INTRODUCTION
No matter how much we choose to talk about technological advancements in recent times, changes in climate, political sphere, nuclear wars, terrorism, etc., though all of significance; one factor that has been under-emphasized is the rate of loss suffered in the aspect of food production, stemming from infestation of crops by plant diseases, transmitted by aphids.
There has been an increasing call to individuals to practice subsistence farming, particularly in underdeveloped and developing countries, in order to reduce the surge of malnourishment and hunger. However, if one, with limited resources still goes ahead to engage in farming, without a knowledge of potential risks on his/her crops, mitigation measures,etc., how can we achieve a set-out goal and kick out hunger?
Even worse is the massive loss suffered in the course of farming for commercial purposes; time invested, money and resources imputed, and corresponding effect as a result of scarcity in the market.
In Nigeria for instance, there was a blight infection of Tomatoes not too long ago that created panic nationwide. The prices of tomato sky-rocketed by nearly 1000%, and the corresponding effect is better left in our memories than explained.
I'll like to focus on plant viruses, transmission agents and effects. Things we need to know, to better our efforts.
HISTORY AND EVOLUTION
There are many hundreds of plant viruses, most of which are dependent upon a vector for transmission between, and inoculation into plant hosts. More than a decade ago, numerous arthropod vectors of plant and animal viruses were identified; and since then, plant-infecting viruses have evolved many interesting and biologically complex association with their vector-carriers.
Over the years, as humans intruded into previously undisturbed ecosystems, in an effort to domesticate the land, we also intruded into virus-vector relationships. The large practice had been monoculture, i.e., planting single monogenic crop on large acres; however, with the advent of modern agricultural techniques and practices, there has been an upsurge in emerging plant viruses, because there are new opportunities for them to exploit and attack available hosts.
Though till date, much has not been discovered about the molecular and cellular mechanisms that regulate the transmission processes and efficiency between plant virus and their carriers; however, in terms of epidemiology, insects are the most important factors in plant virus disease.
WHAT ARE APHIDS?
illustrating aphids on a plant stem
Image Source
They are commonly known as "plant lice", but specifically as whiteflies or greenflies in Britain and the Commonwealth. They are sap sucking insects, and are among the most destructive insect pests on cultivated plants.
The damage they cause to plants has made them enemies to us farmers but the zoologist would tell you they are a very successful group of organisms (quite annoying😏). This success is in part due to the asexual reproductive capability of some species.
There are about 4,400 species, belonging to 10 families, with the family, Aphididae, being the most recognized. Their natural enemies include predatory ladybirds, hoverfly larvae, parasitic wasps, crab spiders, etc.
Aphid Anatomy:
Most of them have soft bodies, which may be green, brown, black or almost colourless. They have antennas with as many as 6 segments. They feed through sucking mouth parts called 'stylets', enclosed in a sheath called 'rostrum', formed from the modification of the mandible and maxilla of the insects mouth parts, as shown in the image below.
They have compound eyes, and an ocular tubercle behind and above each eye, made up of 3 lenses called 'triommatidea'.
When host plant quality becomes poor, or conditions become crowded, some aphid species produce winged offspring, called 'alates' that can disperse to other food sources.
Feeding:
Many aphids feed on only one plant species, others like Myzus persicae, feeds on hundreds on plant species across families. They feed passively on sap of phloem vessels in plants. Once a phloem vessel is punctured, the sap which is under high pressure, is forced into the aphid food canal.
Occasionally, aphids ingest xylem sap, which is a more dilute diet than phloem sap, in that it's concentration of sugar and amino acids are smaller. In this case, active sucking pressure is required as xylem sap is under negative hydrostatic pressure.
Feeding of barley yellow dwarf causing virus
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It is during this course of feeding that aphids often transmit plant viruses to the plants.
Aphid Reproduction:
Some of them have unusual and complex reproductive adaptations, while in others, it is fairly simple. Adaptations include both sexual and asexual reproduction, creation of eggs or live lymph and switches between woody and herbaceous types of host plant at different times of the year.
Life cycle of Pea Aphid
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Many aphids undergo cyclical parthenogenesis. In the spring and summer, mostly or only females are present in the population. Females undergo a modified meiosis that results in eggs that are genetically identical to their mother. The offspring resembles their parent in every way except size.
The process replays all through the summer, producing multiple generations that typically live 20-40 days. Thus, one female hatched in spring may produce many billions of descendants (absolutely crazy🙄🙄).
The males are genetically identical to their mother, except that they have one less sex chromosome. When sexual female and male mates, the female lays eggs that develop outside the mother. The eggs endure the winter and emerge as winged or wingless females the following spring.
Effects on Plants:
Plants exhibiting aphid damage may show a variety of symptoms, including decreased growth rate, yellowing of leaves (as in the image that follows), wilting, low yields and death. The removal of sap creates a lack of vigour as aphid saliva is toxic to plants.
Aphids contributes to the spread of late blight of potatoes and significantly reduces the output and subsequent scarcity.
Cherry aphid causes curly leaf of cherry trees.
The effect of aphids on plants is endless, but for the point still yet to be passed on, I'll stop here.
The damage of plants, and in particular commercial crops, have resulted in large amounts of resources and effort being channeled to attempting to control the activities of aphids.
PLANT VIRUSES
They are simply viruses affecting plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host, and they are pathogenic to higher plants.
The first plant virus to be discovered was the Tobacco Mosaic Virus (TMV), which alongside other viruses caused huge loses in crops yield worldwide.
Subsequent studies have classified plant viruses into 73 genera and 43 families.
Structure of Plant Viruses:
Viruses are extremely small and can only be observed with an electron microscope. The structure of a plant virus is given by its coat of proteins, which surround the viral genome.
Over 50% of known plant viruses are rod-shaped. The length of the particle is usually between 300-500nm (nanometer), with a diameter of 15-20nm. The most common structure amongst plant viruses are isometric particles which are 40-50nm in diameter.
Transmission of Plant Viruses:
For plant viruses to perpetuate themselves, they must be spread from time to time and from one susceptible host to another. Since plant cells have robust cell walls and viruses cannot penetrate them unaided, there is need for wounding of host tissues to establish success; a fundamental difference to their close relatives in the animal world.
There are different means through which they are transmitted and they include the following;
1.) Through sap
2.) Insects
3.) Nematodes
4.) Plasmodiophorids
5.) Seed and pollen
6.) Fungi
The mechanisms involved in each case differs to a great extent.
EFFECT OF PLANT VIRUSES ON FOOD PRODUCTION: A Case Study
Before I take on an instance that nearly brought a popular food item to extinction, I would like to share my recent experience in our little vegetable farm. The farm houses species of Talinium fruticosum, Vernonia amygdalina, Telfairia occidentalis, Cymbopogon citratus, Musa acuminata majorly. In plain English 😂 Water leaf, Bitter leaf, fluted pumpkin, Lemon grass and Banana.
An Overview of my garden
I did a routine check on the leaves of all, only to discover that the bitter leaf had changed coloration, showing specks of yellow as indicated in the next image. The ideal thing was to eliminate them, to avoid spread to other inhabitants.
The others were quite healthy and the yield bounty. Check out the leaf of the pumpkin in next image. Healthy and nutritious, I'm thinking of making vegetable soup by weekend.
Soybean Aphid infection of IOWA
In 2000, the soybean aphid invasion was detected in North America. Since then, outbreaks of this invasive species have become commonplace, threatening U.S. soybean production -- a crop valued at more than $40 billion in 2011. Aphid outbreaks are inconsistent and unpredictable, but they continually threaten food security worldwide because of the plant diseases they spread.
Soybean aphids feed by piercing a plant's phloem, the vein-like sap transport system plants use for circulating nutrients to their near and distant parts. Feeding sessions rob the plant of the nutrients it needs to grow.
Soybean aphids feed by piercing a plant's phloem, the vein-like sap transport system plants use for circulating nutrients to their near and distant parts. Feeding sessions rob the plant of the nutrients it needs to grow.
Additionally, every time an aphid taps a fresh phloem tube, viruses flow through the insect's probing needle-like mouth that draws sap as readily as it releases aphid backwash.
In Iowa, over $60million is spent anually to curtail the effects of these aphids, without which the large expanse of land and produce likewise would be lost.
THE WAY FORWARD
The first step in the solution of any problem is the identification of the problem. Therefore, what are the possible steps to ameliorate, and eventually avoid these viral infestations?
Truth is, aphid vectors cannot always be controlled by chemical means because it may end up killing relevant insects. However, understanding the mechanism of virus transmission is vital to developing key strategies to block virus-vector interactions.
Strategies include;
1.) Exclude or avoid the virus:
Plant virus-free seeds and healthy seedling transplants, cuttings and tubers.
Grow crops in regions where the virus seldom occurs or during periods when virus or aphids are at low level.
For commercial purposes, quarantine regulations should be followed.
2.) Reduce Virus Levels:
Regular control of weeds and other hosts of viruses and aphids around farm buildings.
Destroy old crops promptly, as these often have a high level of viral infection.
Avoid planting new crops next to old ones.
Plant unwinds to reduce number of aphids moving to young crops.
3.) Protect the crops:
Plant virus-resistant varieties.
Use highly reflective mulches and oil sprays to determine aphids from landing and feeding.
Use tall barrier crops and windbreaks on the periphery.
Avoid certain insecticides and seek expert opinion before you make a choice. This is because some insecticide agitate aphids and encourage greater movement and feeding.
SUMMARY
The financial equivalent of losses incurred as a result of plant pathogenic infestations cannot be over-empasized, and with correct measures it can be totally controlled.
For those of us who engage in small-scale farming, we need to be at alert and observant to make sure our crops are faring well.
We all know the importance of food, so let's up our game.
Cheers!!
REFERENCES
[Source 2](Chen. B. et. al. 1990. Cucumovirus transmission by the aphid Myzus persicae is determined by viral coat protein. Journal of General Virology 71:939-944)
[Source 3](Harris K. F. 1990. Aphid transmission of plant viruses. In Plant Viruses vol. II)
[Source 4](Mc.Gavin, G. C. 1993. Bugs of the World. Infobase Publishing)
[Source 5](Mutti N. S. 2006. Molecular studies of salivary glands of pea aphids. Ph.D Thesis, Kansas State University)
[Source 6](Power A. G. 2002.Current opinion on Plant Biology. 3: 336-340)
