A Portable Aerial Spore Collecting System (continued)

Keywords

Introduction

Materials and
Methods

Results and
Discussion

References

Project
Background

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) (52KB color jpg photo). 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) (52KB color jpg photo). A push on/push off switch mechanism (Figure 3) (34KB color jpg photo) 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) (34KB color jpg photo).

The sampler is powered by a 12-volt rechargeable nickel-cadmium battery (Figure 1) (52KB color jpg photo) 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.

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Table of Contents – Spring 1998