Remote sensing carries many different connotations to different individuals, ranging from photography to large satellite platforms. Each day we are provided many frames of remote sensing information through our eyes, which we use to make visual assessments of an object. The local TV weather report uses remote sensing of clouds to point out the passage of storms. Plant pathologists have used remote sensing tools for a number of years and were among the first to use color infrared photography to assess the presence of disease in trees. The application of remote sensing via airborne cameras provided a solution to an issue that might not are possible through ground surveys. In many aspects we have progressed rapidly to our current state of knowledge about the utility of remote sensing.
The intent of this address is to arouse the interest of individuals in discovering how remote sensing could be applied to plant pathological problems of today and tomorrow.
The spectrum of electromagnetic radiation ranges from the from short, high energy wavelengths to the long radio waves. As a receptor, the human eye only measures a comparatively small portion of the spectrum within the visible wavelengths from 0.4 to 0.7 gtm. Remote sensing instruments, on the opposite hand, have utilized wavelengths extending within the microwave region for a spread of applications. For this discussion, we will confine the wavelengths to the region from 0.4 to 14 /im. The region from 0.4 to five /Mm are often represented because the reflected wavelengths.
Reflection is that phenomenon during which an impinging beam of radiation of a specific wavelength is reflected retreat from the thing with none change. This can be contrasted to emittance, which is the emitting of radiant energy at a particular wavelength due to the temperature of an object. Surfaces at the temperature of the earth (300' K) emit mostly in 10-12 4m waveband, while the sun at 60000K emits in the 0.5 pm region. Both reflectance and emittance provide information which will be utilized in applying remote sensing to agricultural problems.
Reflection from individual leaves isn't constant across the wavelengths from 0.4 to 2.5 ym. Leaves have a high reflectance in the visible (0.7-1.2 /m), a low reflectance in the near infrared (0.7-1.2 Mm), and a low reflectance in the middle and far infrared (1.2 Mm) wavebands.
This variation in leaf reflectance has allowed for the differentiation of leaves from soil, which tends to show little variation in reflectance across these wavelengths. Thep primary variation among species is within the visible reflectance and is thanks to species or leaf age.
Reflectance tends to extend in individual leaves because the leaf matures; however, the changes are wavelength dependent. These changes are due to changes in intracellular water content and chlorophyll content. Lesions and reduction in chlorophyll content created by a disease also cause an increase in reflectance. Water stress by reducing the interior water content increases the reflectance from a private leaf. Information gathered from individual leaves provides a basic set of data about the mechanism of the changes occurring within a plant; however, to be of application it must be extended to a canopy or field level.
Composites of leaves or canopies exhibit the same reflective properties of individual leaves; however, there are a series of variables that now must be considered. Leaf orientation, i.e., the arrangement of leaves on the stem and orientation to the sun, provides a source of variation when viewing a cover compared with a private leaf. Also, all leaves aren't exposed to an equivalent level of incoming energy and sometimes don't reflect back to the sky thanks to distortions within the leaf surface. Leaf surfaces often act as polarizing filters and reflect back to portions of the sky that aren't always detected by viewing the cover only the vertical direction.
However, the information contained in bidirectional and polarized reflectance has yet to be fully exploited in the evaluation of canopy response to stress. Leaf fluorescence is another attribute that has been observed in all plants and can be related to the efficiency of the photosynthetic process. It is possible that leaf fluorescence could be used to assess the impact of diseases on the physiological status of the plant. This technique has only been used on individual leaves; however, it might be extended to canopies through the utilization of laser-induced fluorescence. This procedure will have to be adapted to plant canopies but may become a powerful and useful research tool.
Canopies of plants are grown in fields with varying soil, and soil also has some unique reflective properties. The variation across wavelengths is a smaller amount for soil than for leaves; however, the reflectance changes in response to modifications within the surface. The addition of organic matter as residue on the surface reduces the reflectance. Soils vary in reflectance thanks to mineral composition and weathering of the minerals.
Changes in water content within the upper 2 mm cause the most important variation in reflectance. Water features a low reflectance and therefore the addition of a water film round the soil aggregate causes an increased absorption of the incident radiation. As a soil is wetted there's a darkening within the color, which lightens because the soil dries. Since there is a changing amount of plant material both in the adding of new leaves or the senescence of the older leaves, there is a continually changing scene to be viewed.
This challenge must be faced and understood if we are to develop the tools that allow us to assess the effect of a disease or any other stress on the plant. Instruments available for the measurement of reflected radiation adaptable to remote sensing range from the portable spectro-radiometer, which measures all wavelengths between 0.4 and 1.1 /tm to radiometers with multiple channels set for discrete wave- bands. Instruments with individual channels mimic the wavebands available on the current satellites.
A rapidly emerging technology that has yet to be applied is the use of multiple waveband video cameras. This system offers a capability not possible with other radiometers, in that the data are readily available for viewing without intense signal processing and manipulation.
To use the information contained in the reflectance across wavelengths, several vegetative indices have been proposed and evaluated. These indices are based primarily on the ratio or difference between the reflectance in the near-infrared and red wavelengths. The approaches range from simple ratios of near-infrared/- red)/(near infrared + red)] is more appropriately related to the interception of photosynthetically active radiation. To account for the soil background the perpendicular vegetative index was developed to account for a changing soil background due to sur- face soil water content changes. There have been several other indices developed to describe how the changing reflective proper- ties change with growth of the plant.
Observed changes in the vegetative index, in particular, the ratio vegetative index and the normalized vegetative index have shown unique seasonal patterns. The patterns of both indices show an increase with the developing canopy and a hysteresis effect during senescence because plant material remains standing in the field, which has different reflective properties than the soil in the background.The variation is usually 10% of the sector mean. However, the change in spatial variability may be one of the methods that could be effectively used to monitor the changes that occur within fields as a result of disease. Most diseases don't infect an entire field uniformly and thus could induce a change within the field pattern. Even on one sample event this method could provide valuable information given a priori knowledge about the expected level of field variability.
All objects that have a temperature emit radiation consistent with Planck's law . Soil and plant canopies emit energy, and given the temperatures found on the earth's surface, range in the 10-14 bm waveband. Temperature of a plant canopy can be described by the temperature of individual leaves, the temperature of foliage, or the temperature of the canopy that includes the soil. Leaf temperatures that are measured relative to the ccurrence of Verticillum wilt or plant disease of soybeans (Phialo- phora gregata Gans) are measured with attached leaf the Other measurements of Verticillum wilt are made with infrared thermometers. Each method has provided a singular relationship of describing the change in leaf temperature relative to the presence of a disease.
Temperatures of the leaf, foliage, or canopy are a results of the energy exchange process. The observed temperatures are a result of the partitioning between the sensible and latent heat exchanges and therefore are a balance between the energy impinging on the leaf or foliage and the water available for evaporation. Simply stated then, a surface with a free water surface are going to be as cool as possible given the environmental condition while one without water are going to be the warmest possible under a given set of conditions. It is this relationship that has allowed foliage temperature to be effectively used in the estimation of transpiretion from canopies. We have been successful in using foliage and canopy temperature in evapotranspiration models for a variety of crops. In well-irrigated crop canopies, the variation across a field is relatively uniform and the variation increases with increasing soil water deficits.
As with the reflected radiation, the change in field variability could also be useful in defining the characteristics of a given field..
To improve the efficiency of using foliage temperatures, several crop water stress indices are proposed and evaluated since the center 1970s. These became possible at this point thanks to the leaf area index, while the normalized difference [(near infrared development of the accurate, portable, hand-held infrared thermo- meter. At first, the comparison was made between the foliage and air temperature (Tf - Ta), since this type was the integral a part of the energy exchange process. It was found that although the Tf - Ta differences were associated with crop yield induced by water stress, the relationships were site dependent. Further development and study revealed that other environmental variables were needed to completely interpret foliage temperatures and develop less site-specific relationships. The primary variables were net radiation, wind speed, and vapour pressure deficit. These stress indices are supported the energy exchange principles between the foliage and therefore the surrounding atmosphere. Any factor that affects the rate of water movement to the leaf has an impact on the foliage temperature. For example, the addition of high salt content irrigation even in large amounts causes the foliage temperatures to be warmer than those plants irrigated with the same volume of salt-free water.
Recent research has identified that plants have biochemical. temperature optima that outline the optima temperature for plant growth. It has been found that plants with maximum growth during a particular environment have the minimum amount of your time outside of this predetermined thermal range. This range has been defined because the thermal kinetic window and is predicated on the biochemical efficiency of a specific plant. The utility of this stress index has yet to be fully evaluated; however, it offers a way of linking the plant response to an observed parameter. Eg: foliage temperature.
Other methods that can be used are to calculate the canopy resistance to water vapor exchange. It is known that many diseases affect the stomatal resistance and the combination energy balance and observed foliage temperature provide a method of estimating canopy resistance. These techniques, however, are yet to be applied to any measure of disease. They may offer the potential of quantifying the degree of stress or level of infection in ways that have not been possible before.
The instruments available for measuring the foliage temperature range from hand-held portable units, to fixed, battery-powered systems to airborne or satellite thermal scanners systems.
The coverage is obtained, e.g., NOAA or GOES satellites. Smaller areas are covered with systems that provide less frequent coverage, e.g., LANDSAT or SPOT. The variation within a scene of knowledge obtained with an airborne or satellite system won't permit the reliable detection of the onset of a disease. The handheld or fixed units on the bottom could also be useful during a research setting to work out the casual relationships and therefore the development of a monitor program where a problem is suspected. Each of those instruments require some training to most properly collect and interpret the info .
Both the spatial attributes and temporal of remote sensing techniques detect unique features about the surface being observed. Satellites that provide repeated coverage of the earth have allowed assessments to be made of the changes that have occurred over a period of months or years. The same factors apply when applying remote sensing to the monitoring of agricultural fields, forests, and native or managed grasslands. The value of repeated coverage has provided for a singular glimpse at the ecosystem that we try to watch .
Temporal variations can be large because the system to which we are applying may be changing due to the normal progression of growth. However, we all know what patterns to expect and deviations faraway from that pattern provide an investigative tool. Likewise, changes within the spatial patterns may signal a possible problem within a given field or ecosystem. The interpretation of the temporal and spatial pattern will require some experience but may provide an indication of a problem not possible before this information was made available.
There are two avenues during which remote sensing information, either reflected or emitted radiation, are often incorporated into disease monitoring. The two approaches involve either direct or indirect methods of evaluating the disease occurrence and extent. Given the amount of things that cause variation in both the reflected and emitted radiation signals it's unlikely that the direct monitoring method are going to be useful. Both the indirect and direct methods require a priori knowledge that a condition may exist.
Given this data then, one may use an immediate monitor program to live the extent of a disease. eg: Phytophthora spp. on soyabeans or Fusarium spp on beans, which cause a discount in leaf area. The spatial sampling capability provides an assessment impossible with ground monitoring. For example, a rise in foliage temperature during a field with an adequate soil water system would signal a possible problem that would invoke a monitoring effort. An unexplained change in leaf area or wilting leading to a change in reflectance could signal a drag before complete infestation. The utilization of both indirect and direct methods will require imagination and dedication to the problem by a number of researchers.
The intent of this address is to arouse the interest of individuals in discovering how remote sensing could be applied to plant pathological problems of today and tomorrow.
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Regions of The Spectrum
The spectrum of electromagnetic radiation ranges from the from short, high energy wavelengths to the long radio waves. As a receptor, the human eye only measures a comparatively small portion of the spectrum within the visible wavelengths from 0.4 to 0.7 gtm. Remote sensing instruments, on the opposite hand, have utilized wavelengths extending within the microwave region for a spread of applications. For this discussion, we will confine the wavelengths to the region from 0.4 to 14 /im. The region from 0.4 to five /Mm are often represented because the reflected wavelengths.
Reflection is that phenomenon during which an impinging beam of radiation of a specific wavelength is reflected retreat from the thing with none change. This can be contrasted to emittance, which is the emitting of radiant energy at a particular wavelength due to the temperature of an object. Surfaces at the temperature of the earth (300' K) emit mostly in 10-12 4m waveband, while the sun at 60000K emits in the 0.5 pm region. Both reflectance and emittance provide information which will be utilized in applying remote sensing to agricultural problems.
Reflection from Leaves
Reflection from individual leaves isn't constant across the wavelengths from 0.4 to 2.5 ym. Leaves have a high reflectance in the visible (0.7-1.2 /m), a low reflectance in the near infrared (0.7-1.2 Mm), and a low reflectance in the middle and far infrared (1.2 Mm) wavebands.
This variation in leaf reflectance has allowed for the differentiation of leaves from soil, which tends to show little variation in reflectance across these wavelengths. Thep primary variation among species is within the visible reflectance and is thanks to species or leaf age.
Reflectance tends to extend in individual leaves because the leaf matures; however, the changes are wavelength dependent. These changes are due to changes in intracellular water content and chlorophyll content. Lesions and reduction in chlorophyll content created by a disease also cause an increase in reflectance. Water stress by reducing the interior water content increases the reflectance from a private leaf. Information gathered from individual leaves provides a basic set of data about the mechanism of the changes occurring within a plant; however, to be of application it must be extended to a canopy or field level.
Reflection from Canopies and Fields
Composites of leaves or canopies exhibit the same reflective properties of individual leaves; however, there are a series of variables that now must be considered. Leaf orientation, i.e., the arrangement of leaves on the stem and orientation to the sun, provides a source of variation when viewing a cover compared with a private leaf. Also, all leaves aren't exposed to an equivalent level of incoming energy and sometimes don't reflect back to the sky thanks to distortions within the leaf surface. Leaf surfaces often act as polarizing filters and reflect back to portions of the sky that aren't always detected by viewing the cover only the vertical direction.
However, the information contained in bidirectional and polarized reflectance has yet to be fully exploited in the evaluation of canopy response to stress. Leaf fluorescence is another attribute that has been observed in all plants and can be related to the efficiency of the photosynthetic process. It is possible that leaf fluorescence could be used to assess the impact of diseases on the physiological status of the plant. This technique has only been used on individual leaves; however, it might be extended to canopies through the utilization of laser-induced fluorescence. This procedure will have to be adapted to plant canopies but may become a powerful and useful research tool.
Canopies of plants are grown in fields with varying soil, and soil also has some unique reflective properties. The variation across wavelengths is a smaller amount for soil than for leaves; however, the reflectance changes in response to modifications within the surface. The addition of organic matter as residue on the surface reduces the reflectance. Soils vary in reflectance thanks to mineral composition and weathering of the minerals.
Changes in water content within the upper 2 mm cause the most important variation in reflectance. Water features a low reflectance and therefore the addition of a water film round the soil aggregate causes an increased absorption of the incident radiation. As a soil is wetted there's a darkening within the color, which lightens because the soil dries. Since there is a changing amount of plant material both in the adding of new leaves or the senescence of the older leaves, there is a continually changing scene to be viewed.
This challenge must be faced and understood if we are to develop the tools that allow us to assess the effect of a disease or any other stress on the plant. Instruments available for the measurement of reflected radiation adaptable to remote sensing range from the portable spectro-radiometer, which measures all wavelengths between 0.4 and 1.1 /tm to radiometers with multiple channels set for discrete wave- bands. Instruments with individual channels mimic the wavebands available on the current satellites.
A rapidly emerging technology that has yet to be applied is the use of multiple waveband video cameras. This system offers a capability not possible with other radiometers, in that the data are readily available for viewing without intense signal processing and manipulation.
Vegetative Indices
To use the information contained in the reflectance across wavelengths, several vegetative indices have been proposed and evaluated. These indices are based primarily on the ratio or difference between the reflectance in the near-infrared and red wavelengths. The approaches range from simple ratios of near-infrared/- red)/(near infrared + red)] is more appropriately related to the interception of photosynthetically active radiation. To account for the soil background the perpendicular vegetative index was developed to account for a changing soil background due to sur- face soil water content changes. There have been several other indices developed to describe how the changing reflective proper- ties change with growth of the plant.
Observed changes in the vegetative index, in particular, the ratio vegetative index and the normalized vegetative index have shown unique seasonal patterns. The patterns of both indices show an increase with the developing canopy and a hysteresis effect during senescence because plant material remains standing in the field, which has different reflective properties than the soil in the background.The variation is usually 10% of the sector mean. However, the change in spatial variability may be one of the methods that could be effectively used to monitor the changes that occur within fields as a result of disease. Most diseases don't infect an entire field uniformly and thus could induce a change within the field pattern. Even on one sample event this method could provide valuable information given a priori knowledge about the expected level of field variability.
Emited Radiation
All objects that have a temperature emit radiation consistent with Planck's law . Soil and plant canopies emit energy, and given the temperatures found on the earth's surface, range in the 10-14 bm waveband. Temperature of a plant canopy can be described by the temperature of individual leaves, the temperature of foliage, or the temperature of the canopy that includes the soil. Leaf temperatures that are measured relative to the ccurrence of Verticillum wilt or plant disease of soybeans (Phialo- phora gregata Gans) are measured with attached leaf the Other measurements of Verticillum wilt are made with infrared thermometers. Each method has provided a singular relationship of describing the change in leaf temperature relative to the presence of a disease.
Energy Exchange Processes
Temperatures of the leaf, foliage, or canopy are a results of the energy exchange process. The observed temperatures are a result of the partitioning between the sensible and latent heat exchanges and therefore are a balance between the energy impinging on the leaf or foliage and the water available for evaporation. Simply stated then, a surface with a free water surface are going to be as cool as possible given the environmental condition while one without water are going to be the warmest possible under a given set of conditions. It is this relationship that has allowed foliage temperature to be effectively used in the estimation of transpiretion from canopies. We have been successful in using foliage and canopy temperature in evapotranspiration models for a variety of crops. In well-irrigated crop canopies, the variation across a field is relatively uniform and the variation increases with increasing soil water deficits.
As with the reflected radiation, the change in field variability could also be useful in defining the characteristics of a given field..
Crop Stress Indices
To improve the efficiency of using foliage temperatures, several crop water stress indices are proposed and evaluated since the center 1970s. These became possible at this point thanks to the leaf area index, while the normalized difference [(near infrared development of the accurate, portable, hand-held infrared thermo- meter. At first, the comparison was made between the foliage and air temperature (Tf - Ta), since this type was the integral a part of the energy exchange process. It was found that although the Tf - Ta differences were associated with crop yield induced by water stress, the relationships were site dependent. Further development and study revealed that other environmental variables were needed to completely interpret foliage temperatures and develop less site-specific relationships. The primary variables were net radiation, wind speed, and vapour pressure deficit. These stress indices are supported the energy exchange principles between the foliage and therefore the surrounding atmosphere. Any factor that affects the rate of water movement to the leaf has an impact on the foliage temperature. For example, the addition of high salt content irrigation even in large amounts causes the foliage temperatures to be warmer than those plants irrigated with the same volume of salt-free water.
Recent research has identified that plants have biochemical. temperature optima that outline the optima temperature for plant growth. It has been found that plants with maximum growth during a particular environment have the minimum amount of your time outside of this predetermined thermal range. This range has been defined because the thermal kinetic window and is predicated on the biochemical efficiency of a specific plant. The utility of this stress index has yet to be fully evaluated; however, it offers a way of linking the plant response to an observed parameter. Eg: foliage temperature.
Other methods that can be used are to calculate the canopy resistance to water vapor exchange. It is known that many diseases affect the stomatal resistance and the combination energy balance and observed foliage temperature provide a method of estimating canopy resistance. These techniques, however, are yet to be applied to any measure of disease. They may offer the potential of quantifying the degree of stress or level of infection in ways that have not been possible before.
The instruments available for measuring the foliage temperature range from hand-held portable units, to fixed, battery-powered systems to airborne or satellite thermal scanners systems.
The coverage is obtained, e.g., NOAA or GOES satellites. Smaller areas are covered with systems that provide less frequent coverage, e.g., LANDSAT or SPOT. The variation within a scene of knowledge obtained with an airborne or satellite system won't permit the reliable detection of the onset of a disease. The handheld or fixed units on the bottom could also be useful during a research setting to work out the casual relationships and therefore the development of a monitor program where a problem is suspected. Each of those instruments require some training to most properly collect and interpret the info .
Temporal and Spatial Variation
Both the spatial attributes and temporal of remote sensing techniques detect unique features about the surface being observed. Satellites that provide repeated coverage of the earth have allowed assessments to be made of the changes that have occurred over a period of months or years. The same factors apply when applying remote sensing to the monitoring of agricultural fields, forests, and native or managed grasslands. The value of repeated coverage has provided for a singular glimpse at the ecosystem that we try to watch .
Temporal variations can be large because the system to which we are applying may be changing due to the normal progression of growth. However, we all know what patterns to expect and deviations faraway from that pattern provide an investigative tool. Likewise, changes within the spatial patterns may signal a possible problem within a given field or ecosystem. The interpretation of the temporal and spatial pattern will require some experience but may provide an indication of a problem not possible before this information was made available.
Integrating Remote Sensing Into Plant Pathology
There are two avenues during which remote sensing information, either reflected or emitted radiation, are often incorporated into disease monitoring. The two approaches involve either direct or indirect methods of evaluating the disease occurrence and extent. Given the amount of things that cause variation in both the reflected and emitted radiation signals it's unlikely that the direct monitoring method are going to be useful. Both the indirect and direct methods require a priori knowledge that a condition may exist.
Given this data then, one may use an immediate monitor program to live the extent of a disease. eg: Phytophthora spp. on soyabeans or Fusarium spp on beans, which cause a discount in leaf area. The spatial sampling capability provides an assessment impossible with ground monitoring. For example, a rise in foliage temperature during a field with an adequate soil water system would signal a possible problem that would invoke a monitoring effort. An unexplained change in leaf area or wilting leading to a change in reflectance could signal a drag before complete infestation. The utilization of both indirect and direct methods will require imagination and dedication to the problem by a number of researchers.
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