TECHNICAL BULLETIN 121 ISSN 0070-2315

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. 21 DEC \990

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EVALUATION OF SOIL SOLARIZATION FOR CONTROL

OF FUSARIUM WILT OF WATERMELON

N. Ioannou and C. A. Poullis

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AGRICULTURAL RESEARCH INSTITUTE MINISTRY OF AGRICULTURE AND NATURAL RESOURCES

NICOSIA CYPRUS

OcrOBER 1990

EVALUATION OF SOIL SOLARIZATION FOR CONTROL

OF FUSARIUM WILT OF WATERMELON

N. Ioannou and C.A. Poullis

SUMMARY

Two field experiments were conducted to evaluate the effectiveness of soil solarization against Fusarium wilt of watermelon, caused by F. oxysporum f.sp. niveum. The first experic ment was carned out in 1984-85 on a coastal loamy sand soil at Paralimni and the second in 1985-86, on a terra rossa clay soil at Liopetri. Both sites were naturally infested with the Fu­sarium wilt fungus, with soil inoculum density averaging ca 2,000 and 11,000 propagules per gram of soil at Paralimni and Lio petri , respectively. In the first experiment, solarization raised soil temperature (at 15 cm depth) by about 10°C and in the second by about 7°C. At the end of the solarization period (40 days), soil inoculum density was reduced by about 90% in both trials. A similar reduction of inoculum density was also obtained by soil fumigation with methyl bromide, which was tested in the Liopetri trial only. With regard to wilt control on watermelon, solarization was very effective at Paralimni, whereas at Liopetri neither so­larization nor fumigation gave satisfactory results, apparently due to the very high inoculum density pre-existing in the soil.

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To Cl]flooi.£1Jfla auto avacp£Q£taL Otl] cuvatotl]ta XataTCOA£fll]Ol]<; tOU <pousaQi.ou tl]<; xaQTCousLo.<; (Fusarium oxysporum f.sp. niveum) fl£ tll fl£eOCO tll<; l]ALOe£QflavOll<; (llALOaTCo­AUflavOll<;) toU £C>Ucpou<;. fw to OXOTCO auto £YLvav Mo uTCai.eQW TC£LQo.flata, to rrQwto O£ aflflOTCl]AWC£<; TCaQaALaxo £cacpo<; OtO ITaQaAi.flvL xatU tllv TCtgi.oco 1984-85 XaL to C£utcQo O£ aQYLAAwc£<; £c>acpo<; OtO ALOTCELQL XatU tllv TCtgi.oco 1985-86. KaL ota Mo £cuCPl] uTCllQX£ CPUOLX~ flOAUVOl] fl£ <POUSUQLO, Oto TCQWtO YUQw on<; 2,000 TCoAAaTCAaOLaonx£<; flOVUc>£<; (propagules) avO. ygaflflo.QLO £C6.cpou<; XaL Oto C£Ut£Qo YUQw on<; 11,000. H l]ALOaTCOAUflav­all £cpagflOOtllx£ O£ AWQi.C£<; xatu fl~XO<; tWv fl£AAovnxwv ygaflflwv cput£uOll<; XaL CLUQXW£ YUQw on<; 40 flEQ£<;, xan'1 tou<; fl~v£<; IoUALo-Auyouoto. H cpUt£UOl] £yLv£ O£ XaflllAu touvE­ALa xato. to <P£BQOuo.QLO-Mo.Qno tou tnOfl£vOU Xgovou. Lto ltdQafla toU ALOltttQi.ou co­XLflUOtllx£ tnLJtQooena XaL aTCoAUflavOl] tou £C6.cpou<; fl£ BQWflLOUXO fl£8U',LO, ltOU £cpaQflo­Otl]x£ tni.oll<; O£ AWQCC£<; YUQw on<; 25 flEQ£<; ltQLV alto tl] cpUt£1JOll. H llALOaltOAUflavOl] au~l]O£ Ol]flavtLXU tl] e£Qfloxgaoi.a tou £cucpou<;, TC£Qi.ltOU xatU l00C Oto I1agaAi.flvL XaL xatU 7°C Oto ALOlthQL (0£ Bueo<; 15 £x.). Aoyw tl]<; \jJllA~<; e£QfloxQaoi.a<;, Oto t£AO<; tll<; 1ttgLOCOU llALOaTCoAUflavOl]<; 0 ltAl]eUOflO<; tou <Pousagi.ou OtO £cacpo<; fltLWel]X£ xatu 90% It£Qi.TCou XaL Ota Mo lttLguflata. AVUAOYl] fl£i.wOl] tou £cacpLXou flOAUO!J.ato<; tJtECP£Q£ XaL l] aTCoAUflavOl] fl£ BQWflLOUXO !J.£eUALo. Ooov acpoQu tl]v XataTCOAEfll]Oll tl]<; aOeEV£W<; Otll cpu­tda xaQltOUSLU<; TCOU aXOAOUel]O£, II llALOaTCOAU!J.avOll £CWO£ ltOAU LXaVOltOLllnXU aTCOt£AE­oflata Oto ITUQaAi.flvL, OltOU to tJti.mco tl]<; agXLx~<; flOAUVOl]<; tOU £CUCPOU<; ~tav ox£nxu xafll]Ao. Ito ALOlt£tQL, OflW<;, tOOO l] llALOaTCOAUflavOl] 600 XaL l] aTCOAuflavoll fl£ BgwflLOu­Xo fl£eUALO aTC£HLxav va cwoouv LXaVOltOLllnX~ xataTCOAEflllOll tl]<; aOeEvHa<;, ltgocpavw<; Myw tWv ltOAU \jJl]AWV ltAl]8uoflWV <pousaQi.ou ltOU ltQoult~Qxav Oto Ecacpo<;.

2

lNTRODUCfION tion) is an alternative control method for soil-borne

Fusarium wilt of watermelon [CitrulJus lanatus (Thunb.) Natsum & Nakai], caused by Fusarium ox­ysporum Lsp.niveum (E. F. Smith) Snyder & H~­sen, is a limiting factor to watermelon productIOn in many watermelon growing regions of the world. In Cyprus it is most prevalent and damaging on early crops grown under low plastic tunnels in the coastal plains, especially in the southeastern part of the island (Kokkinochoria area). Such crops are usu­ally transplanted in the field during January- March and harvested from April to June. Normal-season or late crops, transplanted in the open field from April onwards and harvested throughout summer and early autumn, suffer comparatively less severe losses (poullis, 1987).

Presently, the most Widely grown watermelon cultivar in Cyprus is Crimson Sweet. which is con­sidered highly resistant to Fusarium wilt. Sugar Baby, a wilt-susceptible cultivar, used to be Widely cultivated, but is nowadays grown only to a limited extent for very early production (January-February planting) on coastal sandy soils. Cultivated on a smaIl scale are also the newly-introduced hybrids Imperial, Madera, Mirage and few others, which are described as "wilt-resistant" by their producers. In practice, however, all watermelon cultivars grown in Cyprus, including Crimson Sweet, are commonly in­fected by Fusarium wilt, apparently due to the wide­spread occurrence of a highly virulent local strain of the fungus, which is able to overcome all known types of wilt resistance (poullis, 1987).

In an effort to alleviate losses from Fusarium wilt in early watermelon, growers use routinely soil fu­migation with methyl bromide and/or post-plant fun­gicide treatments applied through the irrigation sys­tem. These methods, however. besides being costly and environmentally unsafe, do not always control wilt satisfactorily (poullis, 1987). Long-term crop rotation, which reduced the amount of soil inoculum and the incidence of wilt in some countries (Martyn and McLaughlin, 1983; Hopkins and Elmstrom, 1984), is not always practicable under Cyprus condi­tions and it occasionally proved to be ineffective (poullis, 1987). Consequently, more effective, practi­cal and economic control methods against Fusarium wilt of watermelon are urgently needed.

Soil pasteurization by solar heating (soil solariza­

plant pathogens developed in Israel (Katan et aI., 1976, 1983; Katan. 1980, 1981) and California (pull­man et aI., 1981a,b; Stapleton and DeVay, 1982, 1984; DeVay and Pullman. 1984 ). In Cyprus this method has been successfully tested against vascular wilts and other root diseases of tomato. cucumber and eggplant, under greenhouse and open-field con­ditions (Ioannou, unpUblished). The present study was therefore undertaken to evaluate soil solariza­tion against Fusarium wilt of watermelon, particular­ly in early crops grown in low tunnels, and also. to compare it to soil fumigation with methyl br~mld~.

which has recently become a standard practIce III

commercial early watermelon production.

MATERIALS AND METHODS

Two field experiments were carried out, the first in 1984-85 on a coastal loamy sand soil (85% sand. 5% silt and 10% clay, pH 7.2) at Paralimni and the second in 1985-86 on a terra rossa clay soil (25% sand, 20% silt and 55% clay, pH 7.9) at Liopetri. Both fields were naturally infested with F. oxyspor­um Lsp. niveum and had a previous history of dis­ease incidence. The Paralirnni trial evaluated soil so­larization for wilt control in the susceptible cultivar Sugar Baby, while the Liopetri trial compared the effectiveness of solarization with that of methyl bro­mide fumigation, using the wilt-resistant Crimson Sweet as test cultivar. In both trials the experimen­tal design was a randomized complete block with four replications. Individual plots consisted of single rows which were either 30 m long and 2.4 m apart (paralimni) or 50 m long and 2.8 m apart (Liope­tri).

In both trials, solarization treatments were ap­plied in the summer preceding the year of plan~ing:

i.e. from July 11 to August 20, 1984 at Paralunm and from August 3 to September 9, 1985 at Liope­trio The sandy soil of the first site was prepared for solarization through a single. dry rototilling whereas the clay soil of the second site required pre­irrigation and disking, before being rototilled. Pre­paratory tillage practices were applied uniformly on the entire field, irrespective of solarization treat­ments. Mulching for solarization was carried out in 80-cm wide strips, using transparent polyethylene sheets, 70!!m thick. With this mulching procedure

3

only about 30% of the soil surface was treated. During solarization the treated plots were irrigated through a drip irrigation system laid under the plas­tic cover (two irrigation lines per strip). The Para­limni trial received three irrigations corresponding to ca 1,200 m3/ha and the Liopetri trial two irriga­tions corresponding to ca 1,000 m3/ha. The untreat­ed check plots and plots to be fumigated were nei­ther mulched nor irrigated during the solarization period. Soil temperature at 15 cm depth was contin­uously recorded with two mercury-in-steel thermo­graphs, one of which was installed in a solarized and one in a non-solarized plot.

Soil fumigation at Liopetri was carried out in late February, 1986 as in local commercial practice. The fumigation treatment was also applied in 80-cm wide strips, using· 100 g of methyl bromide per square meter. The fumigated plots remained covered with polyethylene sheets for 2 weeks, Le. until about 10 days before planting.

The effect of solarization or fumigation on the population density of Fusarium spp. in soil was de­termined by laboratory assays of soil samples taken from treated and untreated plots in each trial. From each plot, 10 subsamples were taken at 0-20 cm depth and bulked into a composite sample of about 1 liter. These composite samples were assayed for Fusarium propagules using serial soil dilutions on a modified peptone-PCNB agar medium (Papavizas, 1967). Soil sampling was done just before starting the solarization or fumigation treatment, at the end of the solarization period or after fumigation, and at about bimonthly intervals thereafter, till the end of the experimental season.

In both trials, planting was done about six months after solarization, Le. on February 15, 1985 at Paralimni and on March 18, 1986 at Liopetri. Three-week-old seedlings grown in sterilized com­post were transplanted in rows spaced 2.4 or 2.8 m apart. Plant rows occupied the center of solarized or fumigated strips and each contained 66 or 110 plants at Paralimni or Liopetri , respectively. Imme­diately after transplanting, plants were covered with plastic (low tunnels), which was removed in April, allowing plants to adjust to open air conditions. Af­ter uncovering plants were examined for wilt symp­toms at ca lO-day intervals. Samples of wilted plants were also colleCted and used to isolate the

pathogen in the laboratory, to confirm that the dis­ease was caused by F. oxysporum f. sp. niveum. Based on the severity of wilt symptoms plants were grouped into four categories: O=apparently healthy; l=plants with mild wilt symptoms; 2=plants with se­vere wilt symptoms; 3=dead plants. The wilt index (WI) was then calculated using the formula:

Nl+2NH3N3 WI=----­

No+Nl+NHN3

where, No, Nl, N2, and N3, the number of plants in categories 0, I, 2, and 3, respectively. The rate of disease increase during the growing season (infection rate) was calculated from the percentages of wilted plants, after they had been corrected for multiple infections using the logit transformation (Van der Plank, 1963). Marketable yield was recorded at har­vest, on May 25, 1985 and June 10, 1986, at Para­limni and Liopetri, respectively.

RESULTS AND DISCUSSION

Soil temperature

Solarization appreciably raised soil temperature in both trials, its effect being more pronounced in the sandy soil at Paralimni than in the clay soil at Liopetri (fable I). Thus, the mean daily maximum temperature of solarized soil was 43.6oC at Paralim­ni and 40.2oC at Liopetri; these temperatures repre­sent an increase of about 10°C and 7°C over the mean daily maximum temperature of untreated soil, respectively. The highest (absolute maXimum) tem­perature recorded in solarized soil at both locations was again 9°C and 7°C higher than the respective highest temperature in untreated soil. In both trials solarization also raised the mean daily minimum temperature of soil by about 6°C (Table I).

Temperatures recorded in solarized soil in the present study were considerably lower than those reported in Israel (Katan, 1980, 1981; Katan et al., 1976, 1983) and California (pullman et al., 1981a, 1981b; Stapleton and DeVay, 1982, 1984), both of which possess a mediterranean climate similar to that of Cyprus. This difference could be attributed to the fact that soil solarization in Cyprus was car­ried out in the moderately hot coastal zone, which

4

Table 1. Soil and air temperatures (0C) at Paralimni and Liopetri during the solarization periods l

Paralimni, 1984 Liopetri, 1985

Temperature Solarized Untreated Solarized Untreated parameter soH 2 soW Air) soH 2 soH 2 Air)

Mean daily maximum 43.6 34.1 31.5 40.2 33.0 31.9

Highest maximum 45.0 36.0 33.8 41.2 34.0 38.0

Mean daily minimum 36.4 29.8 21.8 33.7 28.4 22.5 Lowest minimum 34.0 26.9 18.9 32.0 27.2 19.7

l July II-Aug. 20, 1984 and Aug. 3-Sept. 9, 1985 for the Paralimni and Liopetri trial, respectively.

2 Soil temperatures at 15 cm depth.

) Air temperatures recorded by the Department of Meteorological Services in respective regional

meteorological stations.

is the main vegetable production area of the island, whereas in Israel and California such studies were mainly conducted in very hot central valleys. The present results also indicate that solar heating may be more effective on sandy soils, a fact which could explain in part the very high soil temperatures achieved by solarization in certain desert areas of Israel.

Soil inoculum

Laboratory assays on soil samples taken just be­fore starting the solarization total Fusarium populations 2,000 propagules per gram

treatment showed that at Paralimni were ca

of soil compared to

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oil 3.,

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1984

11,000 at Liopetri. In both trials, solarization re­duced Fusarium populations by over 90% (Fig. 1 and 2). At planting time populations in solarized soil were still about 80% lower than in untreated soil. At Liopetri, the initial effect of fumigation on Fusarium populations in soil was similar to that of solarization (Fig. 2). During the growing season, however, populations in all plots increased rapidly, such an increase being more pronounced in the fu­migated plots, in which inoculum density at harvest time approached that of the untreated check. By contrast, in both trials Fusarium populations in so­larized soil at harvest time remained 50% lower than in untreated soil (Fig. 1 and 2).

.. 01 C

III01 .,C ... > c ' ­o o

I

F M A J

1985

Fig. 1. Total Fusarium populations in solarized and nonsolarized soil. Paralimni, 1984-85.

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25

c 0... 0N-o

30 'L: 0 0'­_L

~8.

0 Solarized

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Fig. 2. Total Fusarium populations in solarized, methyl bromide-fumigated and untreated soil. Liopetri, 1985-86.

Disease development and fruit yield tal marketable yield in solarized plots was increased by about 75% over the untreated check, reaching a

Wilt was more severe on the resistant cultivar level of production (34 tons!ha) normally expected

Crimson Sweet used at Liopetri than on the suscep­from a successful off-season crop of Sugar Baby wa­

tible cultivar Sugar Baby used at Paralimni (fables termelon. Both the number and the size of pro­

2 and 3). This contradiction was attributed to the duced fruits were increased by solarization.

overwhelming difference in soil 'inoculum density be­tween the two sites (Fig. 1 and 2). In the Liopetri trial, however, neither solarization

nor fumigation appreciably reduced the final inci­At Paralimni, solarization was very effective

dence of Fusarium wilt (fable 3). Both treatments against Fusarium wilt, significantly reducing all dis­

reduced to some extent the severity of wilt symp­ease parameters measured (fable 2). Thus, the rate

toms, as evidenced by small, but significant reduc­of infection, expressing the speed of disease spread,

tions in the WI value. The development of milder was reduced to one sixth of the value determined in

symptoms in treated plots was probably the resultthe untreated check, while final wilt incidence was

of a delay in disease spread, as indicated by lowerreduced to 5.5% compared to 27.5% in the untreat­

infection rates in treated compared to untreateded check (80% reduction). The WI value was like­

plots. Later, however, disease incidence in bothwise reduced by 85%, indicating that the severity of

treatments approached that of the untreated check. wilt symptoms was also reduced by solarization. To­

6

Table 2. Effects of soil solarization on the incidence, severity and rate of Fusarium wilt infection, and on the

yield of Sugar Baby watermelons in a naturally infested loamy sand soil (paralimni, 1984-85)

Soil Final Final Infection Fruit 'LTeatment wilt wilt rate yield

incidence index (kg/ha)

(%)

Solarization 5.5a 0.10a 0.01 34,125a

Control 27.5b 0.67b 0.06 19,750b

This has occurred either because of inoculum build­up during the growing season or because the root system of watermelon plants in treated plots had grown into untreated soil, the latter phenomenon being a widely recognized disadvantage of strip so­larization or fumigation (Katan, 1980, 1981). Yields in solarized and fumigated plots were signifi­cantly higher than in the untreated check plots (Ta­ble 3). Such yields (22-28 tons!ha), however, were not high enough to justify the application of either control method in practice, considering that the norm yield of early watermelons in Cyprus is esti­mated to about 37 tons!ha and that of normal­season crops to about 60 tons!ha (Papachristodoulou et ai., 1987). Also, it must be noted that in the ab­sence of Fusarium wilt, individual growers of Crim­son Sweet watermelons (low-tunnel crops) in the Kokkinochoria area, frequently achieve yields of up to 80 tons!ha (Poullis, 1987).

The available information concerning control of watermelon wilt by soil solarization is very limited and inconclusive. Thus, in Texas, USA Martyn and Hartz (1986) obtained partial disease control by soil solarization treatments, whereas in south-eastern Australia solarization reduced soil inoculum level but not disease incidence on watermelon (porter and Merriman, 1985). These contradicting results, like those of the present study, indicate that the ef­fect of soil solarization on Fusarium wilt of water­melon may vary considerably, depending on initial soil inoculum density, soil type and possibly other factors. Although under certain conditions solariza­tion may give satisfactory control of watermelon wilt, such treatment cannot yet be recommended for commercial practice until all factors involved are elucidated and the efficiency of the technique is im­proved by further research.

Table 3. Effects of soil solarization or fumigation with methyl bromide on the incidence, severity and rate of

Fusarium wilt infection and on the yield of Crimson Sweet watermelons in a naturally infested terra

rossa clay soil (Liopetri, 1985-86)

Final Final Infection Fruit Soil wilt wilt rate yield

treatment incidence index (kg/ha) (%)

Solarization 70a 1.5a 0.26 28,482a

Fumigation 74a 1.8a 0.32 22,021a

Control 83a 2Ab 0.39 12,930b

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ACKNOWLEDGEMENTS

The authors thank A Hadjinicolis and D. Con­stantinou for technical assistance.

REFERENCES

DeVay, J.E., and G.S. Pullman. 1984. Epidemiology and ecology of diseases caused by Verticillium species, with emphasis on Verticillium wilt of cotton. Phytopathologia Mediterranea 23:95-107.

Hopkins, D.L., and G.W. Elmstrom. 1984. Fusarium wilt in watermelon cultivars grown in a 4-year monoculture. Plant Disease 68: 129-131.

Katan, J. 1980. Solar pasteurization of soils for dis­ease control: status and prospects. Plant Disease 64:450-454.

Katan, 1.1981. Solar heating (solarization) of soil for control of soil-borne pests. Annual Review of Phytopathology 19:211-236.

Katan, 1., G.L. Fisher, and A Grinstem. 1983. Short and longterm effects of soU solarization and crop sequence on Fusarium wilt and yield of cotton in Israel. Phytopathology 73: 1215-1219.

Katan, J., H.A Greenberger, and A Grinstein. 1976. Solar heating by polyethelene mulching for the control of diseases caused by soil-borne pa­thogens. Phytopathology 28:28-32.

Martyn, R.D., and T.K. Hartz. 1986. Use of soil so­larization to control Fusarium wilt of watermel­on. Plant Disease 70:762-766.

Martyn, R.D., and R.I. McLaugWin. 1983. Effects· of inoculum concentration on the apparent resis­tance of watermelons to F. qxysporum f sp. niv­eum. Plant Disease 67:493-495.

Papachristodoulou, S., C. Papayiannis, and G.S. Pa­nayiotou. 1987. Norm input-output data for the main crop and livestock enterprises of Cyprus. Agricultural Economics Report 16, Agricultural Research Institute, Nicosia, Cyprus. 225p.

Papavizas, G.c. 1967. Evaluation of various media and antimicrobial agents for isolation of Fusarium from soil. Phytopathology 57:848-852.

Porter, 1.1., and P.R. Merriman. 1985. Evaluation of soil solarization for control of root diseases of row crops in Victoria Plant Pathology 34: 108­118.

Poullis, C.A 1987. Vascular Wilts of Watermelon in Cyprus. M. Sc. Thesis, West Virginia University, Morgantown. 67p.

Pullman, G.S., J.E. DeVay, and R.ll. Garber. 1981a. Soil solarization and thermal death: a logarithmic relationship between time and temperature for the soil-borne plant pathogens. Phytopathology 71:959-964.

Pullman, G.S., J.E. DeVay, R.ll. Garber, and AR. Weinhold. 1981b. Soil solarization: effects on Verticillium wilt of cotton and soil-borne popula­tions of Verticillium dahliae, Pythium sp., Rhizoc­tonia solani, and Thielaviopsis basicola. Phytopa­thology 71 :954-959.

Stapleton, J.J., and J.E. DeVay. 1982. Effect of soil solarization on populations of selected soil-borne microorganisms and growth of deciduous fruit tree seedlings. Phytopathology 72:323-326.

Stapleton, J.I., and J.E. Devay. 1984. Thermal com­ponents of soil solarizations as related to changes in soil and root microflora and increased plant growth response. Phytopathology 74:255-259.

Van der Plank, lE. 1963. Plant Diseases: Epidemics and Control. Academic Press, New York and London. 349 p.

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