Biology, Detection, and Management of Plant Pathogens in Irrigation Water

Biology, Detection, and Management of Plant Pathogens in Irrigation Water Chuanxue Hong, Gary W. Moorman, Walter Wohanka, and Carmen B端ttner, Editors

Part I

Linkages Between Crop Disease and Irrigation Water

Chapter 1

Sources and Distribution Systems of Irrigation Water and Their Potential Risks for Crop Health ●●

Gary W. Moorman The Pennsylvania State University, University Park, Pennsylvania, U.S.A.

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Amanda J. Gevens University of Wisconsin, Madison, Wisconsin, U.S.A.

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Leah L. Granke Mary K. Hausbeck Michigan State University, East Lansing, Michigan, U.S.A.

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Katherine Hendricks Pamela D. Roberts University of Florida, SWFREC, Immokalee, Florida, U.S.A.

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Timothy R. Pettitt Eden Project, Bodelva, Cornwall, U.K.

IN THIS CHAPTER Water Sources Groundwater Ponds, Lakes, and Other Surface Impoundments Rivers, Streams, and Other Free-Flowing Sources Water Collected from Roofs and Paved Surfaces Municipal Water Supplies Water Collected from Fields and Unpaved Areas

Trickle and Drip Irrigation Hydroponic, Nutrient Film, Trough, and Float Systems Subirrigation Components of Recycling Systems Indoor Tanks Outdoor Reservoirs Soil-Lined Channels Pipes

Distribution to Crops Overhead Irrigation Flooded Furrows Flooded Fields

Concluding Remarks

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Many sources of water can be used for irrigating crops, ranging from water from springs and streams to water that has been recycled following industrial, agricultural, or domestic use.

The quality and quantity of the water available from these sources vary greatly depending on the surrounding environment, season, and use by agricultural industries and others. In terms of this book, the quality criterion of greatest importance is the water’s microbial community— more specifically, the presence or absence of plant pathogens.

Growers employ a wide variety of systems to distribute water to crops. The categories and names of the systems used here are those in general use but are also somewhat arbitrary and overlapping. Moreover, it is not unusual for a single grower to use multiple systems and hybrids of the systems described here. Water sources, distribution systems, and recycling networks must be examined on a case-by-case basis. This chapter outlines the factors that must be considered when assessing whether the source and distribution system pose a risk to the crop by harboring and dispersing plant pathogens.

Water Sources

runoff. If the extracted water is not immediately pumped into pipes and stored in a suitably enclosed or covered area, it is at risk of becoming contaminated with plant pathogens. Borchardt et al. (2) found that human intestinal viruses had percolated over 70 m (230 feet) from the surface to the groundwater below a sealed well system and postulated that this transmission had occurred over many years. Thus, work should be done to assess whether plant-pathogenic viruses on long-used agricultural land can percolate into groundwater.

Ponds, Lakes, and Other Surface Impoundments Water impoundments, whether natural or human made (Fig. 1.1), may contain few or no plant pathogens or harbor high populations of plant pathogens. Such sources are at high risk for contamination by plant pathogens dispersed to them by wind currents, by effluent from the streams or rivers that feed them, and by soil and debris carried to them by humans and other animals and in runoff from surrounding crops, other plants, and the land.

In general, impoundments pose a high risk of carrying plant pathogens. The pH, alka­ linity, dissolved oxygen, and algal content of

Groundwater Groundwater and natural springs tend not to harbor plant pathogens (11,18), unless the water table is very high and close to the soil surface.

Conventional wisdom suggests that the soil and soil microbial community above the water table act like a filter that physically or biologically removes most plant pathogens, except those pathogens that are particularly well adapted to the soil environment. As plants’ root systems penetrate the soil, soilborne organisms— including certain plant pathogens—colonize the root surface (i.e., rhizoplane) and may come in contact with shallow groundwater.

When groundwater is extracted using a well that has not been properly encased, there is the potential for contamination by debris or surface

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Fig. 1.1. This pond provides a source of irrigation water for a greenhouse.

Part I / Linkages Between Crop Disease and Irrigation Water

impoundment water vary greatly with the time of year (6), and all of these factors can influence the survival and activity of plant pathogens. The rate of turnover in the impoundment, the depth from which the water is withdrawn for use, and the distance between the intake pipe for the irrigation system and the point at which water enters the impoundment (6) are also factors in the presence of pathogens in the water applied to the crop.

If the impoundment is created by a dam, the characteristics of the water vary with both the impoundment depth and the depth from which the water is withdrawn. Water withdrawn from the middle of a water column or from near the bottom of a deep reservoir tends not to harbor aerobic plant pathogens (3). The status of plantpathogenic viruses at various depths of water has not been assessed.

plant debris-borne pathogens that are blown onto roofs and paved surfaces, washed from surrounding areas into the channeling system or impoundment area, or brought out of the air column during a precipitation event.

Rivers, Streams, and Other Free-Flowing Sources

Water Collected from Fields and Unpaved Areas

Rivers, streams, and canals are other highly variable sources of water and widely considered to be at high risk for harboring plant pathogens (1,5,8–10,12,17,19). The quality of free-flowing water can change rapidly under the influence of relatively distant changes in weather. Plantpathogenic viruses from different crops have been detected in streams in forested areas with little human or no agricultural activity (4). However, in free-flowing water, the vectorless transmission of viruses is very inefficient because of the great dilution of the virus particles.

Precipitation and excess irrigation water applied to fields (known as tailwater) is collected by channels and directed to ponds or other impoundments and used for irrigation. This water poses a high risk for harboring plant pathogens from the natural vegetation surrounding the fields and from crops in cultivated fields. In

Municipal Water Supplies Municipal water is normally quite low in or free of plant pathogens and is considered to pose a low risk. Using water from a municipal source is usually expensive, however, and in areas in which domestic water use is intense, there is the increasing threat of restricting the availability of water for agriculture during periods of drought.

That threat is becoming more serious with the onset of climate change and is motivating farmers to capture and recycle water.

Water Collected from Roofs and Paved Surfaces Precipitation can be collected from the roofs of greenhouses and storage buildings, as well as from other impervious surfaces, and channeled to cisterns or impoundments (Fig. 1.2). This water should harbor few plant pathogens and pose a low risk of being a source of plant pathogens.

Even so, it must be recognized that this water may become contaminated by plant pathogens that are normally airborne or by soilborne and

Fig. 1.2. Water for irrigation is collected from a greenhouse roof and channeled to a pond.

Chapter 1 / Sources and Distribution Systems of Irrigation Water and Their Potential Risks

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addition, the fertilizers and pesticides applied to crops that accumulate during tailwater recovery may influence crop susceptibility to waterborne plant pathogens.

Distribution to Crops Overhead Irrigation An overhead irrigation system is inexpensive to install and operate. In its simplest form, overhead irrigation consists of hand watering using a hose fitted with a nozzle to produce the required droplet size. More complex systems use hoses or pipes connected to stationary, upright pipes that terminate with an oscillating nozzle, which moves from 0–360° across the crop, as set by the operator. Another option is for the water source to be connected to a boom that supplies multiple nozzles and is moved over the crop (Fig. 1.3).

Such systems may require pumps, may be automated to open and close valves and turn on pumps, or may be manually operated by opening a tap. Moreover, these systems may be designed to cover areas of various shapes and sizes.

Overhead irrigation systems generally use large amounts of water and consequently produce large volumes of runoff. Depending on the sprinkler

type, the droplets distributed by overhead irrigation can greatly facilitate the spread of plant pathogens already in the crop in two ways: (a) splashing water disperses spores and bacteria from leaf and stem lesions to other leaves and stems, and (b) the large volume of water applied washes inoculum from the plant roots, stems, and leaves into puddles or runlets, thereby exposing neighboring plants to the inoculum. Another effect of overhead sprinkler irrigation is that it puts moisture on foliage, stems, and roots, where most plant pathogens require it for germination, penetration, and infection. For all of these reasons, overhead irrigation greatly enhances disease development.

Flooded Furrows Water is sometimes distributed by creating furrows in a field that are parallel to the rows of the crop and graded so that water applied at one end of the field flows to the opposite end. Tailwater is then channeled to waste, to a retention pond for reuse, or back to the source (e.g., stream, river, impoundment). This distribution system poses a high risk of dispersing many different plant pathogens, because patho­ gens can accumulate along the furrows and be dispersed by the flowing water.

Flooded Fields Water can also be distributed by flooding an entire field that has been diked to retain the water and graded to channel the tailwater to waste, to a retention pond for reuse, or back to the source (e.g., stream, river, impoundment).

Like the flooded furrow system, the flooded field system poses a high risk of dispersing plant pathogens.

Trickle and Drip Irrigation

Fig. 1.3. Overhead boom irrigation of a greenhouse crop.

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A trickle irrigation system uses significantly less water than an overhead system to deliver sufficient water for plants’ needs without applying any excess and thus minimizing runoff.

Part I / Linkages Between Crop Disease and Irrigation Water

Using drippers or emitters, trickle irrigation increases the efficiency of water application; namely, 44–72% of the water applied is retained by the soil or most soilless media in containers, compared with 13–20% with an overhead system (20). However, when trickle irrigation is used to water crops of vegetables or cut flowers rooted in rock wool, glass wool, coir, or pumice, 30% of the water applied commonly passes through the substrate.

Trickle irrigation systems use seep hoses or emitters to deliver water to the soil surface, thereby avoiding splashing water onto the crop canopy. This is the major advantage of trickle irrigation: Applying water directly on or closely to the growing medium reduces splash dispersal of plant pathogens and avoids placing water on infection courts. Trickle systems also have several disadvantages, however. They generally involve a high initial cost for equipment and installation, and maintaining optimum system performance requires filtration to keep pipes and drippers free of particulates. Also, when trickle irrigation is used in systems that have runoff—for example, when irrigating plants in rock wool—the tailwater collects plant pathogens and returns them to the recirculating tank, which places the crop at risk for repeated inoculation. (This risk is greatly reduced if the tailwater is run to waste.)

Fig. 1.4. Drip irrigation system, in which two emitters are held in place over each container.

Dripper systems tend to be used for highvalue containerized plants—for example, highquality plants grown in the pot-in-pot system (15) or in greenhouses or shade houses (Fig. 1.4).

Because plant spacing is fixed by the distance between emitters, such systems are difficult to manage with small containers. If the emitters are in line and not on the ends of flexible tubes, then the spacing between plants in a row must be fixed. Seep hoses are generally best for field crops, although they can be used to deliver water to capillary sand beds for subirrigation of containerized crops.

Hydroponic, Nutrient Film, Trough, and Float Systems In nutrient film and trough irrigation systems (Fig. 1.5), water flows slowly through a tray or container in which plants are arranged and the excess water is captured and recycled. The application of water can be continuous or intermittent, depending on whether the plants

Fig. 1.5. Trough irrigation system.

(Courtesy Michael Christian, American Hydroponics, Arcata, CA)

Chapter 1 / Sources and Distribution Systems of Irrigation Water and Their Potential Risks

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of plant pathogens moving from the water into the containers depends to a large extent on the composition of the potting medium (7,13).

Sand Beds and Capillary Mats

Fig. 1.6. Float irrigation system. (Courtesy Michael Christian, American Hydroponics, Arcata, CA)

are potted or the roots are bare. Plants may be in containers of soilless potting mix or in slabs of rock wool, coir, or some other solid substrate, or plants may be bare rooted.

In a float system, seedlings are grown in trays of soilless potting mix or trays that hold barerooted plants (Fig. 1.6). Multiple trays float in a common bath, usually in sufficient number to completely cover the water surface.

The risk for plant pathogens in these systems is determined by several factors, including the population of plant pathogens in the water used to charge and maintain the system, the health of the plants put into the system, and the measures taken to prevent the introduction of pathogen-containing stray soil or plant debris into the system. Once a plant pathogen has been introduced, however, the risk of spread within the system is high, particularly for very stable organisms. The vectorless spread of viruses can be highly efficient in these types of systems (4).

Subirrigation A subirrigation system supplies water from below the root system, usually to plants in containers, and water is taken up by the potting medium by capillary action. Water is applied as needed by the plants, not on a continuous basis. The risk

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The sand used for subirrigation must have a balance of fine and coarse particles: fine particles (0.1–0.5 mm (less than 1 ⁄64 inch)) to provide for the capillary movement of water and coarse particles (0.5–4.0 mm (1 ⁄64 –1 ⁄8 inch)) to prevent too much water from being held in wet weather.

In addition, the sand must be free of plant pathogens; sand taken from river beds and areas of standing water are well recognized to harbor plant pathogens. Capillary mats are made of thick fabric that retains water when wetted. The largest pot size that can be grown satisfactorily on a sand bed or capillary mat system is approximately 9.0 L (2.4 gallons).

When used in conjunction with overhead irrigation, a sand or capillary matting bed can draw up the excess water from sprinklers and redistribute it more evenly across the bed. The Efford sand bed (16) is composed of a layer of sand (at least 75 mm (3 inches) deep) laid over a polythene-lined base, and the bed is profiled to a central drain. Generally, there is little or no runoff from a sand or capillary bed and the excess water is not recycled. The main risk posed by this system comes from the presence of plant pathogens in the source of water used.

Flooded Troughs, Floors, and Benches Ebb-and-flow and flood-and-drain subirrigation systems apply water to an impervious diked or slightly sunken floor (Fig. 1.7) or bench (Fig. 1.8), which is essentially a large tray. The water is taken up by the potting mix and plants in containers. In one cycle of watering, two-thirds of the volume of water applied usually returns to a holding tank, whereas one-third is absorbed by the potting medium, used by the plant, or lost to evaporation.

A flooded floor or bench can be viewed as an indoor version of a flooded field. The main difference between the two systems is that with a flooded floor or bench, the indoor

Part I / Linkages Between Crop Disease and Irrigation Water

plants are containerized rather than growing in the ground.

The risk for plant pathogens in this type of system is determined by the population of pathogens in the water used to charge and maintain the system, the health of the plants put into the system, and the steps taken to prevent the introduction of pathogen-containing stray soil or plant debris into the system. Once a plant pathogen has been introduced into an ebb-andflow system, the risk of spread is very high.

Components of Recycling Systems If excess water from any type of irrigation system is collected and then reused or recycled during subsequent irrigation, then components of the system must be assessed for the presence of plant pathogens and the risk of dispersing the pathogens throughout the system. The threat posed by plant pathogens carried in irrigation water that is being recycled can be mitigated to some extent by making wise choices in designing the system (14).

Fig. 1.7. Flooded floor ebb-and-flow irrigation system, before and during flooding (top and bottom, respectively).

Indoor Tanks When the water in a greenhouse is recycled, it is usually impounded in an indoor tank of some sort. The tank may be above the ground or in the ground, and if it is not covered, it is subject to contamination with plant pathogen-containing stray soil and plant debris. The tank collects any plant pathogens leached from containers and then serves as a source of inoculum at each subsequent watering.

Outdoor Reservoirs

Fig. 1.8. Flooded bench ebb-and-flow irrigation system.

Outdoor reservoirs used for recycling systems are subject to contamination from plant pathogencontaining stray soil and plant debris, and they also accumulate plant pathogens leached from containers. Thus, these reservoirs have a high potential for dispersing plant pathogens in recycling irrigation systems.

Chapter 1 / Sources and Distribution Systems of Irrigation Water and Their Potential Risks

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