings,used as rotation plants after the first strawberry crop,were transplanted into pots containing the correspondingrecycled soil from the first cycle of planting. In the non-rotation treatments, pots were replanted to strawberry asper the first strawberry crop. There were two plants perpot and there were eight replicate pots for each treatmentof each plant type. All pots were kept in controlledenvironmental rooms with the same conditions as above.This experiment was arranged in a randomized blockdesign. Plants in each pot were harvested 4 months later.After harvesting, soils from each pot within each treat-ment were pooled, air-dried and then thoroughly mixed.For the last cycle of planting, strawberry plants werepotted into the recycled soils from the second cycle ofplanting. Plants were harvested 2 months later whenstrawberry plants in pots containing the infested soilwere wilted and/or dead. This experiment was repeatedonce more under the same conditions.

Plant assessment

For all experiments, at harvest, the severity of Fusariumwilt on strawberry plants was recorded by two criteria,viz. shoot and root disease of each plant, and the impactof Fusarium wilt on strawberry plants assessed by twoadditional criteria, viz. shoot dry weight (DW) and rootDW of each plant. Whole plants were removed frompots and thoroughly washed under running tap water toremove all attached soil and then plants floated in shal-low trays with DI water to remove any remaining par-ticles. Shoot disease was assessed on a 0–5 severityrating scale, where: 00 plant well developed, no diseasesymptoms; 10 plant slightly stunted; 20 plant stuntedand yellowing; 30 plant severely stunted and/or wilting;40 majority of leaves of the plant wilted or dead; 50plant dead (Fang et al. 2011b). Root disease wasassessed on a 0–5 disease severity scale, where: 00root well developed, no discolouration; 10<25 %root discoloured; 20≥25 %, <50 % root discoloured;30≥50 %, <75 % root discoloured; 40≥75 % rootdiscoloured; 50 all root discoloured (rotted), plantdead (Fang et al. 2011b). All root discolouration inthis assessment refers to combined outside and insidediscolouration of the roots. After rating shoot androot disease symptoms, shoots and roots of each plantwere separated by cutting, and then placed in separatepaper bags and dried in an oven at 69 °C for 1 weekbefore weighing.


In all experiments, re-isolations of the pathogenwere made from segments of freshly harvested dis-eased root tissues from all treatments. These roottissues were superficially disinfested with 1.25 % so-dium hypochlorite for 1 min, and then rinsed threetimes in sterile distilled water before placed onto PDAplates to confirm that disease symptoms were in factcaused by F. oxysporum f. sp. fragariae.

Data analyses

Data analyses were conducted using GenStat (14th edi-tion, VSN International Ltd., UK, 2012). For severityratings on shoot disease and root disease of strawberryplants at each treatment of each experiment, only datafrom infested soil were included in the analyses. Datafrom the control treatment (uninfested soil) were notincluded in the analyses because all plants from thecontrol treatment of all the experiments showed no shootdisease (disease ratings 00), and only few plants from thecontrol treatment of the experiments relating to cropresidue and crop rotation showed root discolourationbut at extremely low levels (disease ratings <0.38). Per-centage reductions in shoot DWand root DWof plants ateach treatment were calculated by comparing plants ininfested soil with those control plants in uninfested soil.Normality of data and homogeneity of variances fromeach experiment were tested before analysis. The ordinaldata on shoot disease and root disease of each experi-ment did not have a normal distribution and similarvariation; therefore, the effect of different levels of soil

pH, manure compost, crop residue and crop rotation onshoot disease and root disease of plants were determinedutilizing the non-parametric Kruskal-Wallis analysis andMann–Whitney comparisons (P00.05). The data onreductions in shoot DW and root DW of plants of eachexperiment had a normal distribution and similar varia-tion; therefore, the effect of different levels of soil pH,manure compost, crop residue and crop rotation onreductions in shoot DW and root DW of plants weredetermined by analyses of variance, and subsequentmultiple comparisons between treatments were madeusing Fisher’s protected least significant differences(P00.05). Standard errors (SE) of means were alsocomputed. For all analyses, data from the two repeatexperiments (i.e., the original and one repeat experiment)of each study were not significantly different (P>0.05);therefore, data from the two repeat experiments of eachstudy were combined and analyzed together.

Results

Effect of soil pH

There was a significant effect (P<0.001) of soil pH onshoot and root disease, and on reductions in shoot androot DW of strawberry plants (Table 1). With increas-ing soil pH, there was a trend for shoot and rootdisease to decrease and a trend for reductions in shootand root DW to decrease. Reduction in shoot and rootDW of plants was related to the severity of shoot and

Table 1 Effect of different soil pH levels on the severity and impact of Fusarium wilt on strawberry plants (cv. Camarosa)

Soil pH Shoot disease severity Root disease severity Reduction in DW (%)c

Ratinga Mean rank Ratingb Mean rank Shoot Root

5.2 4.75a 56.25 4.12a 54.50 71.9±2.6a 84.5±1.8a

5.8 3.00b 36.50 2.62b 32.12 55.6±2.9b 60.4±4.7b

6.7 1.75c 14.00 1.50c 18.00 20.7±4.5d 27.5±2.2c

7.5 2.25c 23.25 2.12bc 25.38 39.7±3.4c 36.2±2.7c

Chi-square probability Chi-square probability F-probability F-probability

Significance P<0.001 P<0.001 P<0.001 P<0.001

a 0–5 rating scale where 00 plant well developed, no disease symptoms and 50 plant dead. Different letters in the same column indicatesignificant differences according to Mann–Whitney comparison test (P00.05)b 0–5 rating scale where 00 root well developed, no discolouration and 50 all root discoloured (rotted), plant dead. Different letters inthe same column indicate significant differences according to Mann–Whitney comparison test (P00.05)c Reduction in dry weight. Data are presented as means±SE (n016). Different letters in the same column indicate significant differencesaccording to Fisher’s protected least significant difference test (P00.05)


root disease of plants, respectively. With increasingsoil pH, the trend for shoot disease severity was sim-ilar to that for root disease severity, and the trend forreduction in shoot DW was similar to that for rootDW. At soil pH 5.2, plants showed the most severedisease with the highest shoot disease rating of 4.75and root disease rating of 4.12, and also showed thehighest reduction in shoot DW (71.9 %)) and root DW(84.5 %). At soil pH 6.7, plants showed the leastsevere disease with the lowest shoot disease rating of1.75 and root disease rating of 1.50, and also showed thelowest reduction in shoot DW (20.7 %) and root DW(27.5 %). However, disease severity for both shoots androots of plants showed a tendency to increase when soilpH was as high as 7.5. Control plants in uninfested soilwere healthy without wilt symptoms.

Effect of soil amendment with manure compost

There was a significant effect (P<0.001) of manurecompost on shoot and root disease, and on reductionsin shoot and root DW of strawberry plants (Table 2).With increasing levels of manure compost added to thesoil, there was a trend for shoot and root disease todecrease and a trend for reductions in shoot and rootDW to decrease. Reduction in shoot and root DW ofplants was related to the severity of shoot and rootdisease of plants, respectively. With increasing levelsof manure compost, the trend for shoot disease sever-ity was similar to that for root disease severity, and thetrend for reduction in shoot DW was similar to that for

root DW. In soil without manure compost, plantsshowed the most severe disease with the highest shootdisease rating of 4.88 and root disease rating of 4.25,and also showed the highest reduction in shoot DW(74.2 %) and root DW (85.5 %). In soil amended withmanure compost at 5.0 %, plants showed the leastsevere disease with the lowest shoot disease rating of2.00 and root disease rating of 1.88, and also showedthe lowest reduction in shoot DW (30.1 %) and rootDW (23.5 %). Control plants in uninfested soil werehealthy without wilt symptoms.

Effect of soil amendment with crop residue

There was a significant effect (P<0.001) of crop residueon shoot and root disease, and on reductions in shootand root DW of strawberry plants (Table 3). Reductionin shoot and root DW of plants was related to theseverity of shoot and root disease, respectively, acrosssoils with different added levels of crop residue. In soilwithout added crop residue or with the highest addedcrop residue of 10.0 %, plants showed the most severeshoot and root disease, and plants showed highest reduc-tions in shoot and root DW in soil without added cropresidue. In soil with added crop residue at 2.5 % or5.0 %, plants showed the least severe disease, and therewas no significant difference in terms of shoot or rootdisease. Reduction in shoot DW of plants at 5.0 % wassignificantly lower than that of plants in soil withoutadded crop residue or with the highest added cropresidue of 10.0 %. Reduction in root DW of plants at


Table 2 Effect of different manure compost levels on the severity and impact of Fusarium wilt on strawberry plants (cv. Camarosa)

Compost (v/v) Shoot disease severity Root disease severity Reduction in DW (%)c


0.0 % 4.88a 56.00 4.25a 54.25 74.2±1.3a 85.5±1.2a

2.5 % 3.38b 35.50 3.00b 33.75 57.9±2.6b 64.0±1.6b

5.0 % 2.00c 11.75 1.88c 16.25 30.1±4.4d 23.5±2.1d

10.0 % 2.88b 26.75 2.50bc 25.75 45.6±3.9c 49.7±2.9c




2.5 % or 5.0 % were significantly lower than that ofplants in soil without added crop residue or with thehighest added crop residue of 10.0 %. For control plantsin uninfested soil, shoots were healthy without wiltsymptoms, but roots showed slight brown discoloura-tion in soil with added crop residue.

Effect of crop rotation

There was a significant effect (P<0.001) of crop rota-tion on shoot and root disease, and on reductions inshoot and root DW of strawberry plants (Table 4).Plants in soil rotated with tomato not only showed

the least severe disease, with the lowest shoot diseaserating of 3.12 and root disease rating of 2.50, but alsoshowed the lowest reductions in shoot DW (50.6 %)and root DW (45.6 %), which were significantly lowerthan those of plants in soil continuously planted withstrawberry (i.e., without rotation) or rotated with cap-sicum. Plants in soil rotated with capsicum onlyshowed a significant difference in shoot disease incomparison with plants in soil without rotation. Forcontrol plants in uninfested soil, while shoots werehealthy without wilt symptoms, roots of strawberryfrom soil rotated with tomato or capsicum did showsome slight brown discoloration.

Table 3 Effect of different crop residue levels on the severity and impact of Fusarium wilt on strawberry plants (cv. Camarosa)

Crop residue (v/v) Shoot disease severity Root disease severity Reduction in DW (%)c


0.0 % 4.75a 50.50 4.38a 49.75 74.5±2.3a 83.0±2.3a

2.5 % 3.00b 20.75 1.99b 14.88 57.5±1.5bc 48.6±3.2c

5.0 % 2.75b 16.75 2.37b 21.12 53.6±2.7c 51.9±2.1c

10.0 % 4.25a 42.00 3.75a 44.25 63.9±2.4b 69.8±3.9b




Table 4 Effect of different crop rotations (SSS, strawberry-strawberry-strawberry; SCS, strawberry-capsicum-strawberry; STS,strawberry-tomato-strawberry) on the severity and impact of Fusarium wilt on strawberry plants (cv. Camarosa)

Crop rotation Shoot disease severity Root disease severity Reduction in DW (%)c


SSS 5.00a 37.50 4.38a 35.50 80.3±1.9a 72.6±1.3a

SCS 4.38b 25.62 3.50a 28.50 73.9±2.6a 68.2±2.0a

STS 3.12c 10.38 2.25b 9.50 50.6±2.1b 45.6±1.6b





Discussion

Soil pH strongly influenced the severity and impact ofFusarium wilt on strawberry plants in this study. Dis-ease was most severe in the very acidic soil with pH at5.2 and least severe in soil close to neutral with pH at6.7 where there was also least impact on the growth ofstrawberry plants. This is consistent with what weobserved in a survey of commercial strawberry fieldswhere high levels of disease occurred at field siteswhere soil pH ranged from 4.7 to 5.4 while there werelow levels of disease at field sites where soil pHranged from 6.3 to 6.7 (Fang et al. 2011a). Increasingsoil pH has been suggested as a strategy for diseasemanagement as there is a direct correlation betweenhigher soil pH with lower levels of Fusarium wilt onother crops such as tomato, cotton, melons and banana(Dominguez et al. 1996; Ghorbani et al. 2008; Jones etal. 1989). It is suggested that soil pH influences plantdisease infection and development directly by effectson soil-borne pathogens (Ghorbani et al. 2008). How-ever, soil pH is also important in relation to soilfertility and nutrient availability, and can weaken theplant host through altered nutrition induced by the soilacidity, affecting the susceptibility to disease. Thegrowth of strawberry plants was not affected in acidicsoils in our study and confirms a previous report thatan optimum soil pH for strawberry production liesbetween pH 4.6 and 6.5 (Niskanen and Dris 2002).Therefore, liming of the acidic soils in strawberryfields to increase soil pH offers significant opportunityfor improved management of Fusarium wilt on com-mercial strawberry production.

In this study, soil amendment with chicken manurecompost greatly reduced the severity and the impact ofFusarium wilt on strawberry. Several previous reportshave shown that the application of manure compostscan reduce Fusarium wilt on other plant hosts. Forexample, the application of chicken, cow and sheepmanure composts in soil reduced Fusarium wilt ontomato (Alvarez et al. 1995; Szczeck 1999), and theapplication of cow manure compost in soil reducedFusarium wilt on cucumber (Kannangara et al. 2000).Similarly, it has been reported that soil amendmentswith chicken manure compost reduced Verticilliumwilt on potato (Larkin et al. 2011), and that poultry/cow and/or dairy manure compost reduced red steledisease of strawberry caused by Phytophthora fragar-iae (Millner et al. 2004). Such disease reductions from

the application of compost likely not only had a resultof direct suppression on the pathogen, but also anindirect effect through improved soil health. The ap-plication of chicken manure compost also offers sig-nificant opportunity for improved management ofFusarium wilt on commercial strawberry production.

This study also demonstrated that soil amendmentwith crop residue reduced the severity and impact ofFusarium wilt on strawberry. It has been previouslyreported that residues of various crop plants, such asrapeseed, broccoli, corn, oat and buckwheat, can re-duce Verticillium wilt (Davis et al. 2010; Larkin et al.2011; Subbarao et al. 1999), and that incorporation ofgrain rye crop residue in soil reduced the severity ofroot rot on field bean (Abawi and Widmer 2000).However, the impact of crop residues on reducingdisease severity of plants can be variable dependingon the type of crop residue (Conklin et al. 2002). Forexample, soil amendment with citrus pulp did notsignificantly reduce severity of disease caused byPhytophthora capsici on bell pepper (Kim et al.1997), and soil amendment with a hairy vetch cropresidue increased the level of root rot on beans (Abawiand Widmer 2000). Clearly, there is much scope tofurther explore the potential of crop residues for re-ducing Fusarium wilt on commercial strawberryproduction.

This study demonstrated that soil amendment witheither compost or crop residue can reduce the diseaseseverity and impact of Fusarium wilt on strawberryplants. While soil organic amendments such as com-posts and crop residue have been applied to agricultureworldwide for centuries, their deliberate applicationfor the purpose of controlling plant diseases is a morerecent phenomenon (Litterick et al. 2004; Vanbruggen1995). The role of soil organic amendments in diseasesuppression may be associated with reduced aggres-siveness and infection of pathogens (Ghorbani et al.2008), and also with improved plant vigour as a resultof physical and/or chemical improvement of the soilalong with increased host resistance (Lampkin 1999).Frequently, incorporation of organic matter into soilcan provide a more beneficial alternative than chem-icals for plant disease control (Ghorbani et al. 2008).Therefore, there is much scope for further evaluatingthe potential use of organic matter for improving man-agement of Fusarium wilt on strawberry. Although soilamendment with either compost or crop residue canreduce the disease severity and impact of Fusarium


wilt on strawberry plants, these effects can be variabledepending on their levels added in soil. Our studyshowed that while soil amendment with compost at5.0 % or crop residue at 2.5 % or 5.0 % significantlyreduced Fusarium wilt on strawberry plants, amend-ment with compost or crop residue at a high rate of10 % did not significantly reduce Fusarium wilt.While excessive soil amendment with organic mattercan increase the severity of Fusarium wilt, compostsor crop residues can also contribute towards better soilhealth and fertility by way of increased nutrients thatmay influence final disease severity. Further studiesare needed to determine the effects from the contribu-tion of composts or crop residue towards soil fertilitycould also have on consequent disease, as suggested byGhorbani et al. (2008). Such an approach is also sup-ported by reports that severity of Fusarium wilt oncyclamen can actually increase when excessive fertilityis introduced into soil from compost (Hoitink et al.1997; Ceuster and Hoitink 1999).

It is noteworthy in this study that rotations ofstrawberry plants with tomato in particular, or withcapsicum, but to a lesser extent, reduced the severityand impact of Fusarium wilt on strawberry plants.Tomato and capsicum are the most common cropsused for rotation with strawberry in Australia. Rota-tion with tomato not only reduced the disease severitymost, it also brought the least impact of Fusarium wilton strawberry plants. Rotating crops is a well-established cultural practice utilised to manage soil-borne diseases in agriculture and has been applied tostrawberry, including rotation of strawberry with graincrops in New England (Elmer and LaMondia 1999);rotation of strawberry with oats or sorgho-sudangrassin the USA (LaMondia et al. 2002); and, rotation ofstrawberry with broccoli and brussel sprouts to man-age Verticillium wilt on strawberry production inCalifornia (Subbarao et al. 2007). Further, it was alsoevident in the earlier field surveys (Fang et al. 2011a)that in strawberry fields involving rotations with othercrop species a degree of disease control had occurredeven without fumigation. The effects of rotation cropson reducing Fusarium wilt are likely a consequence ofreducing build-up of soil-borne F. oxysporum f. sp. fra-gariae and as has been shown for other examples in-volvingF. oxysporum (Cook 2000; Elmer and LaMondia1999; LaMondia et al. 2002). Further, crop rotations, ingeneral, can also provide benefits to crop productionconservation, maintenance, and/or replenishment of soil

resources (Larkin et al. 2010). However, such effectsvary in relation to different plants. We found that inrotation with capsicum, strawberry plants still showedhigh levels of disease, which was also observed in ourearlier field surveys where strawberry plants still showedhigh levels of disease in some fields even when inrotation with other vegetable crops (Fang et al. 2011a;Fang et al. unpublished). This is likely a consequencefrom different rotational crops differentially increasing,decreasing and/or maintaining the population density ofF. oxysporum f. sp. fragariae. It is clear that rotation withtomato offers significant potential for better managingFusarium wilt on commercial strawberry production.There is even greater potential if the benefits from rota-tion are combined with benefits we have highlightedfrom increasing low soil pH and from soil organicamendments.

This study has demonstrated that the manipulationof soil pH, soil amendment with manure compost orwith crop residue, and crop rotation can greatly reducethe severity and impact of Fusarium wilt on strawber-ry. These findings highlight the potential for furtherexploiting soil pH, soil amendments with organic mat-ters, and crop rotation to successfully manage Fusa-rium wilt on strawberry. Such treatments have thepotential not only to control Fusarium wilt, but offera more sustainable means of management of this dis-ease in commercial strawberry production; in particu-lar, decreasing the current almost total reliance uponsoil chemical fumigants. Further, there is additionalscope to further explore and exploit the complimenta-ry roles of soil pH, organic matters, and crop rotationand their possible synergistic effects for managementof different Fusarium wilt disorders and other soil-borne diseases. In particular, the specific mechanismsby which soil pH, organic matter, and crop rotationreduce Fusarium wilt of strawberry may open the wayto develop even better non-chemical and non-host resis-tance means of managing Fusarium wilt disorders.

Acknowledgments We appreciate the funding support for thisresearch provided by the Australia Research Council and theDepartment of Agriculture and Food Western Australia. Thefirst author gratefully acknowledges the financial assistance ofthe China Scholarship Council and the University of WesternAustralia by a jointly-awarded PhD Scholarship. We also appre-ciate the assistance of strawberry growers in Western Australiaallowing us collect plant samples and soil from their fields. Wegratefully acknowledge the provision of half the salary ofMartin Barbetti by the Department of Agriculture and FoodWestern Australia.


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