Effects of a range of fungicides used to control rust (Uromyces viciae-fabae) on spring-sown field beans (Vicia faba) in the UK

In two years a range of fungicides was used to control rust (Uromyces viciae-fabae) on spring-sown field beans (Vicia faba). In 1984 fenpropimorph, maneb, mancozeb, maneb plus mancozeb, propiconazole, thiram, triadimefon and zineb-polyethylene thiuram disulphide plus tridemorph were applied once or twice and compared with no fungicide. In 1985 the experiment was repeated with the addition of benodanil. In 1984 there was very little rust or other foliar disease and the effects of treatments on yield were small. In 1985 rust was first found in late June and became severe on untreated plots in August. All the fungicides reduced the incidence of rust but the amount of control differed with frequency of application and on different dates of disease assessment. In 1985 chocolate spot (Botrytis fabae) also became severe in late August. All the fungicides reduced chocolate spot but maneb plus mancozeb was the most effective. In 1985 yields were increased from 5 · 6 to 7 · 7 t/ha by maneb plus mancozeb applied once. Other fungicides gave smaller yield increases. Yield increases given were attributable mainly to control of rust but partly to control of chocolate spot. Yield increases given by non-systemic fungicides came mainly from increases in the weight of individual grains; those yield increases given by systemic materials came, in addition, from better pod retention.     

Ranking of partial resistance to common rust (Puccinia sorghi Schr.) in 30 sweet corn (Zea mays) hybrids

Thirty sweet corn (Zea mays) hybrids were evaluated for resistance to rust (Puccinia sorghi Schr.) for 3 years in an artificially inoculated field plot at St Paul, Minnesota. On the basis of visual determinations of rust severity, these cultivars were ranked according to level of resistance. All of them allowed at least some rust to develop, but the degree of final rust severity varied considerably. Correlation coefficients between the rust severity scores recorded for the hybrids in all three pairs of years were 0·68 (1978–1979), 0·79 (1978–1980) and 0·86 (1979–1980). Spearman's rank correlation coefficients were 0·76, 0·89 and 0·82, respectively, for the same comparisons.     

Chemical control of faba bean rust (Uromyces viciae-fabae)

The effectiveness of eleven foliar-applied fungicides on faba bean rust (Uromyces viciae-fabae (Pers.) J. Schröt.) and on the seed yield of faba bean (Vicia faba L.) were studied in growth chambers and in the field in Spain. Fungicides were tested at recommended and reduced rates. All the fungicides tested provided very effective preventive control in the growth chamber studies. Triazoles (difenoconazol, epoxiconazol, tebuconazol) and their mixtures with benzimidazoles (carbendazim-flutriafol and carbendazim-flusilazole) provided the most effective curative effect, even at 25% of recommended concentrations. They were followed by dithiocarbamates, copper dithiocarbamate mixture, carboxamide and chlorothalonil. Triazoles, benzimidazole-triazole mixtures and carboxamide maintained their effect until 15 days after fungicide application.Under field conditions, rust infection caused 22-26% yield reduction. All fungicides except mancozeb caused a significant decrease in disease severity under field conditions, but only treatments with triazoles and benzimidazole-triazole mixtures provided significant yield increases (22.7-15.6%) when applied twice. Three applications of oxycarbosin or copper-mancozeb were needed to provide a significant yield increase. Dithiocarbamates (thiram, maneb or mancozeb) or chlorothalonil reduced rust severity but did not provide a significant yield increase.     

The effect of systemic fungicides,applied as seed treatments or early foliar sprays,on speckled leaf blotch of wheat, Mycosphaerella graminicola (Fuckel) Schroeter

Control of speckled leaf blotch (Mycosphaerella graminicola) of wheat (Triticum aestivum) with systemic fungicides used as seed treatments or foliar sprays was studied. Seed treatment with either imazalil (1·125 g/kg seed), nuarimol (0·2 g/kg), thiabendazole (1·5 and 0·75 g/kg), triadimenol (0·3 and 0·2 g/kg) or Shell WL 47675 (0·415 g/kg) reduced the number of infected plants by 28–62% when assessments were made 11 weeks after sowing. Thiabendazole (1·5 and 0·75 g/kg) and triadimefon (0·3 and 0·2 g/kg) slowed the development of speckled leaf blotch for c. 15 weeks after sowing. A single foliar spray of triadimefon (125 g/ha) slowed the development of the disease for c. 10 weeks, but a single spray of benomyl controlled it for the whole season. There was no additional benefit from combining a seed treatment of triadimefon (0·225 g/kg) with the foliar spray.     

Agronomic and economic assessment of intensive pest management of dry bean (Phaseolus vulgaris)

Two common production constraints of dry bean (Phaseolus vulgaris) in Ontario are annual weeds and anthracnose (caused by Colletotrichum lindemuthianum). Dry bean is not considered a competitive crop and weed interference can result in substantial yield losses, while anthracnose is considered one of the most devastating diseases in dry bean production. A study conducted in Ontario Canada, examined the effect of two herbicide programs on weed management, thiamethoxam insecticide treatment on plant enhancement and three fungicide programs on anthracnose development in a navy bean cv. ‘OAC Rex’. The premium herbicide program (s-metolachlor + imazethapyr) reduced percent weed ground cover relative to the economic herbicide program (trifluralin) in five of six locations. Thiamethoxam increased emergence and vigour at only one location, which contradicts reported benefits of thiamethoxam on plant health. The herbicide or thiamethoxam treatments did not affect anthracnose disease severity, visible seed quality, net yield or economic return. The fungicide seed treatment was often superior to the untreated control, for a number of the parameters measured. The application date of the foliar fungicide, relative to the onset of disease, varied between site-years. This dramatically influenced the fungicide’s effectiveness. Foliar fungicides increased seed quality and net economic return compared to the control when applied prior to disease development. The combination of fungicide seed treatment followed by a foliar fungicide provided the largest reduction in anthracnose severity.     

Disease control and yield improvement obtained with fungicide applications to spring wheat in South Africa

Foliar sprays containing benzimidazole and/or compounds inhibiting ergosterol biosynthesis in fungi were tested for their effects on spring wheat at different localities in the Western Cape Province, South Africa. Fungicide treatments caused significant yield increases (P > 0.05) at most of the localities where plants were sprayed at Zadok's growth stages 31 and 39. Plants were affected by physiological leaf spot, which occurred in all localities. At one locality good control of leaf rust (Pucdnia recondita f.sp. tritici) was obtained with triazole-containing sprays, but not with imidazole- and benzimidazole-containing sprays. Lodging due to eyespot (Tapesia yallundae) occurred at another locality where it was partially controlled by benzimidazole-containing fungicide mixtures. Fungicides had little or no effect on yield when planting date was delayed or when the rate of soil-applied nitrogen on fallow land was increased. The study indicates that in the Western Cape Province growth regulating effects, rather than fungicidal effects of fungicides, are often responsible for yield increases of wheat.     

Epidemiology and control of chocolate spot (Botrytis fabae) on winter field beans (Vicia faba)

Dispersal of Botrytis fabae spores and the development of chocolate spot lesions were monitored in crops of winter-sown field beans during the 1980/81, 1981/82 and 1982/83 seasons. The greatest numbers of B. fabae spores collected on horizontal sticky slides exposed in crops were associated with periods of heavy rain and numbers declined if weather was dry. Amounts of chocolate spot developing on leaves of young potted plants exposed in crops near the slides followed the same trends as spore numbers. When numerous spores were collected, abundant chocolate spot developed on the leaves of tagged plants in the crops. Benomyl sprays were applied (at 0·5 kg a.i./ha) either after peaks in numbers of spores collected or according to crop growth stages. Sprays applied in January–April generally gave no increase in yield whether or not timed according to spore peaks. The greatest yields were obtained from plots sprayed in late May, at the midflowering stage of crop growth.     

Grain weight response to foliar diseases control in wheat (Triticum aestivum L.)

Foliar diseases are the main biotic restriction reducing yield in wheat crops affecting both, grain number and/or grain weight, depending on developmental stage at which infection occurs (pre- or post-anthesis, respectively). Grain weight reductions due to foliar diseases were widely reported in the literature mostly associated with decreases on radiation interception during the grain filling period. However, different evidences in wheat showed variations on grain weight responses when fungicide was applied during the grain filling period, probably associated with the timing of fungicide application or with the amount of available resources per grain set when fungicides are applied. The present study was designed to determine the causes of grain weight reduction due to foliar diseases complex (including leaf rust, Septoria leaf blotch and tan spot) in wheat crops growing under contrasting agronomic and environmental conditions (i.e. different years, locations, cultivars and N supply). The experiments were carried out during 4 years under field conditions in different locations of Argentine and France. Five different commercial wheat cultivars were sown on early and late sowing dates; and two contrasting N availability and two fungicide treatments (protected and unprotected) were applied. Grain number was not affected by foliar diseases as their appeared after anthesis. Grain weight was strongly, poorly or not affected by foliar diseases and was not associated individually with both, the sink size and the source size. However, when the grain weight response due to fungicide application was plotted against the healthy area absorption per grain (HAA_G), a significant negative association (r² = 0.81; p < 0.0001) was found for the Argentine experiments. When the HAA_G was corrected by the grain weight potential (HAA_GW) all experiments conduced in Argentine and in France fit well to a common negative linear regression (r² = 0.74, p < 0.0001) for the relationship between grain weight variation and HAA_GW demonstrating that grain weight potential is an important feature to consider in diseases control programs. Foliar diseases forced the crop to use the accumulated reserved increasing the utilization rate of the water soluble carbohydrates (WSC_UR), depleting as a consequence the water soluble content at physiological maturity (WSC_PM) in all experiments. The association between WSC_UR and the healthy area absorption per grain corrected by grain weight of healthy crops (HAA_GW) suggest that foliar diseases in wheat cause source limitation, forcing to the crop to use the WSC reserve which could be insufficient to fill the grains previously formed.     

Crown rust affects plant performance and interference ability of Italian ryegrass in the post-epidemic generation

This study investigated the effects of crown rust (caused by Puccinia coronata f.sp. lolii) in Italian ryegrass (Lolium multiflorum) on plant growth and interference ability in the post‐epidemic generation. Rust‐infected Italian ryegrass plants produced seeds with lower weights than healthy plants (on average proportionately 0·36 lower). Furthermore, the early seedling vigour of plants from rust‐infected parents was reduced by 0·50 compared with seed from healthy parents. Under disease‐free conditions in the post‐epidemic generation, mature plants from rust‐infected parents produced 0·13 less shoot dry weight, 0·15 fewer tillers, 0·11 fewer leaves and 0·25 less leaf area than plants from healthy parents. As such, the effects of crown rust in Italian ryegrass persisted for at least one post‐epidemic generation. Under rust‐infected conditions in the post‐epidemic generation, however, the difference in performance between plants from healthy and rust‐infected parents was minimal. This suggests that any advantage plants gain from having healthy parents, compared with rust‐infected parents, might be lost when the post‐epidemic generation is grown under conditions of rust infection. Irrespective of rust infection in the post‐epidemic generation, the magnitude of intra‐specific interference was greater in monocultures of Italian ryegrass from healthy parents than in those from rust‐infected parents. However, the magnitude of inter‐specific interference and the relative interference ability of plants of rust‐infected and healthy parents did not differ. The implications of these results to plant ecology are discussed.     

The response of onion couch (Arrhenatherum elatius ssp. bulbosum (Willd.) Schub. & Mart.) to glyphosate and other foliage-applied herbicides

Two experiments were conducted in which foliage-acting herbicides were applied to onion couch (Arrhenatherum elatius ssp. bulbosum). The first experiment used an artificially established population and investigated the effects of straw burning on the performance of autumn-applied treatments of glyphosate, aminotriazole and dalapon. Spring barley was sown without cultivation. The second experiment, conducted on a naturally occurring infestation, compared the performance of glyphosate applied at alternative autumn timings and a single application of flamprop-methyl made in the following spring, in the presence of a winter wheat crop. In the first experiment, assessments made after 15 months suggested that straw burning resulted in increased growth. In addition, herbicide performance appeared less effective after burning. In neither case were these differences significant. Glyphosate at 1·0 and 1·5 kg a.e./ha gave the highest levels of control of both shoots and bulbs, whereas control from aminotriazole at all doses (1·5, 3·0 and 4·5 kg a.i./ha) was poor. Dalapon (6·0, 12·0 and 18·0 kg a.i./ha) was intermediate in its effectiveness. In the second experiment, applications of glyphosate at 1·0 and 1·5 kg a.e./ha resulted in high levels of control of both shoots and bulbs with no significant difference between application dates. Flamprop-methyl at 0·52 kg a.i./ha reduced the number of shoots and bulbs by 55% and 58% respectively. All treatments significantly reduced the number of inflorescences present in July.     

Commercial monitoring programme for the onion fly,Delia antiqua (Meigen) (Diptera: Anthomyiidae) in south-western British Columbia

Onion flies were monitored from 1979 to 1982 using onion-baited yellow sticky traps in 88 commercial onion fields in the lower Fraser Valley of British Columbia. Capture rates of onion flies varied widely both between and within farms, and within fields both spatially and temporally. An action threshold of 0·1 females/trap/day was used to determine the need for first-generation adult sprays, beginning 30 days after seeding with a granular insecticide. Over the 4 years, growers applied an average of 1·6 sprays per field per season using the monitoring programme, compared with averages of 4·4 or 9·3 sprays had they followed a production guide schedule, or a 10-day fixed routine schedule, respectively. Growers adhering to the established monitoring programme achieved excellent control of maggot damage.     

Insecticidal control and economic damage of sugar-cane wireworm, Agrypnus variabilis (Candèze) in maize

Six insecticides (terbufos, chlorpyrifos, ethoprophos, phoxim, bendiocarb and lindane) were tested against sugar-cane wireworm Agrypnus variabilis (Candèze) in maize. The first five insecticides were applied at 0·25, 0·5 and 0·75 kg a.i./ha (2·27, 4·55 and 6·82 g a.i./100 metres of row) while lindane was applied at the registered rate of 0·29 kg a.i./ha (2·64 g a.i./100 metres of row). In addition, bendiocarb was applied as a seed treatment at the rates of 4000 and 6000 ppm. The insecticides were applied at planting as insecticide/fertilizer mixtures in 18 cm wide bands, 2–3 cm below the soil surface and 3–5 cm above the seed. The minimum effective rates were terbufos 0·25 kg a.i./ha, lindane 0·25 kg a.i./ha and chlorpyrifos and ethoprophos both at 0·5 kg a.i./ha. These treatments gave at least 5 weeks' residual control. The bendiocarb seed treatments were phytotoxic. No residues were detected in the cobs or grain at the minimum effective rates of the terbufos, chlorpyrifos or ethoprophos treatments. The positive linear relationship between plant population and yield showed that for every 1% loss in plant stand below the optimum range of 60 000–65 000 plants/ha, there is a 0·72-0·74% loss in grain yield respectively. Control measures would be economically justified with only a 2·3% reduction in plant stand (assuming $20/ha for cost of control and $150/tonne for maize).     

Residual effects of Bacillus thuringiensis and chemical insecticide treatments on spruce budworm (Choristoneura fumiferana Clemens)

Biochemical analysis has shown that populations of Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae) which have survived treatment with chemical insecticides (organophosphates and carbamates) have greater energy reserves and vigour than those from untreated areas: conversely, populations surviving treatment with Bacillus thuringiensis (B.t.) are considerably less vigorous. Surviving pupae from untreated, organophosphate-treated and B.t.-treated areas gave, respectively, the following results on biochemical analysis: average weight, 69·5, 82·0 and 46·9 mg; Ca⁺⁺, 80·0, 114·5 and 54·0 mg/kg; total proteins, 27·6, 36·3 and 19·2 mg/kg; 3-hydroxybutyrate dehydrogenase, 1004, 1091 and 200 mU/g; alkaline phosphatase, 541, 580 and 251 mU/g. These analyses confirmed that B.t. treatments had a detrimental effect on survivors whereas chemical insecticides encouraged the resurgence of vigorous populations of C. fumiferana as a result of stimulation of these insects by sub-lethal dosages (hormoligosis).     

Biological efficiency of small dicofol droplets against Tetranychus urticae (Koch) eggs,larvae and protonymphs

The influence of droplet size on mortality of Tetranychus urticae (Koch) eggs, larvae and protonymphs was evaluated using sprays of an oil-based 1% dicofol formulation. The effect of using droplets of fixed size but with varying concentrations of active ingredient (0·05%–4·0% a.i.) against eggs was also investigated. The relationship between LD₅₀ and droplet diameter was positive and curvilinear for the three developmental stages, with an LD₅₀ of 12 ng/cm² against eggs, 5 ng/cm² against larvae and 6 ng/cm² against protonymphs when applying droplets of diameter 20 μm. With droplets of 40 μm the LD₅₀ value increased approximately twofold against larvae, protonymphs and eggs: thus an increase in droplet size generally resulted in a decrease in the efficiency of sprays against all three developmental stages of the mite. A U-shaped relationship was found between LD₅₀ and concentration with a theoretical optimum concentration of 1·18% a.i. (i.e. 11·8 g/litre) when droplets of a fixed size were used against eggs. The efficiency of droplets with the minimum active ingredient content (0·05% a.i.) and those with the maximum concentration (4·0% a.i.) was reduced by more than three times compared with the efficiency of droplets with the optimum concentration.     

Efficacy of Bacillus subtilis MBI 600 Against Sheath Blight Caused by Rhizoctonia solani and on Growth and Yield of Rice

Rice sheath blight disease(ShB),caused by Rhizoctonia solani,gives rise to significant grain yield losses.The present study evaluated the efficacy of Integral?,the commercial liquid formulation of Bacillus subtilis strain MBI 600,against rice ShB and for plant growth promotion.In greenhouse studies,four log concentrations of Integral(from 2.2×106 to 2.2×109 cfu/mL) were used as seed treatment(ST).After 25 d,seedlings were dipped(SD) into Integral prior to transplanting.At 30 d after transplanting(DAT),leaf sheaths were inoculated with immature sclerotia of the pathogen.At 45 DAT,a foliar spray(FS) with Integral was applied to some treatments.The fungicide control was 50% carbendazim at 1.0 g/L,and a nontreated control was also included.Overall,there were 10 treatments,each with five replications.ShB severity was rated at 52 DAT,and seedling height and number of tillers per plant were rated at 60 DAT.In 2009,two field trials evaluated Integral at 2.2×108 and 2.2×109 cfu/mL.Integral was applied as ST,and seedlings were produced in a nursery bed.After 32 d,seedlings were treated with Integral as SD and transplanted into 10 m2 blocks.Foliar sprays were given at 45 and 60 DAT.There were seven treatments,each with eight replications arranged as a factorial randomized complete block design.At 20 DAT,the plots were broadcast inoculated with R.solani produced on rice grains.Seedling height before transplanting,ShB severity at 90 DAT,and grain yield at harvest were recorded.Integral at 2.2×109 cfu/mL provided significant increase of seedling heights over other treatments under greenhouse conditions.The Integral treatments of ST + SD + FS at 2.2×109 cfu/mL significantly suppressed ShB over other treatments.In field studies,Integral provided significant increase of seedling height in nursery,and number of tillers per plant,compared with the control.ShB severity was significantly suppressed with higher concentrations of Integral compared to lower concentrations.Grain yield were the highest at an Integral concentration of 2.2×109 cfu/mL.Overall,Integral significantly reduced ShB severity,enhanced seedling growth,number of tillers per plant and grain yield as ST + SD + FS at the concentration of 2.2×109 cfu/mL under the conditions evaluated.     

Increased severity of Rhizoctonia root rot in sugar beet treated with systemic insecticides

The systemic insecticides aldicarb 15G (15% granules) at 2·6 kg active ingredient (a.i.)/ha and phorate 1OG (10% granules) at 1·7 kg a.i./ha, applied as side dressings about 1 month after planting in 1979 and 1980, significantly increased the severity of root rot, caused by Rhizoctonia solani Kühn, in sugar beet (Beta vulgaris L.). Carbofuran 10G (10% granules) at 2·2 kg a.i./ha also increased root rot, but not significantly. Numbers of harvestable roots were reduced by all treatments but significantly by phorate only. Both aldicarb and phorate were slightly fungistatic to R. solani when the pathogen was grown on potato-dextrose agar incorporating 0·5, 5·0, and 25 μg a.i./ml. Trichoderma sp., a potential antagonist of Rhizoctonia, was slightly inhibited by aldicarb and phorate initially, but soon overcame the effect. Thus, the increase in disease severity in the field may be attributable to some metabolic or physiological effect of the chemicals either on the host or on the infection process of the pathogen. Indiscriminate use of these insecticides should therefore be avoided in areas where root rot is prevalent but where insects are not a problem.     

Efficacy and timing of application of fungicides for plum rust control in Turkey

Little information is available regarding the efficacy and timing of modern fungicides for the control of plum rust. In this study, modern fungicides, including triazoles, strobilurins, and benzimidazoles, and a classic fungicide (chlorothalonil) were tested under both greenhouse and field experiments. Excellent disease control was obtained by the pre-inoculation application of all fungicides in the greenhouse experiments. However, only triazoles (tebuconazole, difenoconazole, flusilazole and hexaconazole) were effective when applied up to 6 or 7 days after inoculation. Unexpectedly, strobilurins (azoxystrobin and trifloxystrobin) and benzimidazoles (thiophanate-methyl and carbendazim) were not effective when applied at any time after inoculation. In field experiments, early-season fungicide applications effectively inhibited primary infections; however, these applications provided no apparent benefit in preventing secondary infections and premature leaf fall later in the season. All fungicides provided better control over the season and significantly reduced premature leaf fall when applied immediately after rust pustules (uredinia) were first observed. It is suggested that all fungicides tested in this study can be used for plum rust control. Disease monitoring data indicated that disease severity remained low throughout May but dramatically increased in early June. Mid-May is evidently the best time to initiate fungicide sprays against rust on plums; the spray should be applied before or when rust pustules are first observed and when rust is present at low severity.     

Integration of environmental and fungicidal control of Bremia lactucae in a glasshouse lettuce crop

The effect of combinations of night ventilation regimes and fungicide treatments on levels of downy mildew (Bremia lactucae) was investigated in a November-sown glasshouse lettuce crop. Environmental regimes were allocated in a randomized-block design to glasshouse compartments in which fungicide treatments were applied to sub-plots from which sub-sub-plots were harvested on each of four dates. The benefit of night ventilation was strikingly shown. It was almost as effective as the systemic fungicide metalaxyl which gave complete control of B. lactucae for 20 weeks after its incorporation in the peat blocks. A ‘heat-purging’ treatment increased the incidence of downy mildew, while other fungicidal regimes failed to control late infection. In the period close to cutting, when fungicide usage is not permitted, night ventilation may be the only effective means of controlling the disease.     

Toxicity of biofungicide Timorex 66 EC to Cladobotryum dendroides and Agaricus bisporus

Twenty Cladobotryum dendroides isolates from Serbian Agaricus bisporus farms collected during 2003–2007, and the F56 strain of A. bisporus were tested in vitro for sensitivity to tea tree oil (Timorex 66 EC), a biofungicide, in comparison with prochloraz–manganese (Octave WP). The efficacies of tea tree oil and prochloraz–manganese were evaluated in a mushroom growing room, after application at standard product application rates and a combination of the two at respective proportion of 20:80%. Tea tree oil was considerably less toxic than prochloraz–manganese in vitro to C. dendroides isolates (ED₅₀ 112.9–335.8 mg l⁻¹) and A. bisporus F56 strain (98.0 mg l⁻¹), although neither fungicide was lethal to the pathogen. The biological efficiency of tea tree oil was higher than in treatments with the reference formulation of prochloraz–manganese. In our in vivo trials, no negative interference of the biofungicide with A. bisporus physiology was observed. Tea tree oil applied at the standard product application rate caused a significant reduction in cobweb disease levels in the A. bisporus growing room. Timorex 66 EC should be tested further in combination with other biofungicides to investigate the effectiveness of various mixtures for A. bisporus disease control.     

Effect of leaf rust (Puccinia recondita tritici) on yield of four isogenic wheat lines

Four near-isogenic lines of wheat (Thatcher and Thatcher backcross lines with Lr3b, Lr16 and Lr9) with different levels of resistance to leaf rust were used to determine the relationship between disease severity and yield loss in the greenhouse and the field. Larger areas under the disease progress curves were associated with greater host susceptibility. Yield losses varied from 21% to 47% depending on the susceptibility of the lines and the severity of the epidemics. Yield loss measured varied with the size of the plot harvested. Percentage yield losses from each line in the field experiment were pooled and regressed against the area under the disease progress curve of disease severity on flag leaf and the mean severity per tiller. Both regressions were highly significant with R² of 0·89 and 0·88, respectively. The ratio of area under the disease progress curve to yield loss was higher in susceptible than resistant cultivars; thus it was difficult to ascertain whether a linear or S-shaped curve was more appropriate for the disease-loss relationship.