Introduction

Materials and
Methods

Results and
Discussion

References

Project
Background

A Portable Aerial Spore Collecting System

T.L. Friesen, E.D. De Wolf, and L.J. Francl
Department of Plant Pathology
North Dakota State University


Abstract
Sampling the atmosphere for spores of fungal pathogens can help determine the importance of dispersal in a crop epidemic and can provide us with the means to estimate the dispersal distance of a pathogen. We designed and built a spore sampler that was lifted to various heights by a tethered helium balloon. Sampling was possible in remote areas including, but not limited to, fields during harvest. Portability allows this collecting system to be transferred from field to field or repositioned within a field. Tests showed that the balloon could be operated at heights up to 150 ft., in winds up to 10 mph, for up to 6 hrs per day. Spores of Bipolaris sorokiniana, the cause of wheat spot blotch and common root rot, were found 50 ft. above and 300 ft. downwind from wheat windrows being combined. This suggests the sampler can detect plant pathogen propagules being dispersed from a source. This article is only available online at http://www.ag.ndsu.nodak.edu/ndagres/ndagres.htm

Impact — A portable low-altitude sampler was devised as a tool to study plant pathogen dispersal. Crop disease management may be improved if dispersal becomes better understood and quantified.
Audience — Plant pathologists, allergists, aerobiologists, microbiologists

Keywords
Epidemiology, aerobiology, dispersal, NDAWN

Introduction
In the 1920s, E.C. Stakman established in the minds of agriculturalists the concept of the "Puccinia Pathway" (Stakman 1934). Puccinia graminis f. sp. tritici and P. recondita f. sp. tritici are the causative pathogens of wheat stem rust and wheat leaf rust, respectively. By sampling the atmosphere at an elevation of 16,400 ft. (5000 meters), Stakman proved that spores of these fungi traveled the breadth of the Great Plains from south to north. Thus, as wheat matured across the continental plains, so also did the rust diseases spread. Since rust inoculum often arrived early during the wheat growing season in North Dakota, yield losses of susceptible cultivars were substantial in years when the environment was suitable for a rust epidemic. Under a project sponsored by the military, Asai (1960) later provided additional details about the long-distance movement of rust spores and dispersal from inoculated plots. The Puccinia Pathway was the first demonstrated continental dispersal phenomenon in botanical epidemiology and had an enormous impact on how rust diseases were managed during the 20th century.

For spores to migrate long distances, they first must reach elevations substantially above the canopy. Once the spores have reached a certain height they travel a measurable horizontal distance before being deposited (Nagarajan and Singh 1990). A simple physical model of spore dispersal distance includes air speed, height, and sedimentation rate. Random diffusion can be incorporated by a three-dimensional Gaussian plume model (Campbell and Madden 1990). In reality, spore movements are also governed by turbulence, convection currents and other air movements, facts recognized by Stakman as early as 1923. Knowledge of the vertical profile of airspora is important to both plant pathologists and allergists for a better understanding of spore and pollen distribution (Lyon et al. 1984).

The agricultural landscape of the Great Plains has changed considerably since the pioneering work of Stakman and the cold war-inspired research of Asai. Now, wheat and durum cultivars widely grown in the northern Great Plains are largely resistant to stem and leaf rusts. Healthy tissue thus has become available for colonization of other wheat pathogens. Moreover, widespread adoption of conservation tillage practices, which overtook traditional tillage practices nationwide in 1993 (McMullen et al. 1997), has had a profound effect on other diseases affecting wheat. This effect has come in part because of the increased survival ability of inoculum on crop residues that were previously plowed under.

Tan spot and scab are examples of wheat diseases caused by fungi that overwinter on plant residue left on the soil surface (Shaner 1981, Khonga and Sutton 1988). Prior to the early 1970s, tan spot of wheat and durum was rarely mentioned in North Dakota; recently, tan spot was ranked as the most serious foliar disease of wheat (McMullen and Nelson 1992). In the 1990s, scab or Fusarium head blight (FHB) epidemics have caused over a billion dollars in lost small grain production, destroying an estimated 607 million bushels of wheat and barley in the United States and Canada from 1991-1996 (McMullen et al. 1997). FHB epidemics were rare in the northern Great Plains prior to the 1980s. In addition to tan spot and FHB, black point and septoria leaf blights have caused large economic losses in recent years (McMullen and Nelson 1992, Sheehy 1969).

Combine harvesting can play a significant role in the dispersal of pathogens to neighboring fields (Buchwaldt et al. 1996, Rowe et al. 1974). Dispersal of this inoculum may render crop rotation only partially effective as a disease management option. The role of harvesting in the long distance dispersal of wheat pathogens has not been studied previously, although some conjecture appears in the literature (Fletcher et al. 1953).

Our objective was to build a portable sampling device to help us better understand the dispersal of inoculum from currently important wheat pathogens. We have devised a helium balloon sampler that can collect spore samples at various heights without being limited to permanent tall buildings or towers. This device will allow us to study the elevation that different wheat pathogen spores can ascend, and from those data be able to model distances the spores can travel horizontally. Information about the numbers of spores will also be useful for predicting diffusion of the pathogen and assessing the epidemiological significance of long-range dispersal.

Materials and Methods
The aerial spore collector consists of the balloon, the Rotorod spore sampler, and the platform (Table 1).

The Balloon
A 7 ft. (2.13 m) diameter round vinyl balloon filled with helium provided the lift for the platform and collector. The balloon has a lift capacity of approximately 8.5 lbs.(3.86 kg).

The Rotorod Sampler
The Rotorod sampler model 20 is a rotating arm impactor that collects spores on rapidly moving (2400 RPM) polystyrene rods. This type of sampler has become widely accepted as a device to collect fungal spores since its development (Asai 1960, Perkins 1957, Campbell and Madden 1990). The model 20 has two rods, each with a collecting surface with dimensions of 0.06 X 1.4 inches (1.5 x 36 mm). The collector provides a volumetric sample of 9.2 ft³ (0.26 m³) per hour. Therefore, knowing the sample time results in a quantitative estimate.

Retracting heads were purchased separately. Springs in these heads hold the rods in a protective sheath when the collector is not spinning. Centrifugal force exposes the rods when the collector spins.

The Platform
The platform included an attachment for the balloon, tethers, attachments for the tethers, an attachment point for the Rotorod sampler, a 12-volt power supply, an on/off switch, and an indicator buzzer. The platform base is a triangular piece of 1/4" plywood cut to 16 in. (40 cm) on a side. A triangular shape was chosen over a square because it offered greater stability with fewer tether lines. Eye bolts were installed at each corner of the plywood for tethers and a U-bolt near the center served as the balloon attachment (Figure 1). Nylon rope approximately 0.1 inches in diameter was used for tethers. Tethers were 164 yards (150 m) long with a 100 lb. tensile strength. A 3/4 inch diameter, 12 inch long threaded aluminum pipe, capped with a threaded nut and inserted into the middle of the board, was the attachment for the Rotorod sampler (Figure 2). A push on/push off switch mechanism (Figure 3) allowed the sampler to be turned on and off from the ground. The string that is attached to the switch mechanism is suspended down the aluminum pipe to avoid tangling with the sampler and tethers. The string was attached to a hinge that runs over the switch to press the switch on or off. A spring also supports the hinge to maintain the on or off mode as needed (Figure 3)

The sampler is powered by a 12-volt rechargeable nickel-cadmium battery (Figure 1) with the switch in series and an indicator buzzer and sampler in parallel. The indicator buzzer signals the on mode of the sampler with an intermittent 3.5 kHz sound.

Table 1. Materials, cost and suppliers for construction of a helium balloon spore sampler.

Results and Discussion
Preliminary flights were held within Fargo city limits to test system design. The balloon lifted the sampler, which had a total weight of 3.5 lbs. The excess lifting capacity was enough to reach an elevation of 150 feet and keep the platform stable under low wind (<10 mph) conditions. The audio signal could be clearly heard by ground personnel from more than 300 feet. The nickel-cadmium battery performed for at least 6 hours on a charge and the switch worked well but required an extra person to turn the collector on and off.

Sampler operation was tested near Prosper, ND in a wheat field that was being harvested from windrows. For three samples, test flight parameters ranged from 150 to 750 yards downwind of the combines, from 16 to 75 feet elevation, and three to six combine passes. Collection heads were disassembled and surfaces examined under a microscope. Three conidia of Bipolaris sorokiniana were found in the sample taken at 50 ft. above the ground and 300 yards downwind from the harvested windrow. This fungus causes common root rot and spot blotch of wheat leaves. Test results suggested that spores of a wheat pathogen can be detected with this sampling methodology.

Wind produced the most difficulty for balloon stability. According to North Dakota Agricultural Weather Network (NDAWN) records from the station closest to the sampler trials, average wind speeds at 10 ft. during testing ranged from 6 to 11 mph with gusts up to 17 mph. During gusts, the balloon was forced down so that sampling at a specific altitude could not be maintained. To maintain sampling within predetermined limits, the sampler must be turned off as it moves below the accepted sampling height and turned back on when the altitude increases again.

The problem of collection at a specific altitude was addressed after the field trials. The push on/push off switch was replaced with a push on/release off switch (Table 1) that would turn the collector on when the switch was compressed and off when decompressed. The same hinge and spring setup was used and a 1 lb. (0.45 kg) weight was added to the end of the string that is attached to the switch mechanism. The weight was attached at the length of the desired elevation. When the weight comes off the ground the switch is compressed and the collector starts to spin. When the weight comes in contact with the ground it puts slack in the string and the switch decompresses, shutting off the device.

This collecting system is portable and can be transferred from field to field and from site to site within a field. The balloon was transported in the back of a full-sized pickup truck (Figure 4). A poly-type tarp was put under the balloon to prevent punctures. Another tarp was placed on top of the balloon and was tied down to the corners of the truck to secure it. Both tarps were tied to the corners and middle of the truck box. This allowed transport at normal highway speeds.

The performance of the sampler could be improved, but ideas were excluded to reduce costs. A lightweight radio-controlled servo-mechanism to turn the sampler on and off could be purchased for around $200. This improvement would eliminate the need for an extra person and the string controlling the switch. An enclosed 7 ft. wide trailer would allow for faster transport by eliminating time for tie down and concern about accidental release. The trailer would also eliminate concern for puncture in transport and allow for faster travel. Finally, a second sampler would allow measurement of spores upwind of the field under study to determine background levels of air spora.

The sampler has several advantages over other systems. Any crop and growth stage can be sampled due to the portability of this system. The sampler is useful in remote areas where fixed towers or buildings are not available to measure spore dispersal height. The balloon can reach an elevation of 150 ft., unlike portable towers that are limited to a length of about 30 ft. As height increases, so does the frequency of laminar air flow; therefore, the accuracy of the sample is affected less by nonlaminar air flow caused by ground obstructions such as buildings or trees. Cost of materials is less than $1,000; some volumetric samplers cost more than $4,000. The major disadvantage of a helium balloon sampler is the dependance on clement weather with low wind.

We are particularly interested in the liberation and subsequent dispersal of wheat fungal pathogens during harvest in the southern Great Plains. Our hypothesis is that newly important pathogens are part of the air spora being transported long distances across the Great Plains. This device can give us sample data to test this theory.

Literature Cited
Asai, G.N. 1960. Intra- and interregional movement of black stem rust in the upper Mississippi Valley. Phytopathology 50:535-541.

Buchwaldt, L., R.A. Morrall, G. Chongo, and C.C. Bernier. 1996. Wind born dispersal of Colletotrichum truncatum and survival in infested lentil debris. Phytopathology 86:1193-1198.

Campbell, C.L., and L.V. Madden.1990. Introduction To Plant Disease Epidemiology. Pp. 83-87. John Wiley and Sons, Inc.

Fletcher, R.C., E.B. Lambert, K.M. Nagler, and E.C. Stakman. 1953. The origin and extent of a regional spore shower of wheat stem rust. Phytopathology 43:471 (Abstr.).

Khonga, E.B., and J.C. Sutton. 1988. Inoculum production and survival of Gibberella zeae in maize and wheat residue. Can. J. Plant Pathol. 10:232-39.

Lyon, F.L., C.L. Krames, and M.G. Eversmeyer, 1984. Vertical variation of airspora concentrations in the atmosphere. Grana 23:123-125.

McMullen, M.P., R. Jones, and D. Gallenberg. Scab of wheat and barley: a re-emerging disease of devastating impact. Plant Dis. 81:1340-1348

McMullen, M.P., and D.R. Nelson. 1992. Tan spot and five years of disease survey. Pp. 80-85 in: Advances in Tan Spot Research. L.J. Francl, J.M. Krupinsky and M.P. McMullen, eds. North Dakota Agricultural Experiment Station Special Publication.

Nagarajan, S. and D.V.Singh. 1990. Long distance dispersion of rust pathogens. Annu. Rev. Phytopathol. 28:139-153.

Perkins, W.A. 1957. The rotorod sampler. Second semiannual report of the aerosol laboratory, Stanford University, Palo Alto, CA, 66 pp.

Rowe, R.C., S.A. Johnson, and M.K. Beute. 1974. Formation and dispersal of Cylindrocladium crotalariae microsclerotia in infected peanut roots. Phytopathology 64:1294-1297.

Shaner, G. 1981. Effects of environment on fungal leaf blights of small grain. Annu. Rev. Phytopathol. 19:273-296.

Sheehy, J. 1969. Aerobiology and epidemiology of organisms associated with black point of durum wheat. M.S. Thesis North Dakota State University.

Stakman, E.C., A.W. Henry, G.C. Curran, and W.N. Christopher. 1923. Spores in the upper air. J. Agric. Res. 24:599-606.

Stakman, E.C. 1934. Epidemiology of Cereal Rusts. Proc. Pac. Sci. Congr., 5th, 1933 vol. 4, Pp. 3177-3184.

Project Background

Authors
T.L. Friesen, Research Specialist II
Department of Plant Pathology
North Dakota State University
Fargo, North Dakota 58105
tfriesen@badlands.nodak.edu

E.D. De Wolf, Graduate Student
Department of Plant Pathology
North Dakota State University
Fargo, North Dakota 58105
edewolf@plains.nodak.edu

L.J. Francl, Associate Professor
Department of Plant Pathology
North Dakota State University
Fargo, North Dakota 58105
francl@badlands.nodak.edu
http://www.ndsu.nodak.edu/instruct/francl/

Corresponding Author
L.J. Francl

Location where research was (primarily) done
Fargo, North Dakota

Funding source
State appropriations

Table of Contents – Spring 1998

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