Wheat yellow rust (Puccinia striiformis
f. sp. tritici
), also known as wheat stripe rust, is a fungal micro-organism which blights wheat crops worldwide. It is so named due to the yellow striations it forms along venations on leaf blades, which resemble the surface texture and discolouration attributed to oxidation in ferrous metals. Though the spores primarily target leaf blades, leaf sheathes and spikes can also be affected in conditions of high humidity and rainfall, or during epidemics. The onset of disease typically occurs early in the growth season, when the ambient temperature falls within a range of 2 to 15 °C, but can occur up to a maximum of 23 °C [1
]. Symptoms include stunted development and weakening of affected crops, reduced numbers of spikes, shriveled grains occurring in fewer numbers per spike than on healthy plants, and losses in grain mass [1
]. Temperatures during the time of winter wheat emergence and the coldest period of the year are crucial for the development of epidemics. As such, in countries where wheat is grown during the winter or at high elevations, yellow rust is a common threat.
We consider Ethiopia as an example to highlight how yellow rust affects harvested wheat crops. Ethiopia’s climate is amenable to wheat cultivation all year round, and 81% of its exports are accounted for by agricultural products, which constitute 34.8% of its gross domestic product [2
]. In addition, close to 72.7% of its labor force work in the agricultural sector. More specifically, its annual wheat production accounts for the dominant share (41.6%) of that of all Sub-Saharan African (SSA) countries. Yellow rust routinely destroys, or otherwise renders unusable, between 5% and 25% (in rare instances) of wheat crop harvested [3
]. Thus, there exists significant scope for developing a low-cost, low-power, user-friendly technological means for early-warning spore detection in the region. Many of the symptoms described earlier are not apparent to the naked eye until after the infection has progressed to a point where intervention is ineffective, leading to inevitable crop degradation or losses. Furthermore, whatever remedial action is taken has deleterious ecological effects and is costly.
This project, named Sentinel, aims to provide a cheap, reliable, and globally networked “24/7” in-field bio-alarm system to detect, within a matter of hours, fungal and, potentially, bacterial pathogenic attacks on crops. This system would effectively act as a proxy extension worker with an analytical lab, positioned day and night in all susceptible fields. The data then form a reliable dynamic feed into existing mechanistic spore transport models providing a ground-truth and dramatically increasing the temporal and spatial surveillance accuracy for disease outbreak management. The Sentinel sensor-nodes will be located in the “hot spots” as identified by those models [4
]. Parallel ongoing research by the authors has demonstrated that a disposable 1 cm2
biomaterial structure and volatile release mechanism can be produced for volume manufacture, with extended-life (i.e., a full growing season), within a biosensor film cartridge. The roadmap being to deliver, within each sub-US10¢ film cartridge, an array of independently addressable “biomimic” detection elements that are “tuned” to specifically trigger the germination, directional growth, and hyphae penetration mechanism of specific fungal species and families. In this way one small coin-sized cartridge can simultaneously detect multiple spore germination events, minimizing false positives, and enabling the Sentinels to identify numerous fungal and bacterial diseases. The resulting wireless Sentinel networks, and data services, can then be developed for delivery as a sustainable SSA business model initially exemplified in Ethiopia against Puccinia striiformis
f. sp. tritici
) followed by P. graminis
f. sp. tritici
), i.e., wheat yellow and stem rusts, respectively.
Due to the limited number of tolerant cultivars to Pst
, the use of triazole fungicides remains the most widely used solution in SSA, following crop infection, with some alternates, such as succinate dehydrogenase inhibitor (SDHI) and strobilurin fungicide, which have varying degrees of cost and effectiveness which reduce their suitability for use in lower-to-middle income countries (LMIC). Of particular concern is the prospect of rust tolerance developing to single and mixed combinations of triazoles, and other fungicides, through genetic mutation of the pathogens, which is exacerbated by their over usage [5
]. As the curative and preventative modes of actions of the range of fungicidal treatments are both highly dependent on the accurate spatial and temporal application on affected or susceptible crops, respectively, the Sentinel network, in combination with spore transport forecasting, enables precision optimized application of the crop protection chemistries. For all farming nations, and in particular LMICs, the reduced usage of fungicides enables inventory control to be effectively managed such that adequate chemical and labor resources are available to effect the treatments. The quantitative monetary value and yield security, per growing region, from such a policy may be projected but will be highly dependent on a range of factors such as seasonal variations in climate, giving rise to pathogen growth, as well as the nature of daily spore transport giving rise to acute or chronic disease development. Suffice to say in Ethiopia alone, the partner country for this research, over a quarter of a million smallholder farming families are reliant on their wheat production for subsistence and livelihood security and government-funded text messaging system had to be introduced as an interim measure to attempt to stem the potential for a major disease event [6
]. In partnership, this research underpins the future strategy for crop protection and supports the introduction of traceable integrated pest management (IPM) policies to minimize the likelihood of genetic tolerance of the pathogens to the limited suite of regulatory-approved fungicidal actives.
A recent example design for a spore sampler is the so-called “Spornado” device [7
]. The sampler aims to detect fungal disease such as late blight, sclerotinia, fusarium head blight, and powdery and downy mildews. The sampler has a funnel like shape with a vane to allow weathervaning. Spores are trapped into specialised filters hosted in the sampler which can then be removed and sent to a lab for targeted DNA testing [7
]. This is different from our proposed Sentinels which can provide faster and more representative information on spores, as described above. Another example is the latest model of spore traps manufactured by Burkard Manufacturing Ltd. operating using 25 W vacuum pumps running on either a 240 V alternating current supply or 12 V batteries [8
]. This Burkard spore trap is shipped within a case measuring 570 × 650 × 650 mm, where its nominal working area is 0.89 m2
through a 530 mm radius, with an overall height of 940 mm once fully constructed and installed [9
]. It is evident that this approach requires large power supplies, maintenance of moving machinery, and training of users to operate reliably. A key objective, therefore, is to eliminate these requirements by designing a system that can operate with little-to-no human intervention beyond simple installation within a crop to be monitored, whilst requiring considerably less input power.
This communication will focus on the design and testing processes involved in the development of a passive particle sampler in order to increase the probability of spores interacting with an artificial biomimetic surface. This passive sampler will act as an air multiplier, increasing the effective number of spores impinging on the measurement area. This constitutes a novel design objective as most of fluidic devices are usually designed to modulate flow behavior such as tailoring adverse pressure gradient along a device [10
]. A prototype preliminary
design was developed and simulated using computational fluid dynamics (CFD) with additional Lagrangian particle tracking and the performance was validated using wind tunnel testing. Particle interaction probabilities with the biomimetic sensor for different Stokes number values were estimated to evaluate the added value of employing the developed sampler design.
It is anticipated that this developed sensing technology would bring about a sizable positive impact, both to East Africa and beyond. Possible benefits of the successful application of this technology include easing economic and commercial hardship in nations whose markets and development are underpinned by exports of agricultural products, reducing the deleterious ecological impact of spraying fungicide through more effective targeting by limiting use to crops which require such intervention, as well as improving the overall quality and amount of crop harvested, increasing food stocks and availability to populations worldwide. Moreover, such sensing technology could also be generalized to detect many forms of airborne pathogen, even those which are harmful to humans. As a result, one possible future application beyond that which is currently being pursued is monitoring of bacterial, fungal, or viral illnesses in enclosed environments such as hospital wards that spread via airborne transmission.
The rest of the communication is organized as follows: Section 2
explains the methods used; results are then presented and discussed in Section 3
; and, finally, some concluding remarks are highlighted in Section 4