lil :
Crop Provction Vol. 14. No. 4. pp. 315-320. 1995 Copyright @ 1995 Elsevier Science Lfd Pnnted in Greilt Britain. All rights reserved
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The border effect of soil solarization A. Grinstein,* G. Kritzman,’ A: Hetzroni,* A. Gamliel,* M. Moth and J. Katan” *Institute
of Agricultural
Nematology, Pathology
ARO, and
Engineering,
‘Department
of Plant
The Volcani Center, Bet Dagan 50 250,
Microbiology,
The Hebrew
University
Pathology,
*and
Department
Israel; and “Department
of Jerusalem,
Faculty
of
of Plant
of Agriculture,
Rehovot 76 100, Israel
A gradient of reduced effectiveness of solarization toward the edge of the plastic mulch is known as a ‘border effect’, corresponding with decreasing soil temperatures from the middie toward the edge of the mulched area. The cyst nematode Heteroderu avenue, of wheat was completely controlled by solarization at 90 cm or more inward from the edge of the mulched plots. A gradient of decreasing effectiveness of nematode control in the mulched plot, toward the edge of the mulch, was observed. There was a similar gradient with height of wheat planted after solarization, and in the incidence of pod wart disease of peanuts caused by Streptomyces spp., planted as a second crop after soil disinfestation. Disease reduction was correlated with a decrease in population of streptomycetes. A similar, but less pronounced, border effect was observed when solarization was combined with methyl bromide or formalin at reduced dosages. Keywords: fumigation; methyl bromide; soilborne pathogens
Introduction Soil solarization is a procedure used for controlling a wide spectrum of soilborne pathogens and weeds (Katan, 1987, Katan et al., 1976). It is achieved by mulching tilled and irrigated soil with transparent polyethylene sheeting. The soil is heated by the solar irradiation, and many soilborne pests are killed due to physical and biological processes. Soil mulching can be done with separate plastic film strips (Katan et al., 1976, 1980), or alternatively with a continuous film covering on the whole plot applied either manually or by laying separate strips which are melded together, depending on the technology available (Grinstein and Hetzroni, 1991; Grinstein et al., 1979, Jacobsohn et al., 1980). Combining solarization with other control agents, e.g. pesticides at reduced dosages may improve solarization performance (Katan, 1987; Stapleton, Lear and DeVay, 1987). Studies carried out in wide plots (4-7 m) showed that, although solarization was effective in controlling various soilborne pathogens, plant growth was gradually retarded towards the edge of the mulched plots, indicating only partial control. This effect appears to be due, at least partially, to heat loss to the adjacent nonsolarized plots, resulting in reduced solarization efficacy (Grinstein and Hetzroni, 1991; Mahrer, 1991; Mahrer and Katan, 1981). This ‘border effect’ was first shown by Jacobsohn et al. (1980). In that experiment, solarization effectively controlled broomrape, but broomrape infestation along ahe edges of solarized plots was only partially reduced. As a result, carrot plants grown in these plots were stunted in the marginal 60 cm of the 6-m-wide plots. The possibility to minimize the border effect by improved solarization was not reported.
The purpose of this study was to investigate the short- and long-term effects of the border effect of soil solarization and fumigation on plant growth and the control of the cyst nematode Heteroderu avenue of wheat (Brown, 1984), and of the pod wart of peanut (Kritzman et al., 1989) caused by Streptomyces spp.
Materials and methods Soil disinfestation
A long-term field experiment was conducted in the southern part of Israel in a sandy soil with a long history of intensive cropping of potatoes, peanuts and wheat. Three solarization treatments were performed: solarization alone, and solarization combined with either methyl bromide (98% a.i., Bromine Compounds Ltd., Beer Sheva, Israel) at 250 kg ha-’ or formalin (Fordor 37-37% a.i. Dor Chemicals Ltd Haifa, Israel) at 200 1 ha-’ both at dosages lower than those recommended. For each plot, the adjacent nontreated plot served as the control. Formalin alone at 200 1 ha-’ was tested for comparison purposes. Soil solarization of the entire plot was accomplished by using a continuous mulching machine (Technohak, Israel), essentially as described by Grinstein and Hetzroni (1991). The soil was irrigated to the depth of 60 cm prior to mulching with transparent polyethylene sheets (40 pm thick). The polyethylene mulch was laid on 3 August 1988, and removed on 15 September 1988. Methyl bromide for the combined treatment with solarization was applied 24 h after polyethylene mulching by the standard hot gas fumigation technique, using perforated polyethylene tubes (one per bed) that were laid on the soil before mulching. With the combined treatment of formalin plus solarization, five tines placed 22 cm apart were
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The border effect of soil solarization: A. Grinstein et al.
in front of the mulching machine to inject formalin to a depth of 15 cm. Soil temperatures at distance of 25 cm outside the solarized plots, at the plot edge (0 cm), and at 25 cm increments up to 125 cm into the plot at depths of 10 and 20 cm, were recorded by Ttype thermocouples (+0.5”C) connected to a data logger (21X; Campbell, Logan, UT, U.S.A.). Temperatures were measured every 5 min and recorded on an hourly average, during the whole solarization period. Soil and plant samples were taken from various locations inside and outside the mulched plot. For samples outside of the mulched plot only one location was reported, since the results were similar to those from other locations outside the mulched plot.
attached
Crops The first crop after disinfestation was wheat (Triticum durum Desf. ‘Inbar’), which was sown on 10 November 1988. The plants were cut and harvested for hay at the milk stage, on 19 April 1989. Each plot was diskharrowed along the beds and planted with peanuts (Aruchis hypoguea L. ‘Shulamit’) on 2 May 1989. The experimental area was located within a commercial field and was treated according to the common agricultural practices. Assessment of nematode infection Sampling sites were located at various distances from the edge of the mulched plots, as indicated. Ten plants from each sampling site were uprooted and examined for roof infection with H. avenae, 90 days after planting. The roots were gently washed free of soil and dipped for 3 min in 2% sodium hypochlorite. The plant roots were then carefully washed and stained overnight at 50°C in a mixture of lactic acid:glycerin:distilled water (1:3:3 v/v) and 0.005% (w/v) acid fuchsin (Mor, Cohen and Spiegel, 1992). This procedure makes it possible to clarify the root system and observe nematode feeding sites and the early stage of juvenile penetration (Mor et al., 1992). Root infection index was assessed under a stereomicroscope with a rating system similar to the rating by Simon (1980) on a O-5 scale, where 0 = no infection and 5 = maximal coverage of the roots (2120 cysts g-’ fresh root). The average root infection index was calculated for each sampling location. In addition, per cent of infected plants, namely, plants with roots showing infection at any level, was calculated.
replicates were collected from each tested point inward and outward from the edge of the plastic mulch. Each sample was homogenized and subsamples of 10 g were placed in plastic bags containing 90 ml of sterile water supplemented with 0.1% agar and 372 ppm (a.i.) .of metham sodium (Agan Chemical Manufacturers, Ashdod, Israel) as selective agent. After 30 min of homogenizing in a Stomacher 400 (Laboratory Blender, Seward, U.K.), a series of lo-fold dilutions in saline were made and 100 yl from each dilution was spread evenly on the surface of a solidified medium selective for streptomycetes (SKSM) (G. Kritzman, unpublished). The plates were incubated for 5 days at 27°C. SKSM was prepared as follows in g 1-l distilled water): soluble starch (Sigma, U.S.A.), 15.0; yeast extract (Bacto, Difco, U.S.A.), 4.0; peptone (Oxoid, U.K.), 0.6; proteose peptone (Oxoid, U.K.), 0.6; K2HP04,1.0;MgS04~7H20,0.5;NaCl,40;andagar,18. After 20 min of autoclaving, the following compounds were added to give the final concentration (pg ml-’ or yl ml-‘): naladixic acid 20 (should be first dissolved in O.OlN NaOH); penicillin G, 200; cycloheximide, 350; Nystatin (suspension, Sigma), 20. SKSM supports the growth and sporulation of all plant-pathogenic streptomycete isolates as well as many of the nonpathogenic ones. These include common scab streptomycetes isolated from soil and from potato tubers, pitted scab streptomycetes isolates from our collection, and a series of other streptomycetes. Some bacteria can grow on this medium, but their colonies can be easily distinguished. The percentage of recovery of this medium for the tested streptomycete isolates is within the range of 87-94%. Statistical analyses The experiment was carried out in four replicates in a randomized block design, divided into 7.8 (four beds) wide x 6.0 m long plots. Statistical analyses included analysis of variance and mean separation by Student multiple range test at P s 0.05, and calculating the standard errors, as indicated. Regression analyses were conducted to test the relation between the border effect and growth or disease parameters. Regression lines representing the best fit were drawn.
Results
Effect on soil temperature Evaluation of disease severity in peanuts Severity of peanut pod wart was assessed by determining the wart index. Four degrees were established based on the number of warts on the surface of each pod, as follows: 0 = none; 1 = 1; 2 = 2 or 3; and 3 = 4 or more warts. The yield from each plot was pooled and mixed. A sample of 100 pods was taken at random from the bulk of each plot. Evaluation of populations of Sfreptornyces spp. in soil Soil samples were collected at two depths, O-20 and 2040 cm, using a soil sampler (10 cm diam.). Five
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Crop Protection 1995 Volume 14 Number 4
Soil temperatures within the mulched plots were higher than those outside the area (Figure 2). At a distance of 50 cm and more inward from the edge of the mulched plot, the daily average maximal soil temperatures, at depths of 10 and 20 cm, were approximately 42°C and 38”C, respectively. A trend of decreasing temperatures in the mulched plots, from the middle toward the edge, was observed. Thus, at the edge of the mulched plot (0 cm) and at depths of 10 and 20 cm, soil temperatures were, respectively, 1°C and 5°C lower than those 50 cm inward from the edge. This gradient in soil temperatures was observed also during other periods of the day and throughout the measurement period. The difference in temperatures between the non-mulched area (25 cm)
The border effect
of soil
solarization: A. Grinstein et al.
and the distant points from the edge of the mulch was 9°C and 7°C at depths of 10 and 20 cm, respectively. The daily average maximum ambient temperature during this period was 34°C. Effect on nematode
control and wheat growth
The roots of all wheat plants in the non-mulched area of the plot were infected with the cyst nematode, H. avenue, and had the highest root infection index (Figure 2). In contrast, the three soil disinfestation treatments reduced infection to zero, 90 cm inward from the edge of the mulched plots. With each of the three treatments, a gradient of increased root infection index in the mulched plots, toward the edge of the mulch, was observed. This trend (Figure 2) was in accordance with that observed with soil temperature (Figure I). Thus, 10 cm inside the mulched plot, root infection index was reduced by only 50-60% as compared with @HO% and 100% at distances of 45 and 90 cm, respectively, inside the plot. Nematode control was evaluated also by assessing percentage of infected plants. The findings (Figure 3) confirmed those (Figure 2) regarding decreasing effectiveness of disinfestation from the middle toward the edge of the mulch. The same trend was obtained with the three disinfestation methods. Percentage of infected plants outside the mulched plots and at the edge of the mulched plot were 90-100% and did not differ significantly from each other. The formalin treatment (without mulch), injected at the reduced rate of 200 1 ha-‘, was not effective in H. aveltae control and did not differ significantly from the nonmulched control (results not shown). A significant linear relationship (p = 0.05) was established between temperature levels (t) at various distances from the edge of the plastic and percentage of plants infected by nematodes [percentage of infected plants = 425.45 - 10.016(t), 2 = 0.9571.
0 -50
-25
Distance
0
-50
Distance
1 0
J 25 from
50 the
75
100
mulch edge
the
50 mulch
75 edge
100 (cm)
Figure 2. Effectiveness of three soil disinfestation treatments in controlling Heterodera avenae in wheat, evaluated outside and into the polyethylene-mulched plot. Location ‘0’ indicates the edge of the mulched plot. Positive numbers indicate distances from the edge into the mulched plot; negative numbers indicate distances from the edge outside the mulched plot. Root infection index is recorded on a scale of O-5, where 0 = no infection and 5 = maximal covering of the roots (cl20 cysts g-’ root). The index in the nontreated plots was 5. MBr = methyl bromide; S = solarized; For = formalin. The regression equation for solarized is Y = -0.0308X + 2.6567 (,’ = 0.931), for S + For is Y = -0.0291X + 2.7049 (3 = 0.912) and for S + MBr is Y = -0.025X + 2.204 (? = 0.935).
-25
Disatance
25 ’ -25
25
from
0 from
25 the
l
s+l=or
n
s+mr
l
Nontreated
75
100
mulch edge
50
(cm)
Figure 3. Effectiveness of three soil disinfestation treatments in controlling Heterodera avenae in wheat, evaluated outside and into the polyethylene mulched-plot. Infected plants are defined as those with roots showing infection with nematodes at any level regardless of severity. Distance ‘0’ indicates the edge of the mulched plot. Positive numbers indicate sites inward from the edge of the mulch. Disease incidence in the nontreated plot was 100%. MBr = methyl bromide; S = solarized; For = formalin. The regression equation for solarized is Y = 144.22 - 33.61111X (r’ = 0.975); for S + For is Y =230.39 - 51.34lnX (r’ = 0.962); and for S + MBr is Y = 108.55 -23.9lnX (12 = 0.960).
125 (cm)
Figure 1. Average daily maximal soil temperatures outside and into the polyethylene-mulched plot. Temperatures were recorded at depths of 10 cm (circles) and 20 cm (triangles). Distance ‘0’ indicates the edge of the mulched plot. Positive numbers indicate distances from the edge inward; the negative number indicate the distances from the-edge outside the mulched plot. The rearession eauation for 10 cm death is Y = 37.606 + 0.9626lk (I’ = 0.‘812), and for 20 cm depth is Y = 32.574 + 1.2015lnX (/’ = 0.901).
Nematode infection was also associated with various degrees of retardation of wheat growth, reflecting the gradients in temperature and nematode control. The plants in the nonmulched plots were severely stunted, wtih heights only 38-41% of those of plants 90 cm inward from the edge of the disinfested plots (Figure 4). Plant height at 50 cm inward from the edge was 57% and 55% of the maximum, in the solarization and
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The border effect of soil solarization: A. Grinstein et al. solarization + formalin treatments, respectively. The border effect was less pronounced in the solarization combined with methyl bromide treatment. At a distance of 25 cm from the edge of the plastic, the beneficial effect of the disinfestation was greatly reduced with the three mulched treatments. Again, the formalin treatment (without polyethylene mulch) had no beneficial effect, and was not significantly different from the nonmulched control (results not shown).
1.2 Warization
I’
1.0
9 9
0.0
! ._
0.6
8 8
0.4
I Solarization
+
formalin
I
Solarizatiw
!
+ MBr
i5 0.2
The border effect in the second crop after disinfestation
0.0
Peanuts were planted after the wheat harvest in order to detect a possible long-term effect of disinfestation as related to the border effect. At 90 cm inward from the edge of the mulch, the pod wart disease index was reduced by N-87% by solarization, solarization combined with formalin and solarization combined with methyl bromide (Figure 5). A border effect, namely, reduced effectiveness of control at 20+ cm inward from the edge, was significant only in the treatments of solarization and solarization combined with formalin, but not in the solarization combined with methyl bromide treatment. Formalin alone (without mulch) was ineffective in controlling the disease (results not shown). Populations of streptomycetes The total population of streptomycetes was determined at two soil depths 20 days before planting peanuts, namely, 7 months after the termination of solarization. Streptomycete populations, at O-20 and 2040 cm deep were less as the distance soil samples were taken increased from the edge of the plastic mulched plot (Figure 6). The border effect, as was expressed by other
A s
- . S + n
For
n
s+bBr
..
Nmtreated
‘* 20 ’ -50
I -25
Distance
I
0 from
25 the
50
mulch
75 edge
loo
(cm)
Figure 4. Effect of three soil disinfestation treatments on height of wheat plants, evaluated outside and into the polyethylenemulched plot 140 days after emergence. Distance ‘0’ indicates the edge of the mulched plot. Positive numbers indicate distances from the edge into the mulched plot; negative numbers indicate distances from the edge outside the mulched plot. MBr = methyl bromide; S = solarized; For = formalin. The regression equation for solarized is Y = 33.47 -0.105X + O.O* - 0.000~ (? = 0.993); for S + For is Y = 30.02 +0.014X + O.OZ@ - O.OOOZ6 (/’ = 0.990); and for S + MBr is Y -37.02 + 1.076X + o.OOOw2 - 0.0001~ (3 = 0.965).
318
00 Distance
-4s 20-46 PO -46 20-M from the mulch ecbs (cm)
w
Figure 5. Effectiveness of solanzation and combination with formalin or methyl bromide in controlling pod wart disease of peanut outside and into the polyethylene-mulched plot, of peanuts grown as a second crop after disinfestation. Disease index recorded on a scale of O-3, where 0 = no symptoms; 3 = 4 or more warts per pod. Positive numbers indicate distances from the edge into the mulched plot; the negative number indicates the distance from the edge outside the mulched plot. MBr = methyl bromide; Within each soil treatment, different letters indicate significant differences among distances from the edge of the film according to Student’s test (P = 0.05).
parameters in the first crop after disinfestation, was also evident. Thus, population density of streptomycetes in the nonmulched sections of the plots was 4.3-5.3 log cfu g-’ soil and was reduced by the three treatments by 26-40%, 4658% and 68-S3% at 15, 60 and 120 cm, respectively, inward from the edge of the mulched plots. Again, formalin alone was not significantly different from the non-mulched control, and was not effective in reducing streptomycete populations, which ranged, in this treatment from 5.2 to 5.6 log cfu g-r soil, at the two soil depths tested.
Discussion
100
80
-46 -5
Crop Protection 1995 Volume 14 Number 4
The border effect of soil solarization was expressed in a gradient of increasing effectiveness from the edge of the mulched plot inward, in respect to (a) control of the cyst nematode of wheat; (b) growth of the wheat plants; (c) control of pod wart in peanut as a second crop after solarization; and (d) reduction in populations of streptomycetes. The effect was nullified at a distance of approximately 60 cm inward from the edge of the polyethylene mulch, similar to the findings of Jacobsohn et al. (1980) on broomrape control. Percentage of plants infected by nematodes was significantly correlated with decreasing soil temperature (Figure I) from the middle toward the edge of the plot. The border effect can be partially explained by heat loss near the border. A similar border effect occurred in another study (Mahrer and Katan, 1981), in which there was a 24°C difference in temperature between the edge of the polyethylene mulch and the center of a 2-m bed. A twodimensional numerical model (Mahrer and Katan, 1981) enabled prediction of the spatial soil temperature regime. Observed and measured temperatures agreed to within 1°C. An additional explanation for the border effect would be the reinfestation of the solarized plots from the adjacent non-solarized infected area.
The border effect of soil solarization: A. Grinstein et a/. 6 I
8
I
-50
-25
0
25
50
75
100
125
0 -50
-25
0
25
50
75
100
125
mulch
edge
=
Distance
from
the
(crnI
Figure 6. Effect of three soil disinfestation treatments on populations of streptomycetes, outside and into the polyethylene-mulched plots, at two soil depths. Populations were determined 7 months after the termination of disinfestation. Location ‘0’ indicates the edge of the mulched plot. Positive numbers indicate distances from the edge into the mulched plot; negative numbers indicate distances from the edge outside the mulched plot. Symbols to the left of the zero line represent the nonmulched control plants adjacent to the respective treated ones. MBr = methyl bromide; S = solarized; For = formalin. At O-20 cm depth, the regression equation for solarized is Y = 1
[email protected] (6 = 0.99); for S + For is Y = 9.99f13868 (r’ = 0.824); and for S + MBr is Y = 15.08X0.5744 (r’ = 0.879). At 20-40 cm depth the equation of regression line representing solarized is Y = 8.97~-2g7 (I’ = 0.99); for S + For is Y = 8.46p.38g (,’ = 0.98); and for S + MBr is Y = 9.53K”.47 (r’ = 0.973).
The border effect is frequently identified by the presence of a strip of diseased plants along the edge of the mulched plot, showing partial or no control, as demonstrated elsewhere with broomrape (Jacobshon et al., 1980). The width of the border effect narrows as the pathogen’s susceptibility to heat increases, or if the inoculum is concentrated in the upper soil layer (Grinstein and Hetzroni, 1991). Thus, all microsclerotia of Verticillium dahliae were killed at a depth of 10 cm within a few days at both 100 and 10 cm inside the mulch, but a depth of 30 cm, the respective mortality percentages was 99% and only 20% (Mahrer and Katan, 1981). The pink root disease in onions is effectively controlled by strip solarization without any obvious border effect (Katan et al., 1980), probably
because the causal agent, Pyrenochaeta terestris, is easily controlled by soil solarization. The border effect was less marked in this study when solarization was combined with other agents. A border effect was recorded also after soil fumigation (Greenberger, Katan and Keren, 1986). This phenomenon is related to gas leakage from the edges of the mulched plots (Klein, 1989). In contrast to the effect on broomrape, no border effect on weeds was observed (Jacobsohn et al., 1980). Horowitz, Regev and Herzlinger (1983), found that the soil temperature at 15 cm depth was 5°C lower under narrow (20-100 cm) than under wider strips. At 5-cm depth, the width of plastic sheeting had no effect on soil temperature. Control of annual weeds was complete under plastic at all widths, apparently since under these conditions solarization was used effectively in controlling the weeds. Increasing the width of the mulched plot reduces the proportion of the border and consequently the proportion of the partially solarized strips (Grinstein and Hetzroni, 1991). In most instances it is necessary to use a wide mulch to ensure the control of the pathogens to the desired depth. Moreover, a continuous mulch, which also reduces the probability of reinfestation, is especially desirable in order to obtain a long term effect by solarization for more than one crop. In the present study, a significant reduction in streptomycete population by solarization was evident 7 months after solarization, despite the fact that the soil was exposed to common agricultural practices and consequently to reinfestation. Possibly, a shift in microbial population was created, thus delaying reinfestation (Gamliel and Katan, 1993). Reduction in peanut wart disease by disinfestation (Figure 5) was associated with a concomitant decrease in streptomycete population (Figure 6). It is not known, however, what was the proportion of the pathogenic population of streptomycetes in the population of streptomycetes determined in our study. In a different study (Gamliel and Katan, 1991), in a soil with no history of diseases caused by actinomycetes, solarization did not have a significant effect on the actinomycete population. The border effect was found to become less pronounced when solarization was combined with fumigants. Solarization in combination with formaldehyde improved the control of Pythium ultimum but not of nematodes (Stapleton et al., 1987); it also increased the yield of carrots. Integrated control programs were recommended to achieve effective control of H. avelzae in cereals (Brown, 1984). Combined methods of control employing pesticides at reduced dosages, should be the goal, since they might increase and extend the control effectiveness and broaden the spectrum of target pests which can be controlled. Acknowledgements
We thank members of the team working on soil disinfestation in the Hevel-Ma’on area for their help; and the Israel Council of the Vegetable Growers and the Israel Union of Peanut Growers, for partial financial support of this research. We also thank Yehudit Riben and Bracha Steiner for their assistance in conducting the research. Contribution No. 3536-E,
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1995
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14 Number
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The border effect of soil solarization: A. Grinstein et al.
series, from the Agriculture Research Organization, The Volcani Center, Bet Dagan, Israel. 1994
Brown, R.H. (1984) Cereal cyst nematode Australia. PI. Dis. 68, 922-928
and its chemical control in
A. and Katan, J. (1991) Involvement of fluorescent pseudomonads and other microorganisms in increased growth response of plants in solarized soils. Phytopathology 81, 494-502 Gamliel,
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Greenberger, A., Katan, J. and Keren, Y. (1986) Control of Fusarium wilt in watermelon. (Abstr). Biol. Cult. Tests Contr. 1, 25 Grinstein,
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Received 28 March 1994 Revised 14 October 1994 Accepted 24 October 1994
