Polyethylene mulching of soil to reduce viability of sclerotia of Sclerotium oryzae

Soil Biol. Biochrm. Vol. 14, pp. 203 to 206. 1982 Printed in Great Britam All rights reserved

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0038-0717~82~030203-04$03.00/O 0 1982 Pergamon Press Ltd

POLYETHYLENE MULCHING OF SOIL TO REDUCE VIABILITY OF SCLEROTIA OF SCLEROTIUM ORYZAE S. M. Department

HAROON USMANI and A. GHAFFAR

of Botany.

University

of Karachi,

(Accepted 10 Noremhrr

Karachi-32,

Pakistan

1981)

Summary~Mulching of Sclerorium myue infested soil (moist or dry) with polyethylene sheets during hot summer days of May and June increased the soil temperature at 5 cm from 36 C (unmulched) to 48’~C (wet) and from 44 to 52’C (dry) and at 20 cm from 32 to 38~C (wet) and from 35 to 39’ C (dry). In artificially-infested soil, the sclerotia were not eradicated but 955loo”,, loss in viability was observed at 5 cm by a mulch treatment for 1 week and at 20 cm by mulching for 8 weeks, Mulching effects were not influenced by moisture content of soil or by amendments with lucerne or wheat straw. Mulching of naturally-infested soil at a second site did not eradicate S. orJz~ but reduced sclerotial viability by 93”“.

INTRODUCTION Sclrrotiurn oryxe Catt., the cause of stem rot of rice occurs in most rice-producing countries (Shah 1970; Ou, 1972). The fungus survives as sclerotia, either free in the soil or on plant debris which serve as a primary source of inoculum for subsequent rice crops. Survival of these sclerotia for periods varying from 0.33 to 6 yr have been reported (Park and Bertus, 1932; Tullis and Cralley. 1941; Nisikado and Hirata, 1937). but the rates of decline in number of sclerotia or their germinability after various periods are not known. We found more sclerotia with greater viability on surface soil than in soils at a depth of 15-25 cm (Usmani and Ghaffar, 1974). Removal of infected culms or turning of the soil to a depth of 15 cm reduced the sclerotial inoculum from the surface soil. Webster et al. (1976) found that moldboard ploughing resulted in the burial of a large proportion of sclerotial inoculum. But this inoculum is brought back to the surface during puddling before rice seedlings are transplanted. Burning rice stubble after harvest is effective (Webster er a/., 1976) but it contributes to air pollution. Lack of resistant varieties (Ou, 1972) and acceptable chemical control suggest that efforts should be directed towards inexpensive and non-hazardous methods of control. In Israel, Katan et al. (1976) have developed a new method to control soil-borne plant pathogens and weeds by means of solar heating of the soil. This was achieved by mulching soil with transparent polyethylene sheets during the hot season. Since rice is followed by wheat or lucerne or the field is left dry fallow. the organic substrate from these crops change the nutrient status of soil stimulating or suppressing the disease-causing pathogens. We report results of our experiments to adapt the polyethylene mulching technique to reduce the inoculum of S. oryzae with or without natural plant residue under wet and dry soil conditions. MATERIALS AND METHODS

Experiments the University \.“R 133

<

were performed at two field locations: Experimental Farm. Karachi and the 203

Rice Research Institute, Kala Shah Kaku, Lahore. At Karachi the soil was a sandy loam. pH 7.2. Sclerotium oryae isolated from stem-rot disease of rice (K.U. Bot. Dept. Cult. No. 158) was used in this study, One month-old sclerotia produced on sterile rice culms were separated and mixed with soil at 10 sclerotia g-i soil. In another set, infested soil was amended with lo-mm long dried stem and leaf fragments of lucerne (Medicago satioa L.) or wheat (Triticum aestimm L.), 1% (w/w). Ten-g portions of soil containing 10&l 15 sclerotia were placed in 15 x 15 cm nylon net (150 pm pore) bags. These bags were buried in soil at depths of 5 or 20 cm. Plots, 3 x 1 m. were mulched with transparent polyethylene plastic sheets. 40 pm thick (Sehgal Plastic Industry, Karachi) to improve the thermal conductivity of the soil and to increase the thermal sensitivity of sclerotia of S. oryae to solar heating. A comparable non-mulched control was used. Soil in plots was maintained at field capacity by drip irrigation from a system of plastic tubes located beneath the polyethylene mulch. Both mulched and non-mulched plots were irrigated daily. Mulched and non-mulched plots in which the soil was not irrigated but kept continuously dry (2--3”, m.h.c.) throughout the experiment were also used. There were two replicates of each treatment and all the plots were randomized. Periodically (Fig. 1). the mulched polyethylene sheets were removed and 4 bags of each treatment containing sclerotia were recovered both from the surface and at 20cm. Soil was washed through an 120 pm screen on which sclerotia were retained. The populations of sclerotia were determined by wet sieving and floatation (Usmani and Ghaffar. 1974). Viability of sclerotia was tested by transferring sclerotia on to water agar containing penicillin G and streptomycin sulphate, 3 mg ml- ‘. Germination of sclerotia was myceliogenic (Coley-Smith and Cooke, 1971). Sclerotia germinated within 24 h and were counted after 3 days at 30°C. Soil temperature and moisture were measured by inserting thermometers and embedding gypsum blocks at 5 and 20 cm depths. Readings were recorded at every 2 h from 0600 to 2400 h for 1 week (Fig. 2).

204

S. M. HAROOK USMANI and A. GHAFFAR

Wet

RESLILTS

Non-mulched

Mulched -

Surface

.-a

Depth

(O-5cm) (15-2Ocm)

,/I

012

4

012

8

4

0

weeks

Time,

Fig. 1. Effect of a transparent polyethylene mulch on the viability of sclerotia of Sclerorium oryzae in moist soil.*

Data is for the period content of dry soil: errors (SE) are small ments having similar at P = 0.05 according

from 7 May-2 July 1980. *Moisture 2-3”“; wet soil: 96100”,. Standard and are covered by the points. Treatlettering per assay time do not differ to Duncan’s new multiple range test.

A clay loam, pH 7.3 was used at the Rice Research Institute, Kala Shah Kaku, Lahore. Soil artificially infested with sclerotia of S. oryzue lOOg-lo soil. was placed in dry soil (3-5’?0 m.h.c.) at 5 and 20 cm depths. Mulching with polyethylene sheet was carried out in 3 x 1 m plots. Soil not mulched by polyethylene was kept as a control. The treatments were replicated and randomized four times. Soil temperatures were measured every 2 h from 0600 to 1800 h for 1 week (Fig. 3).

Sclerotia buried at 5 cm depth in wet soil showed complete loss in viability after 1 week of mulching (Fig. 1). Similarly 1 week of mulching of dry soil reduced the viability by 95”;. The effect of mulching was not influenced by amendment of soil with lucerne or wheat (P = 0.05). Sclerotia buried at 20 cm depth under mulched wet soil conditions for 8 weeks showed a decline in viability by 90”/; in lucerne. 82p/, in wheat and 69”, in unamended control. A gradual reduction in numbers of viable sclerotia was noticed in soil mulched and kept dry, and after 8 weeks none were viable. In non-mulched treatments there was no significant reduction (P = 0.05) in viability of sclerotia at 5 and 20 cm depths, sclerotia at 20 cm depth in dry soil, however, lost their viability after 8 weeks. irrespective of amendments. Amendment of soil with lucerne or wheat under mulched and non-mulched wet or dry soil gave similar results and did not show any significant differences (P = 0.05) in the viability of sclerotia as compared to non-amended soil both at the surface and 20 cm depth. It was therefore possible to reduce the viability of sclerotia from soil at 5 cm by a mulch treatment for 1 week and at a depth of 20 cm by mulching for 8 weeks, Results on the reduction in viability are well correlated with temperature measurements (Figs 1 and 2). The highest soil temperatures (48-52’C) were attained daily between 1400 and 1600 h in the upper layer of 5 cm depth where soil was kept mulched (52’C in dry and 48~C in wet soils). Greatest temperature differences (12’C in the wet soil and of 8’ C in dry soil) were noticed in the mulched and non-mulched treatments at the peak temperatures of surface soils. In nonmulched treatment the temperature did not exceed

Non - mulched

Mulched

Lange of

-0 l

2ok 0%

0

230-O

0

176-

609

Dry 12

A 0

Surface

0

0.141-O

Depth

.

0

0

0 224-0564

.

0

122-O

Mean

SE

0345 489

155-0406

403

I 18

SE,

184-0507

0 303

0

0 740

P,r

0 282 0 235

0389 502

0 223

I 24 noon

n 00”

Time,

IOOh

Fig. 2. Effect of a transparent polyethylene mulch on the temperature of moist (96100”,) and dry (2-3”“) soil at depths of 5 and 20 cm. Temperatures were recorded every 2 h from 0600 to 2400 h daily. Data is an average temperature recorded at each interval for the first week (7-13 May 1980) at the Karachi University experimental plots. Standard errors (SE) are small and are covered by the points.

Mulching

and sclerotial

205

viability

Non-mulched

Mulched

Range

u *

6o

Air

bO.508-I

o

0381-0962

0 761

Surface

0 0,462-O

l

0

0

Depth

.

487-0718

605

0.693

951

0.530

0.404-0.633

r

r

noon

noon Time,

100 h

Fig. 3. Effect of a transparent polyethylene mulch and 20 cm. Temperatures were recorded every 2 h tures recorded at each interval for one week (16-22 Kaku. Lahore. Standard errors (SE)

at 2Ocm depth the maximum temperature reached was 42 C. Air temperature at the ground surface ranged between 27 and 37-C, maximum temperature being recorded between 1200 and 1400 h. Similar increases in soil temperature were recorded at the Rice Research Institute, Kala Shah Kaku, Lahore (Fig. 3). Viability of sclerotia in soil at 5 cm was reduced to zero in mulched soil as compared to 94”; in non-mulched treatments (Table 1). Sclerotia buried at 20 cm depth irrespective of mulching showed 959, viability. Soil samples collected from the experimental plot with a natural infestation of sclerotia of S. or~~rrr showed 4:,, viability after mulching as compared to 55 “i in non-mulched surface soil.

44 C. Similarly

DISCL’SSION

Mulching with polyethylene increased soil temperature and considerably reduced the viability of sclero-

Table

Mean I 129

568

on the temperature of dry soil (3-5”“) at depths of 5 from 0600 to 1800 h daily. Data are average temperaJune, 1980) at the Rice Research Institute, Kala Shah are small and are covered by the points.

tia (Figs 1 and 2). A pronounced reduction in the populations of Verticillium dahliae and Fusarium oxysporum f. sp. lycopersici (Katan et al., 1976) Sclerotium rolfsii (Grinstein et al.. 1979). PIasmodiophora hrassicae (White and Buczacki, 1979) as well as the diseases caused by these fungi have been reported by mulching of soil with polyethylene sheets. A soil mulching practice can be considered superior to fumigation since it is cheaper and safer with no phytotoxic or pesticidal residues. The high temperature reached in the upper layers of mulched soil would appear to be lethal to S. oryzae either directly or indirectly by breaking the Elton principle and thus encouraging establishment of antagonistic biotic elements in soil (Baker, 1970; Wilhelm, 1973). This hypothesis is further substantiated by the greater proportion of bacteria and actinomycetes inhibitory to S. oryae as found in soil after incubation in oven at high temperatures (S. M. H. Usmani and A. Ghaffar. unpublished results). High

1. Effect of polyethylene mulching of soil on the viability of sclerotia naturally and artificially infested soil* O0 Viability of sclerotia Artificiallyt infested

Treatment Mulched Surface Depth Non-mulched Surface Depth

of S. oryxr

in

in soil after 7 days mulching Naturally: infested

5cm 20 cm

0” 95”

4 0”

5cm 20 cm

94’d 95d’

55h 57h

* Moisture content 3-5”,,. t Number of sclerotia was 100 per 10 g of soil. Viability of sclerotia at 0 days of mulching was 100” 0. $ Number of sclerotia recovered was I I sclerotia per 50 g of soil at surface layer and 0.2-l sclerotia per 50 g at 20 cm depth. Values in each column represent mean of 4 replicates per treatment. Means in each column or means from both columns with like letters are not significantly different (P = 0.05) according to Duncan’s new multiple range test.

S. M. HARNN

206

USMANI and A. GHAFFAR

temperature of 52-C was not attained in non-mulched soil and did not reduce the viability of sclerotia. At 20cm depth, the temperatures were lower and the viability was not reduced in non-mulched wet soil series. The reduction in the viability of sclerotia observed in mutched wet and dry soil and in nonmulched dry soil series, could not therefore be attributed to temperature effects alone. Reduction in the viability of sclerotia in deeper soil layers may be due either to anaerobiosis or accumulation of toxic metabolites since these can not diffuse away under mulched or anaerobic conditions (Katan. 1980). The reduction may also be biological in nature since sclerotia

of

S. oryxe

recovered

from

2%25cm

depth

showed 66”” bacterial colonization as compared to 38”, from &5cm depth in paddy fields (S. M. H. Usmani and A. Ghaffzdr, unpublished results). Detrimental effects of high temperature by polyethylene mulching can be applied to the field to eIiminate stem-rot disease of rice. Mechanical ploughing which removes the inoculum from the surface layer and deposits it to a depth of 20-25cm can be brought back to the surface and thus eliminated by subsequent mulching. A~,kr2o~olrdyrrnrnts-This work was carried out under the PL-480 Programme of the LISDA and Pakistan Agriculture Research Council which is sincereIy acknowledged. We thank Dr J. A. Lewis. Soilborne Diseases Laboratory. USDA, Beltsville for helpful suggestions in the preparation of the manuscript and Dr A. Majeed for providing the field plots at the Rice Research Institute. Kala Shah Kaku, Lahore. Pakistan.

REFERENCES BAKER K. F. (1970) Selective killing of soil microorganisms by aerated steam. In Root Disccws Soilborne Plmt

P~)~j7~)~~~t1.s (T. A. Toussoun,

R. Bega and P. E. Nelson, Eds). pp. 234-239. Univ. of California Press. Berkeley. COLEY-SMITH J. R. and COOKE R. C. (19711 Survival and germination of fungal sclerotia. .iln,utrcl Rc~Gcw of Pltytoputi~olm~y 9, 65 92. GRINST~IN A.. KAT*?J J.. ABCXIL Razrtc A.. ZEYUAN 0. and ELAII Y. (1979) Control of Sckroriurn rctifsii and weeds in peanuts by solar heating of the solI. PImt Disrusr Reporter 63, 1056-l 059. K,XTAN J. (1980) Solar pasteurization of soils for disease control: Status and prospects. P!LIIZIDisetrw 64, 45&454. KATAU J., CREENRERGER A.. ALON H. and GRINSTEIN A. (1976) Solar heating by polyethylene mulching for the control of disease caused by soilborne pathogens. Phrtopciflzotoy!, 66, 683-688. NISIKADO Y. and HIRATA K. 11937) Studies on the longevity of sclerotia of certain fungi under controlled environmental factors. Berichtr Ohuru Insritltrr, Landrcirdsllclfilic,hc> Fo,schlrny 7, 535-547. Ou S. H. (1972) Rice Diseu,se.s. Commonwealth Mycological Institute. Kew. PARK M. and BERTUS L. S. (1931) Scleroizxl diseases of rice m Ceylon. 2. Scierotium oryx~’ Catt. Ceyl0~1 Jourr~c~l 01 Scirncr A. II, 343-359. SHAFI M. (1970) Ten-years of rice findings. Kala Shah Kaku. Rice Research Station. Lahore, pp. 3 4. Tri~rts E. C. and CRALLE~ E. M. (f941) Longevity of sclerotia of the stem rot fungus. Leptosphorri