There are many species under the genus Striga, most notably Asiatica, Gesnerioides, and Hermonthica. However, not only these categories exist as studies show there could be more than thirty. In this article, we will delve into witchweed varieties, devoid of focusing on a single species. In this way, this article will provide valuable information to anyone facing challenges owing to the presence of witchweed.
The witchweed plant is a parasite. That is to say that it cannot survive where it does not have a host. It competes with the host for water, nutrients, and carbohydrates from the root of the host plant. Some examples of affected plants include rice, wheat, maize, sugarcane, sorghum, and cowpeas. As it competes with the plant, it drains it and results in reduced yields in crop fields.
This parasitic plant is commonly found in semi-arid areas where the soils are dry and infertile. You will find that most marginal communities reside in these lands where they engage in farming. With this weed in play, they take very little crop home, and this affects their financial ability.
Eradicating this plant uses up a lot of money. Take the United States as an example. The presence of this weed has led to an expenditure of over a quarter billion US dollars in control methods. That money could have gone into use in other sectors, were it not for the invasive nature of this parasite.
And that is not all. One established plant gives rise to thousands of seeds in a lifetime. These seeds can remain viable for over a decade before they establish. Once they take root, getting rid of them is hard, and it often requires prolonged and costly methods. Thus, this plant is a threat, and treating it as such would be fitting. Let’s get into it.
Witchweeds, which also go by the name parasitic witchweeds, fall under the Striga species. They belong to the Orobanchaceae family. The Striga species is thought to have at least thirty species in it. However, the number is not precise as there exist taxonomic uncertainties as to the same. Of the more than 30 species, three have proven to be problematic. They are Asiatica, Gesneriodes, and Hermonthica.
These species are annual plants, and they feed off grasses and legumes, showing a preference for grasses. Identifying them is hard as some of them remain underground for most of their life. Here, they can rob their nutrients of water and nutrients, out of sight. This behavior led to the name witchweed. They emerge from the ground during the flowering seasons before disappearing under the growth at later stages. In most cases, they will grow at the base of their host plants.
Origin of witchweed
These weeds are native to the tropical and semi-arid regions in the African continent. However, there have been sightings of the same in more than 30 countries across the globe. Take the Asiatica as an example. This species is native to tropical regions in both Africa and Asia, with China and Thailand making good examples. However, it has been reported in the United States.
The Gesnerioides is native to Asia, African, and Arabia, yet it has been sighted in Florida in the United States. Hermonthica is native to the semi-arid regions of Africa, mostly in the northern parts such as Senegal. Yet there have been sightings of the same in other parts across the globe.
Identification of Striga
The height of the witchweed at maturity will depend on the species. Most of them will attain heights of 6-8 inches. However, some species can be as tall as 25 in.
Their stems are bright green with square cross-sections. Along the stems are short white hairs with a coarse texture. The leaves occur in opposite positioning, and they are narrow with lanceolate shapes. They can measure anything from 0.4 to 1.2 inches in length, and they occur in pairs.
The flowers are often small, and they vary in color depending on their region. In Africa, they could be orange, yellow, or red. In Asia, they could be pink, purple, white, or yellow. These weeds also give rise to fruits, which are in the form of winged capsules. In the capsules are seeds, which could be as few as 250 or as many as 500. They are as small as dust, and dispersal is therefore quite easy.
Underneath the ground surface, the stems are white with round cross-sections. They are devoid of hairs, and they attach to the roots of their hosts.
Here is how you can tell the main witchweed varieties apart.
This species can attain a height of up to 25 inches, and it features square-shaped, hairy stems. Its flowers range in color from pink to purple.
This species grows to a height of at least 12 inches and has a purplish/ brownish hue on the branched stem. The root swells up to 1.2 inch in diameter. The flowers on this species could be purple, pink or creamy.
This species results in bright red flowers in most cases. However, in some regions, they will be pink, yellow, or white.
Identifying these species is not easy as they often grow out of sight. However, taking a walk around your compound or crop field could help in the detection of the same.
Preferred Habitat of witchweed
These species do best in semi-arid regions as they can withstand hot and dry conditions. In the presence of rain, they can establish. Asiatica has been sighted in warm temperate areas, as well as dry tropics and subtropics. The same goes for the Hermonthica. But when it comes to the Gesnerioides, there is a variation. This species has been seen to do well even in seasonally wet regions.
Usually, these species prefer infertile soils. However, assessments show that Asiatica can do well in a range of soils. Be they sandy, clay, shallow, or dry, it will establish in the presence of rain.
All of these species have a preferred host and will not establish in the absence of these plants. For the Asiatica and Hermonthica, the preferred hosts are cereals. But in the case of Gesnerioides, legumes take the day.
Reproduction of witchweed
These species reproduce through the production of seeds. Think about it like this. One capsule could hold as many as 500 seeds. But a plant does not give rise to one capsule at a time. Instead, it produces a higher number. Thus, one plant can release as many as 25,000 seeds into the environment. Where the plant has established well and has ideal conditions, it can give rise to as many as two hundred thousand seeds. Can you picture this number of seeds making its way through crop fields? It would be a disaster.
When the ideal conditions present themselves, the seeds establish. Seeds grow when they are near suitable hosts, which is one of the required conditions. Suppose a seed is near a wheat plant, it can establish. It would need ideal temperatures in the ranges of twenty degrees Celsius. It would also require at least one week of moisture to germinate. However, ideal conditions do not lead to the germination of all seeds as some will remain dormant, only to establish at later dates.
As the seed germinates, the seedling grows towards the root of the suitable host plant. It moves through the outer layer of the root and penetrates the plant. This process is possible, thanks to a structure known as the haustorium. Without penetrating a host, the seedling cannot survive. That is because the endosperm of the seed can only provide nutrients for up to a week. If this time lapses before the seedling finds a host, it will die.
With the haustorium in place, the seedling can now feed off the host, and as it does so, the structure thickens. It does so to allow for the passing of water and nutrients from the host to the parasite. The seedling grows underground for up to four or six weeks. After this, it emerges from the ground and produces its first green leaves, which can now make food for the plant. But that does not happen in all cases. Some Striga species prefer to remain under the ground surface and derive all their nutrients from the host, thus weakening it.
Flowering will depend on the species as well as the prevailing conditions. Take an example of the Asiatica. In this case, the species would emerge from the ground and remain there for about four weeks. After this period, flowering would start if the environment provided suitable conditions for this. But with the Gesnerioides, it is a different story. Witchweed starts flowering as it emerges from the ground. The Hermonthica crosses with other of its species and will, therefore, require pollination to aid in the formation of seeds. But the other two species are self-pollinating.
Dispersal of Striga
The seeds get dispersed through various methods as they are quite light. From the wind to water sources through to human soil and animal movement, the dispersal methods are in plenty. For animals and humans, all they need is to attach themselves to the fur, feathers, feet, or clothes. For the seeds that fall on the soil, these can remain viable for up to fifteen years.
Signs of witchweed infestation
How can you tell you have a witchweed problem on your farm or outdoors? The good thing is that though you may not see the plants with ease, you can watch out for symptoms. A good example would be stunted growth. If your farm has everything the host plant would need, and the plants seem to be doing poorly, you probably have an underlying problem. Get the pun?
Also, you should be on the lookout for wilted and yellow leaves. As long as the environmental conditions are favorable and you have done all that you can, this could be an indication of an infestation.
Some of the plants which often fall victim to this parasite are wheat, sorghum, and corn.
Witchweed has a long history of causing losses in farms. Let us take a look at Africa, where these species lead to losses in the ranges of seven billion US dollars yearly. Witchweed affects over 300 million livelihoods and are thus a problem to the society and the economy.
These weeds have been a deterrent in sustainable food production, more so in the Sub-Saharan region, where they affect at least 30% of cereal farms. Where a farm has witchweed infestation, yield can fall by as much as 70%. In some cases, farmers have lost all their crops to this species.
In most cases, the biggest losers to the witchweed are small-scale farmers. Given their scale of production, they are unable to deal with this weed effectively. Chemical control is expensive, and they lack the means to use this form of management. Adopting modern agricultural practices is also not an available option for them. To make matters worse, these farmers often plant crop species that are most vulnerable to witchweed.
In the United States, the Asiatica species took over at a fast rate, invading over 175,000 hectares within eight years. The government, seeing that it was a growing problem, put measures in place to deal with witchweed for good. As it stands at present, the reduction stands at 99%. This control set the government back a whopping 250 million US dollars.
This weed affects economies significantly. Where export grain has an infestation, it gets rejected, and this affects the economic well-being of farmers.
How to prevent witchweed
It is always best to deal with weeds early before they get a chance to spread. A good example would be in Florida, where there were reports of Gesnerioides in reclaimed land. The authorities acted fast, and within no time, witchweed was no longer a problem. Different people can take up varying preventative measures to keep the witchweed from establishing. Here is what you can do:
Farmers are often the most hard-hit by infestations, as this takes a toll on their economic returns. Taking practical steps in the prevention of witchweed infestations is, therefore, necessary.
Have a system in place
It all starts with having adequate information as to this weed. Learn what you can about witchweed by interacting with the local authority as well as other interested groups. You can learn how you can identify witchweed and deal with it effectively. Also, come up with a plan as to how you would deal with the weed if it were to occur. Where you come across witchweed, it helps to use integrated methods in its eradication.
It is quite easy to introduce witchweed into your farm without knowing it. You could get plant material from another country, only to later find out that they had witchweed seeds. It thus helps to certify that the plant material is weed-free and to abide by the rules applicable in your region.
Monitor vehicles and equipment
Where there are infestations in a field, you should avoid the area, whether on foot or by car. You should also designate roads which people can use when moving around the farm. Always avoid disturbing the infested areas as this encourages seed dispersal. Where you have to pass by an infested area, ensure that you clean machinery and cars afterward.
Where you do not have infestations on your farm, you should be wary of visitors. Insist on having vehicles entering the field cleaned before they can make their way through the farm. The same goes for equipment. You should also observe this when entering other people’s farms.
Contain your farm
It is always best to keep weeds at bay by creating containment areas on your farm. Start by having a boundary through which seeds from neighboring farms cannot get to your field. Also, clear paths around the farm to ensure they remain free of weeds.
Check feed before giving it to livestock to ensure that it does not have any contamination. Where animals leave your farm, do not allow them to mix with others upon return. Instead, hold them in a separate area where you can check them for seeds. It also allows them to pass anything they consumed while they were away.
Deal with witchweed
If you do have an infestation in your land, you should ensure that you dispose of witchweed in the prescribed manner. That means that all cuttings should be in landfills or burned. Follow the guidelines as per the reigning authority to ensure you are in line with the law.
While you may not know this, home gardens could also be a source of weeds that could wreak havoc on nearby fields. With witchweed in your garden, dispersal would take place at a fast rate.
Ensure that you buy soil that is free of weed. It helps to check with authorities before getting plant material from another country. Also, be sure to get a lot of information as to possible weeds in your area. Where you come across witchweed, do not dump them in your garden or another such place. Always dispose of them in the prescribed manner, which may include transport to a landfill or burning.
Now, let us look at how you can control this weed:
How to control witchweed
This method involves altering the environmental conditions to make them less favorable to witchweed. Where the soil is dry, you can water it as often as possible to keep the seeds from establishing. Where the soil nutrients are deficient, you can make them better by adding manures. You could also practice crop rotation with legumes that would enrich the soil. Using fertilizers has also proven to reduce the emergence of weeds and increase crop production.
Where low humidity coupled with high transpiration reign, you could alter the conditions by closely planting the crops. You could also add leafy crops to the field to deal with this. Studies show that planting when the rains have set in also helps in reducing the emergence of witchweed. Care should be taken, though, to ensure that these practices do not interfere with the productiveness of the crops.
Transplanting also works in reducing the chances of an attack. The cultural means in place will ultimately depend on the prevailing conditions.
While you could use physical means to eradicate witchweed, it would not work effectively. That is because underneath the ground level, the buds could regenerate after disturbance. If you choose to dig up the witchweed, you have to do it repetitively, with breaks of two weeks. However, if you plan the digging to the mid-season, the results could be better.
Other than digging, you could pull the weeds out by hand. This method has proven to be far more effective. Note that it can only work for small infestations as it is labor-intensive. Where you are dealing with a large farm, this would not be a practical solution.
There have been several attempts to control Striga species through the use of biological agents. However, none of the options has shown great promise, and research is ongoing on the same.
You can use post-emergence herbicides on witchweed, with great success. The Asiatica species got eradicated in the USA through the use of chemicals applied to the weeds upon emergence. There are disadvantages to this method, which may make it unappealing to farmers in developing nations. For one, it is expensive and takes a considerable chunk of the profits. Two, this method only works when witchweed has emerged and already damaged the host plant. As such, dealing with the weeds at this point will have little effect on the productivity of the host plant. Three, the chemical could damage non-cereal crops on the farm.
And given that inter-cropping is also a means of control, the risks are quite high. The other option would be to treat the weeds before emergence, though this is not always a reliable means. With all these side effects, using post-emergence chemicals remains to be one of the best forms of control. Where the level of infestation is high, this could be your best bet in protecting the next set of crops.
Another means of control would be injecting ethylene into the soil to stimulate germination. Where seeds establish in the absence of a host, they die. As is the case with chemical applications, it is quite expensive and would therefore not work in some regions.
Using effective methods in combination, has proven to work in most cases. Gambia would be a good example of this. Farmers work on hand pulling to reduce witchweed populations as they invest in less vulnerable species. They also practice crop rotation, delayed planting, and intercropping. The use of chemicals has occasionally taken place. With these methods in place, the weeds are unable to produce seeds, and this affects their ability to reproduce.
The chosen means of control will depend on the level of infestation, the available capital, and the prevailing environmental conditions.
Educating farmers on the effects of witchweed would go a long way in aiding early detection and eradication. If left to establish, this weed has devastating effects that last through the years, owing to the viability of its seeds. It is thus best to act immediately upon sighting this plant. All the best!