1. Parasitic weeds- striga
spp
By Masara Polite
R161055Z

2. Introduction
 Parasitic weeds are a serious problem in agriculture, causing large crop losses
in many parts of the world.
 Types are holoparasites and acquire all nutrients and water from
hemiparasites but, although they have chlorophyll and a basal photosynthetic
activity, basically also behave as holoparasites

3. Types of parasites
 Hemiparasites
 Holoparasites
 Obligate
 facultative

4. Definations
 ParasitismIt is a relationship between organisms in which one lives as a
parasite in or on another organism.
 Parasitic weeds are plants that grow on living tissues of other plants and
derive part or all of their food, water and mineral needs from the plant
they grow on (host plants)
 Hemi parasite (Semi parasite) a plant which is only partially parasitic,
possessing its own chlorophyll (green colour) and photosynthetic ability
(may be facultative or obligate). E.g Striga hermonthica
 Holo parasite – a plant which is totally parasitic, lacking chlorophyll thus
unable to synthesize organic carbon. E.g Orobanche spp
 Obligate parasite – a plant which cannot establish and develop without a
host
 Facultative parasite – a plant which can grow independently but which
normally behaves as a parasite to obtain some of its nutrition.

5. Striga species
 Striga asciatica
 Striga gesnoroides
 Striga hermonthca

6. Striga hermonithca
• Striga hermonthica is a problem to cereals particulary
sorghum, maize and millet but is also found in
sugarcane and rice fields

7. Striga gesneroides
 This species occurs widely in Africa
 mainly a problem to wild plants only although in west Africa it has been
found in cowpeas

8. Striga asiatica
 Found in Africa
 This has red flowers.
 It’s a problem under cereal production(maize,sorghum,millet and upland
rice).

9. Life cycle of striga spp
 The life cycles of Striga involve a number of mechanisms ensure the coordination of the parasites' life cycles to that of their
host).
 The important steps in the life cycle are:
 germination, -response to the secretion of sec metabolites
 radicle growth to the host root, -so as to form a haustria
 haustorium formation- to establish a xylem to xylem connection with the host to withdraw water and nutrients.
 and attachment to the host root, the successful establishment of a xylem connection and compatible interaction,
 and production of seeds.
 Conditioning(dormancy)
 The interaction between host and parasite begins with the secretion of secondary metabolites(strigolactones) from the roots of
the host (and some false nonhosts) that induce the germination of the parasite seeds.
 The seeds of Striga spp. contain only little reserves, and they can survive for a few days only after germination unless they
reach a host root and a xylem connection is established

10. Life cycle of striga

11. Conditions that favor development
 Overused, depleted, and infertile soil
 Poorly manage soil
 Monocropping, planting of the same crop on the same area for a long period
of time
 Improper weed management
 Seeds from the previous harvest are mixed with Striga seeds

12. Methods of control
 Many methods used fall in two broad categories: cultural and seed-based.
Each has its own strengths and weaknesses therefore calling for integrated
management practices.

13. Cultural control
 Crop rotation: This is a low-cost technology that addresses the two interrelated
problems of low fertility and Striga infestation. However, its potential for wide
adoption depends on whether the break crop is a high-value crop that fits into the
cropping system and its seeds are readily available locally. It is also limited by land
availability, and its effectiveness is limited if neighbors do not adopt the system.
 Use of trap crops:Crops such as cotton and legumes such as cowpea, pigeon pea,
groundnut, and soybean can be intercropped with maize to reduce the Striga seed
bank in the soil. These crops are used as trap crops as they cause the Striga seed
to germinate but since it cannot attach itself to them, it dies.
 Intercropping: Similar to crop rotation, the intercrop has to have a high value, fit
into the current systems, and the seeds widely available and affordable. One
example is soybean intercrop which was found to lower the density of the Striga
and improve soil fertility. However, there has to be a demand for the soybean for
farmers to readily invest in it.

14.  Soil fertilization using organic or inorganic fertilizer: This method is limited
by availability and affordability of the fertilizers and it also increases labor
costs for application. Fertilizer availability is a major obstacle to African
farmers as it is expensive.

15. Seed-based technologies
 include germplasm based-Striga resistance, use of herbicide coated seeds,
and biological control. One major pitfall of these technologies is cash-
strapped farmers’ access to the seeds.

16.  Developing host resistance/tolerance: Breeding crop varieties that are
resistant (prevents or limits Striga attachment or growth) or tolerant (variety
still gives acceptable yields despite Striga attack) is the most widely spread
seed-based and highly effective method. It is the most feasible and
environmental friendly method for small-holder farmers to control Striga. It
has been especially successful in sorghum where great advances have been
made in understanding how the tolerance/resistance work through
biotechnology and the knowledge used to develop Striga-tolerant sorghum
varieties using marker-assisted breeding.

17.  Herbicide dressing: This is a highly effective method if available and
affordable for farmers who have to buy the dressed seeds each season. Seeds
with resistance to herbicides such as imidazolinone herbicide imazapyr
(registered as StrigAway) are coated with it and distributed to farmers for
planting with specific guidelines such as hand washing after planting to
prevent herbicide to move to other non-herbicide resistant maize or other
crops.

18. Biological control
 Fusarium biocontrol:
 This method uses indigenous fungi that are natural enemies of the weed.
 It attacks the weed in all its growth stages—from seed to germination, from
seedling to flowering shoot thus protecting the current crop and preventing
Striga seed germination.
 The method is environmentally friendly, safe to users, specific to the target,
is long lasting, and economically viable in the long run.