Abstract in Spanish

About the Author

Neil C. Gudmestad received his Ph.D. in Plant Pathology from North Dakota State University in 1982. Gudmestad has worked on potato and potato diseases for 19 years. His initial involvement with potato was as a Research Assistant in the Department of Plant Pathology at NDSU in 1977 with the late Joseph Huguelet. Gudmestad also worked on seed potato pathology as a Plant Pathologist in the North Dakota State Seed Department, the state agency in charge of seed certification within the North Dakota Department of Agriculture. He was responsible for establishing the pathogen-free, tissue culture seed stock system currently in place for that state. Since 1985, Gudmestad has been a faculty member of the Department of Plant Pathology at NDSU with the current rank of Professor. His research responsibilities include management of seedborne diseases such as ring rot, blackleg, silver scurf, and Rhizoctonia; management of storage diseases including dry rot, pink rot, and silver scurf and; management of the foliar diseases late blight and early blight. Gudmestad was presented with the Researcher of the Year award by his peers at NDSU's Agricultural Experiment Station in 1991 and most recently was awarded the Meritorious Service Award by the Red River Valley Potato Growers Association in 1996.

Forecasting Late Blight and Fungicide Application Technology

Neil C. Gudmestad
Department of Plant Pathology
North Dakota State University

Introduction. A number of factors affect the efficiency of fungicides used for the management of late blight in a potato crop. Some of these factors include, the timing of the initial fungicide application; the interval between subsequent fungicide applications; the rate of the fungicide to be applied and; the extent of fungicide coverage throughout the plant canopy. Since most effective fungicides used today are protectant, they must be in contact with foliar surfaces of the plant prior to the infection process. Subsequent fungicide applications must be made to ensure that new foliage is adequately protected and to replace fungicide residues that have been lost due to "weathering." The aim of late blight forecasting is to improve the timing of fungicide application relative to the onset of disease development and to improve the overall efficiency of fungicide applications. To a lesser degree, models that have been developed to forecast late blight can be used to schedule fungicide applications throughout the growing season.

Potato leaves must be thoroughly covered with fungicide to maintain an adequate level of protection. Fungicide residues must not only cover individual leaflets and leaves, but must also be present in high enough concentration throughout the canopy to be effective. Microclimates favorable for the development and infection of Phytophthora infestans are most likely to occur in lower plant canopies where deposition of fungicides is most difficult.

Renewed emphasis has been placed on achieving adequate fungicide protection since the introduction of immigrant late blight genotypes into North America during the 1980s. Considerable improvements have been made also in the design of mechanical sprayers. Considering the increased use of aerial application and chemigation of fungicides, a number of studies throughout the U.S. have been initiated to evaluate fungicide application technology.

The objective of this discussion is to review the status of late blight forecasting throughout the world and to present the current information available regarding the application of fungicides.

Late Blight Forecasting. A number of excellent reviews have been written on approaches for predicting the development of plant diseases (Bourke 1970; Fry and Doster 1991; Krause and Massie 1975; Miller and O'Brien 1957). These reviews generally divide predictive systems into two categories. Models that predict the infection period of a disease causing agent and those that predict the development of disease, of which there are many including most of those forecasting late blight. Disease forecasting models can also be further subdivided into empirical predictive systems or fundamental predictive systems (Krause and Massie 1975). Empirical predictive systems are usually developed by studying historical records of disease development and the concurrent weather record for a specific geographic area. Most late blight forecasting models fall into this category. Fundamental predictive systems are developed from information developed in the laboratory or other controlled conditions. The relationship of biological and environmental significance that affect the host-parasite interaction are then quantified.

A great number of predictive systems have been developed to forecast the occurrence of late blight, perhaps more than with any other plant disease. A complete review regarding the chronological development of late blight forecasting systems is not possible but is discussed in considerable detail elsewhere (Miller and O'Brien, 1957; Fry and Doster, 1991). Late blight forecasting models currently in use that exemplify a diverse approach to this area of plant pathology will be discussed in further detail.

‘Irish Rules' The use of a synoptic weather chart as a tool for late blight forecasting was pioneered by Austin Bourke (Bourke 1970; Keane 1995). In this approach, correlations were made between late blight development and surface weather maps. Bourke reasoned that under Ireland's climate, the meteorological factors associated with the spread of late blight were prolonged humid periods with raised temperatures that would promote sporulation and periods of free moisture that would favor germination of sporangia and infection. He further reasoned that three to five cycles of the fungus were required before the disease could be detected. Bourke felt that the primary objective of the Irish Rules was to be able to use the model in conjunction with synoptic weather forecast charts so that these conditions could be predicted ahead of occurrence.

Bourke identified two synoptic environmental conditions that gave rise to late blight occurrences in Ireland. The first condition was a wave of warm moist tropical air. The second condition was stagnant or slow-moving depressions that give rise to lengthy periods of wet, overcast weather. To be effective, at least 12 hours of humid air (>90% RH) with a temperature of not less than 10 C and precipitation within 7 to 15 hours after the initiation of an effective blight period were necessary. Potato growers are warned that weather conditions are conducive for late blight spread and that conditions are favorable for spraying. Late blight warnings are broadcast on national radio and television services as well as being available in regional telephone recordings.

NEGFRY The NEGFRY late blight forecasting model was developed in Denmark and is used for timing the initial fungicide application and the scheduling of subsequent sprays throughout the growing season (Hansen et al. 1995). This system is the combination of the "Phytoprog" system (Schrodter and Ullrich 1966) and a modification of the Blitecast model developed by Fry et al. (1983). The Phytoprog system (Schrodter and Ullrich 1966) uses five environmental parameters to forecast late blight occurrence in potato. The parameters used include meteorological influences on sporangium production, germination, infection, and colonization, the magnitude and duration of high humidity periods, and the effect of dry periods on disease development. Using this data, Schrodter and Ullrich provided a negative prognosis" or negative forecast because it defines late blight-free periods. Fungicide recommendations are made only after the risk value has exceeded 160 ratings (150 ratings is the threshold for blight-free period prior to which there is no risk of a primary late blight attack). When 270 ratings have accumulated, blight is forecast to occur within 15 days before or after this threshold is reached.

Fry et al. (1983) modified Blitecast (discussed below) to more precisely schedule fungicide applications once initiation of chemical sprays to manage late blight has begun. Host resistance and effects of fungicide weathering were incorporated into the modified Blitecast model which generally reduced the number of sprays required during the growing season. NEGFRY has been tested in Denmark, Norway, and Sweden. On the average, the NEGFRY model has reduced fungicide applications by 50% (Hansen et al. 1995). The reduction is primarily due to the delay in the time of initial fungicide application and to a lesser degree, longer application intervals compared to routine spray systems. However, fungicide applications near the end of the growing season have been concluded as being too lengthy and further studies are being conducted.

Columbia Basin System Late blight occurs sporadically in the semi-arid south-central portions of Washington. Because of this, many growers are ill-prepared to initiate early fungicide applications when the disease is likely to occur. The Columbia Basin System (Johnson et al. 1996) was developed to provide potato growers in that production area with sufficient time to monitor fields and initiate fungicide applications.

Relationships between weather and late blight outbreaks over a 25-year period were studied using linear discriminant and logistic regression analyses. Although two models were developed, the most useful model selected variables such as the occurrence of late blight the previous year, number of days with rain during April and May (greater than or equal to 0.25 mm) and total precipitation during May (minimum temp. greater than or equal to 5 C) as the predictors of late blight development. Using these indicators, the model correctly identifies years in which late blight will occur, 92% of the time with at least 14 days advance notice.

Blitecast Blitecast (Krause et al. 1975) is an example of a forecasting system that combined two previously developed predictive models for late blight. One model, frequently referred to as the "favorable day" model, was developed by Hyre (1954, 1955) for northeastern United States, which in turn was modeled after Cook's (1947, 1949) forecasting system. The favorable day model is based on records of daily rainfall and minimum and maximum temperatures. Late blight is forecast to appear 7 to 14 days after the occurrence of 10 consecutive blight favorable days. A favorable day occurs when the 5-day average temperature is <25.5 C (<78 F) and total rainfall for the 10-day period is 23.0 cm (1.2 in). A day is considered unfavorable when the minimum temperature drops below <7.2 C (<45 F). Hyre (1954) referred to this system as the "moving" graph system because it is based on average temperature for the last 5 days and total rainfall for the last 10 days.

Blitecast contains a second model referred to as the "severity valve" model developed by Wallin (1962) for weather conditions found in Midwestern United States. In this system, severity values are assigned to specific relationships between periods of high relative humidity (>90%) and the average temperature during those humid periods. Late blight is forecast to occur 7 to 14 days after 18 severity values have accumulated following plant emergence. Wallin's severity value method is similar to Beaumont periods (Beaumont 1947) except that in the later example consecutive hours of relative humidity >75% are used when the temperature is 10 to 27 C (50-80 F). The high relative humidity criteria (290%) of Wallin's severity value method is, however, similar to that of Smith (1956).

Blitecast, or modifications of it, are widely used throughout the United States. It is available in WISDOM, potato crop management computer program developed by the University of Wisconsin. Although Blitecast, and the models contained within it, are intended to be used with weather data collected from within the crop canopy, it has been successfully used in areas such as North Dakota using macroclimate data collected from out-of-field sensors (Gudmestad et al. 1995). Minor modifications in data input that allow for differences between the height of sensors above the crop canopy (Hirst and Stedman 1956) have provided adequate forecasting of late blight in North Dakota over a five year period (Table 1).

Table 1. Dates of late blight forecast and occurrence in North Dakota and Minnesota during 1992-1996.

*Original forecast date and (forecast date after recalculation accounting for row closure).

Late blight forecasting summary A large number of late blight forecasting models exist and are currently used in many potato production areas throughout the world. Although the approaches used to predict the onset of late blight development are diverse, most have been demonstrated to adequately forecast the initial appearance of the disease. Thus, it would appear that the timing of the initial fungicide application is being adequately addressed by current late blight forecasting models. However, a number of authors have noted that the knowledge and experience of the forecaster can be of comparable importance to the criteria used in the model itself (Bourke 1970; Hirst and Stedman 1956).

An area that could use considerable improvement is in the scheduling of fungicide applications throughout the season. Although a number of studies have attempted to improve fungicide spray programs throughout the growth of the crop, (Fry et al. 1983; Hansen et al. 1995; Hims et al. 1995; MacKenzie 1981), no clear method of fungicide scheduling has surfaced.

Fungicide Application Technology. Costs associated with the production of potato have risen dramatically in the U.S. in recent years. A significant portion of this increase in production cost is associated with the concomitant increase in fungicide usage to manage immigrant genotypes of the late blight fungus. As a result, a more detailed understand how foliar fungicides can be used more effectively and efficiently is warranted. Additionally, new technologies in pesticide application technology have been developed since many studies were published. A review of the literature is necessary, however, to understand a few of the factors involved in fungicide deposition, redistribution and 0degradation dynamics.

Several studies have been performed that relate to fungicide deposition onto potato and tomato plant canopies (Bruhn and Fry 1982a, 1982b; Courshee 1967; Deonier 1955; Ebeling 1963; Luken and Ou 1976; Shoemaker 1979). Fungicide coverage depends on a number of factors including leaf position in the canopy, canopy density, leaf shape and size, leaf texture, and pubescence and growth habit. Generally, fungicide residue levels decline exponentially with increased distance from the top of the plant canopy (Bruhn and Fry 1982a; Courshee 1967). This appears to be especially true for crops with a uniformly dense canopy, such as potato, than with other crops (Courshee 1967).

Several other factors also affect fungicide coverage in potato crops. Bruhn and Fry (1982a, 1982b) found application method, potato cultivar, and application dosage affected coverage. They found that fungicide coverage applied by helicopter resulted in fairly uniform coverage of foliage throughout the upper half of the canopy but poor coverage in the lower half. Fungicide applied with a tractor mounted sprayer declined exponentially. Bruhn and Fry (1982a) felt that the differences in fungicide coverage pattern between the two methods were probably the result of differences in droplet size and the relative importance of impaction and sedimentation of droplets to fungicide deposition. Their results agree with those of others (Courshee, 1967; Ebeling, 1963). Although residues in the upper half of the canopy were directly related to fungicide dosage (Bruhn and Fry, 1982a), fungicide residues in the lower canopy were not changed by fungicide dosage rate. Although cultivar did not significantly affect distribution of fungicide residue levels in the studies of Bruhn and Fry (1982a, 1982b), this may be due to the choice of cultivars used. Katahdin and Monora are both determinate in growth habit and may not reflect potential cultivar differences that may exist if an indeterminate growth habit cultivar had been included. Bruhn and Fry (1982b) did find, however, that fungicide residues were lost more rapidly from Katahdin than from Monona.

More recently, studies examining fungicide coverage have been performed in the Pacific Northwest by Hamm and Johnson; in Wisconsin by Stevenson and James and in North Dakota and Minnesota by Secor et al. These studies have examined factors affecting fungicide coverage such as: application method (ground, air, chemigation), water volume, pressure of delivery, nozzle type and ground speed.

Studies performed in the Pacific Northwest have generally shown that levels of chlorothalonil residues throughout the canopy are highest when applied with a ground application, compared to aerial application and chemigation (Hamm and Johnson, personal communication). Interestingly, application of fungicide via chemigation results in very uniform coverage throughout the canopy but does not appear to change significantly with water volume. For example, chlorothalonil residues throughout the canopy range from 0.2 to 0.4 1g/cm2 regardless of whether the fungicide was delivered with 0.06, 0.12, 0.23 or 0.5 in/a. Apparently, with these volumes of water, most fungicide residue ends up on the ground rather than on the plant canopy. Their studies clearly show that water volumes of 0.003 in/a must be used to significantly improve fungicide residues applied via chemigation.

Fungicide application studies performed in Wisconsin have concentrated on factors affecting ground application of fungicide, i.e., conventional hydraulic sprayers versus air assist or electrostatic sprayers. Additionally, nozzle type, water volume and fungicide dosage have been examined for their effect on fungicide coverage (Stevenson and James, personal communication). Hollow cone and extended range (1100) flat fan nozzles provided more uniform coverage throughout the canopy compared to flood jet nozzles. Fungicide coverage in the potato canopy was greater with water volumes of 20 gpa compared to water volumes of 13 gpa. Although the air assist sprayer and electrostatic sprayer provided for fairly uniform coverage throughout the canopy, results with these types of sprayers was not significantly different from conventional sprayers when hollow cone and extended range flat fan nozzles were used. In general, the high label rates of fungicide resulted in higher fungicide residues throughout the plant canopy when compared to the low fungicide dosage, regardless of leaf position. In the presence of high disease pressure, higher deposition of fungicide was generally associated with improved control of early blight and late blight and higher crop yields.

Studies performed in ND and MN (Secor et al.) have also attempted to compare fungicide coverage as affected by method of application, water volume, and pressure of delivery. Chlorothalonil residues throughout the canopy were generally higher with ground applied fungicide compared to aerial or chemigation applied fungicide.

These results agree closely with those obtained by Hamm and Johnson. No significant differences were observed in fungicide coverage, however, among the methods of aerial application (Thrush vs. Ag Cat vs. Helicopter or 5 gpa vs 7 gpa). In a separate study, coverage of residues as determined using a fluorescent dye were influenced more by water volume and the amount of pressure during delivery than by method of application within a category. In other words, water volume and pressure were more important variables with ground application than the type of sprayer employed. Higher water volumes and increased pressure delivery generally increased coverage throughout the canopy regardless of leaf position. Few differences existed among conventional sprayers and air assist type sprayers at comparable water volumes and pressures.

Few differences existed among method of air application (Ag Cat vs. Thrush vs Air Tractor) which is in agreement with our previous study. Furthermore, Increasing water volume from 5 to 10 gpa did not increase coverage in the lower third of the plant canopy. These results may be due to the method by which increased water volumes were obtained. Water volumes delivered by aircraft were increased by increasing pressure delivery without re-nozling. This may have caused droplet size to decrease with increased pressure causing the sedimentation properties of the droplets to change. Clearly, more studies are needed to improve both ground and aerial application of pesticides.

Summary of Fungicide Application Technology A number of factors have been identified that clearly impact the efficiency of fungicide applications to a potato crop. While a number of these factors involve the crop itself (growth habit, leaf position, leaf shape, size, texture, and pubescence), other factors such as method of application, water volume, and amount of hydraulic pressure play an important role in fungicide efficiency regardless of potato cultivar. Recent improvements with ground application technology may provide a better method of fungicide delivery and management of late blight, but results to date have been inconclusive. Clearly, improvements in the delivery of fungicides via overhead irrigation equipment is needed.

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