Published in Agron J 100:315-319 (2008)
DOI: 10.2134/agrojnl2007.0127
© 2008 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
ECONOMIC ANALYSIS
Estimates of Yield and Economic Losses Associated with White Mold of Rain-Fed Dry Bean in North Dakota
Harikrishnan Ramasubramaniama,
Luis E. del Río Mendozaa,* and
Carl A. Bradleyb
a Dep. of Plant Pathology, North Dakota State Univ., Fargo, ND 58105
b Dep. of Crop Sci., Univ. of Illiniois, Urbana-Champaign
* Corresponding author (luis.delrio-mendoza{at}ndsu.edu).
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ABSTRACT
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White mold [caused by Sclerotinia sclerotiorum (Lib.) de Bary] of dry bean (Phaseolus vulgaris L.) is the most important disease affecting rain-fed dry bean yields in North Dakota. The economic impact of this disease in North Dakota was assessed through a survey of 250 fields during 2003 to 2005. Profitability of a single fungicide application for managing white mold also was examined. Yield loss was estimated by regressing the mean of white mold incidence per county and year with their respective estimated yields. The slope of the resulting equation was divided by its intercept and multiplied by 100 to express yield loss as percentage of potential yield. Results of these analyses indicated a loss of 14 kg/ha (R2 0.50; P = 0.03) for every unit of white mold incidence, which represents a loss of 0.8% of the potential yield. The economic impact of the disease was assessed using estimated hectarage with 
20% white mold incidence in conjunction with yield and dry bean prices as reported by National Agricultural Statistics Service for North Dakota. Over the 3-yr period, economic losses were highest in 2004 and least in 2005; yield loss averaged 524 kg/ha (range 424–722 kg/ha) while production loss averaged 6433 t (range 1,789–10,438 t). These represent an average economic loss of U.S. $2.95 million (range U.S. $0.63–5.74 million) per year. Profitability of a fungicide application to manage white mold was heavily influenced by disease pressure and varied widely between years. In years with high disease pressure, like 2004, a fungicide application would have produced a positive return in 72% of the fields; but under moderate to low disease-pressure, applying fungicides would have been a profitable practice in <20% of fields.
Abbreviations: NASS, National Agricultural Statistics Service • PL, loss of potential yield
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Received for publication April 9, 2007.
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INTRODUCTION
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NORTH DAKOTA is the U.S. leader both in area cultivated and production of dry bean. Dry bean is primarily grown as a rain-fed crop in North Dakota with minor production under irrigation. Approximately, 216,000 ha per annum are harvested in the state, with an estimated production value of U.S. $120 million. The counties of Grand Forks, Pembina, and Walsh along the Red River Valley dominate with little over 60% of the planted dry bean hectarage in the state (USDA- National Agricultural Statistics Service, 2004, 2005, 2006).
White mold is endemic in North Dakota with an annual mean incidence (% of plants affected) ranging from 6 to 28% (Harikrishnan et al., 2006a), and is the leading disease constraint for dry bean production in the state (Bradley and Luecke, 2004). Sclerotia produced by S. sclerotiorum enable the fungus to overwinter for long periods of time. Under prolonged cool and wet conditions, sclerotia germinate by producing apothecia. Epiphytotics of white mold usually start when ascospores, released by the apothecia, cause infection as and when they land on senescent tissues or blossoms (Abawi and Grogan, 1979). Field symptoms exhibit random wilting of plants generally in groups. Upon closer examination of the affected plants, characteristic fluffy white mycelial growth can be seen at the point of infection. Sclerotia are produced both on the outer and inner surfaces of infected plants. These sclerotia are deposited on the ground during the harvesting process and can thus increase the soil-borne inoculum over time. Sclerotia are reported to survive between 5 and 7 yr in soil (Abawi and Grogan, 1979).
As complete physiological resistance to white mold is presently lacking, recommendations to manage white mold in dry bean include the use of wider row spacing (Blad et al., 1978) and growing cultivars with upright architecture (Schwartz and Steadman, 1978) to avoid favorable conditions for disease development under rain-fed conditions. However, the most commonly used control measure is the application of fungicides at flowering (McMullen and Bradley, 2006).
The intensity of white mold epiphytotics is heavily influenced by weather variables. This variability causes uncertainty in making decisions about control measures especially fungicide usage for disease management. This uncertainty is further compounded by financial risks especially with annually fluctuating dry bean market prices. More often, these risks and final selling price of dry bean are unknown at the time of decision-making. Knowledge about the expected returns from use of fungicide would benefit the producers and other agriculture-related industries by enabling them to make better-informed decisions, thereby increasing their probability of making a profit.
The economic impact of white mold on the dry bean industry in North Dakota has not been explored thoroughly. Gross estimates of several 10s of millions of dollars in losses have been attributed to white mold in the early 1990s, while in 2003 yield losses were estimated at about $1.9 million for pinto beans using a yield loss model developed at a research center (del Río et al., 2004b). Researchers and agriculture economists have used several methods to estimate production and economic losses due to diseases in other crops; such methods include field surveys, production statistics, and questionnaires for growers, extension specialists, and field workers (Cowger and Sutton, 2005; Lamey et al., 2001; Wrather et al., 2003). In this report, we assess the yield, production, and economic impacts of white mold on dry bean production primarily under rain-fed conditions in North Dakota using white mold incidence data collected by field surveys and using production statistics obtained from the National Agricultural Statistics Service (NASS) (USDA-NASS, 2004, 2005, 2006). Additionally, percentage of fields above economic action threshold also was calculated.
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MATERIALS AND METHODS
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White Mold Incidence Assessment
Data on white mold incidence (% of plants infected) was obtained by conducting surveys of 250 rain-fed dry bean fields over a 3-yr period from 2003 to 2005. These field surveys were predominantly conducted in the counties of Grand Forks, Pembina, and Walsh along the Red River Valley. In each field, 10 sampling sites were observed in a zigzag manner with 20 plants per site to give a total of 200 plants per field. Each site was spaced at least 10 m apart. Fields were selected arbitrarily with a distance of 2 to 3 km from each other and geo-referenced using GPS coordinates. Surveys were conducted during late August to early September of each year when the crop was in R4 to R6 (mid- to late pod filling) growth stage.
Yield, Production, and Economic Loss Assessments
Impact of white mold on yield is expressed as loss of potential yield (PL). To calculate loss of potential yield, first we developed a regression equation wherein, white mold incidence across fields within each county were averaged per year and regressed against estimated yield for each county per year as reported by NASS (USDA-NASS, 2004, 2005, 2006). Then we arrived at PL by dividing the slope by the intercept of the above equation and multiplying by 100.
Yield loss (L; kg/ha) due to white mold incidence was calculated using the following equation:
where WMI represents mean white mold incidence (%), PL represents loss of potential yield per unit of white mold incidence (kg/ha/incidence), and YLD represents estimated average yield per county in kg/ha. Then the yield loss per hectare was multiplied by the estimated dry bean hectarage with 20% white mold incidence to arrive at production loss. In this article we used 20% white mold incidence as the damage boundary (DB) (Higley and Pedigo, 1996) because white mold incidences below 20% gave inconsistent and nonsignificant yield loss relationships in our observations. Similar lack of significant yield loss relationship under low white mold incidence has been observed previously in S. sclerotiorum–soybean system (Yang et al., 1999). Economic loss was assessed by multiplying price of dry bean with production loss. These calculations were made for each county per year.
Profitability
The cost of single fungicide application for managing white mold was estimated to be U.S. $50/ha. This estimated cost of a single fungicide application was derived following consultations with area growers and commercial fungicide applicators. The estimated cost of a single fungicide application includes both the costs of the fungicide and its application, but excludes machinery depreciation. The current recommendation for white mold management in our rain-fed dry bean production system is a single fungicide spray at 30 to 50% flowering. Currently, thiophanate-methyl (Topsin), iprodione (Rovral), and boscalid (Endura) are the only fungicides registered to manage white mold in dry bean in North Dakota. Of these three, thiophanate-methyl is the most popular (McMullen and Bradley, 2006).
The concept of economic injury level (EIL) as defined by Higley and Pedigo (1996) was adapted to identify the level of white mold incidence required to offset the cost of protecting the plants with fungicides and was calculated using the following equation:
where C represents the cost of protecting the plants with a single fungicide application ($/ha), P represents the market value of a kg of dry bean ($/kg), YLDC represents the yearly average yield of each county (kg/ha), and PL represents loss of potential yield per unit of white mold incidence (kg/ha/incidence).
The economic action threshold (EAT), defined here as the level of disease above which the use of fungicides would provide a positive economic return, was calculated as
where EIL represents the economic injury level (incidence) and DB represents the damage boundary level (incidence).
The profitability of using fungicides to control white mold in each county and year was expressed as the percentage of fields within each county and year that had white mold incidences > EAT.
Data Analysis
Data were analyzed using general linear models procedure in SAS (version 9.1; SAS Institute, Cary, NC). The least significant difference test was used to make comparisons of white mold incidence, yield loss, production loss, economic loss, and profitability of fungicide applications for all 3 yr and three counties.
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RESULTS
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White Mold Incidence
Over the 3-yr period, white mold was detected (prevalence) in 72, 100, and 60% of the fields, respectively, in 2003, 2004, and 2005 and was significantly different among years. White mold incidence (% plants infected) differed significantly (P < 0.0001) between years and ranged from 0 to 96% in individual fields; with mean incidences of 14, 27, and 8%, in 2003, 2004, and 2005, respectively (Fig. 1
).
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Fig. 1. Prevalence (% of fields affected) and incidence (% of plants affected/field) of white mold in rain-fed dry bean fields during 2003 to 2005 in North Dakota.
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Yield, Production, and Economic Loss
Regression analysis indicated that for every unit of white mold incidence yield was reduced by 14 kg/ha (R2 = 0.50; P = 0.03; Fig. 2
); which is equivalent to 0.8% of the potential yield. Yield loss varied significantly (P < 0.0001) from year to year but not among counties. Over the 3-yr period, yield loss due to white mold incidence averaged (weighted by affected area) 524 kg/ha (range 424–722 kg/ha). Production loss varied significantly between years (P < 0.0001) and among counties within years (P < 0.0001). Production loss averaged 6,433 t with the range being 1,789 to 10,438 t. This production loss resulted in a total economic loss of U.S. $8.85 million over the 3 yr in the top three dry bean-growing counies of the state with an average of U.S. $2.95 million per year. Both, production and economic losses were greatest in 2004 followed by 2003 and 2005 (Table 1
).
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Fig. 2. Regression analysis showing the relationship between white mold incidence and yield loss in rain-fed dry bean.
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Table 1. Estimated production and economic losses associated with white mold incidence (WMI) of rain-fed dry bean in the top three dry bean growing counties during 2003 to 2005 growing seasons in North Dakota.
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Profitability
The EAT values did not change much between the 3 yr of the study even though the price of dry bean was one and a half times higher in 2004 than in 2003 and 2005 (Table 2
). Expressed differently, in 2004 an average dry bean yield increase of 91 kg/ha would have been required to recover the cost of fungicide application, whereas in 2003 and 2005 the required yield increase would have been 143 kg/ha. The profitability of using fungicides for control of white mold, however, varied wildly from year to year and exhibited significant year x county interaction (P < 0.0001) (Table 2). In 2004, a year with high disease pressure, applying fungicides for white mold control would have been a profitable activity in 60, 74, and 96% of the fields scouted in Grand Forks, Pembina, and Walsh counties, respectively (Table 2). In a year with moderate disease pressure, like 2003, fungicide use would have been profitable in 37, 10, and 13% of the fields in Grand Forks, Pembina, and Walsh counties, respectively, whereas in years with low disease pressure, like 2005, fungicide use would have provided a positive return in <10% of the fields. On average, fungicide use would have produced economic returns in 72, 20, and 4% of the scouted fields in 2004, 2003, and 2005, respectively.
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Table 2. Economic action threshold (EAT) levels for white mold incidence in rain-fed dry bean and fields above EAT levels during 2003 to 2005 growing seasons in the top three dry bean-producing counties of North Dakota.
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DISCUSSION
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Yield, production, and economic losses due to white mold incidence on rain-fed dry bean were estimated over 3 yr in the three predominant dry bean-growing counties of North Dakota. Based on these assessments, white mold incidence resulted in an average yield loss of 524 kg/ha, which equated to a total production loss of 19,299 t over the 3-yr period. This production loss was equivalent to U.S. $8.85 million over the same time. Even though we used potential yield loss per unit of white mold incidence and not the per se yield reduction per unit of white mold incidence as described by the regression equation (Fig. 2), our estimate of yield loss is similar to those that have been described previously in other S. sclerotiorum-host systems (Grau, 1988; Yang et al., 1999; Thomson et al., 1984) and in dry bean (del Río et al., 2004b; Kerr et al., 1978). However, unlike in the above reported studies where the yield loss estimates due to white mold were derived from field experimentation, our findings of yield loss estimate originated by conducting extensive field surveys covering areas that are representative of major rain-fed dry bean production centers in North Dakota. To arrive at estimates of production and economic losses, we used only area with white mold incidence 20% since below this level plants are able to compensate yield. In doing so, we were able to provide a more accurate and realistic estimate of production and economic losses and avoided overestimation of losses associated with white mold incidence. Nonetheless, white mold does cause extensive damage to the dry bean crop on a yearly basis in North Dakota. White mold is endemic to dry bean production areas in North Dakota. In most years, temperature and moisture are not limiting factors for white mold development in our region. Temperature averages in the mid-20s (+C) and adequate moisture is available during flowering and post flowering periods, which are essential for ascospore infection of senescent tissues or blossoms and for white mold development (Abawi and Grogan, 1979). Throughout the 2004 growing season, cooler than average temperatures increased the potential for greater white mold incidence, which resulted in higher white mold incidence and greater yield losses. The estimated economic impact of the disease was further aggravated by above-average dry bean market prices.
The decision to use fungicides to manage white mold is often difficult to make because the nature of the disease occurrence is so spatially and temporally variable. During the 3-yr survey, several fields, within the same county and year, were totally decimated, while others had low white mold incidence. This was especially evident in Grand Forks County where in 2004, a year with high disease pressure, a number of fields had WMI <10% while others had WMI >60%. Thus, even though white mold is endemic in North Dakota, spraying fungicide to manage the disease is not always economical. These observations reiterate the need for increased understanding of the epidemiology of white mold in general and more specifically of the spatial attributes of inoculum (sclerotial) distribution and ascospore dispersal and their relation to white mold development. Our analysis showed high variability in the profitability of a fungicide application for white mold control among years with 20, 72, and 4% of the fields inspected in 2003, 2004, and 2005, respectively, giving a positive return. These findings support the need for an integrated disease management approach to manage white mold including, the need for a white mold forecasting system which can aid growers in making fungicide spray decision in relation with prevailing and future (near) market price forecasts of dry bean; growing cultivars with erect architecture (Blad et al., 1978) and use of wider row spacing (Schwartz and Steadman, 1978), all of which helps eliminate favorable canopy conditions for white mold development; and the use of biological control agents that target the sclerotia of S. sclerotiorum, such as Coniothyrium minitans (Harikrishnan et al., 2006b) and Sporidesmium sclerotivorum (del Río et al., 2002). These biological entities have been found to remain active and survive for at least 2 yr in upper Midwest conditions (Martinson and del Río, 2001; del Río et al., 2004a). Yield, production, and economic loss estimates furnished here verify the importance of white mold on dry bean in North Dakota. Finally, white mold continues to be a management challenge and is one of the leading yield reducing and economic loss factors which hampers maximizing yield potential of dry bean in North Dakota.
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ACKNOWLEDGMENTS
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Research was partially supported by the USDA-ARS National Sclerotinia Initiative Program and Northarvest Bean Growers Association.
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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ABSTRACT
INTRODUCTION
