Weeds can interact with crops and other organisms in an agroecosystem in various ways. They can serve as hosts or reservoirs for pathogens, viruses, nematodes, and insects. Some examples provided include weeds hosting wheat stem rust or potato blackleg disease pathogens. Weeds also provide food and habitat for insects, earthworms, slugs, and beetles. Weed management like herbicide use can directly impact fungi, insects, and indirectly influence disease incidence by changing soil microbes, plant physiology, or insect behavior. While herbicides may reduce some diseases, they can also increase the severity of others such as glyphosate increasing Corynespora root rot in soybeans. More research is needed

1. Effects of weeds and weed
management on other organisms
Abolfazl Hajihassani
Department of Soil Science, University of Manitoba

2. Robert Norris (retired weed scientist from the
University of California, Davis)
``Weeds live in communities and they are forced
to interact with crops and other organisms. In
ecological terms, weeds are producer organisms,
whereas other pest organisms are
consumers``(Weed Science, 2005).
Weeds can serve as a source of increased diversity in agroecosystems

3. Interaction of weeds with other components of the
biotic and abiotic community of an ecosystem
Fungi
Temperature
Virus
Bacteria
Light
Water
Nematodes
Host crops
Soil
inhabitants
Annuals
Insects
Perennials
Biannual

4. Dynamics of weeds population in plant-pest systems
• Weed abundance (i.e. density)
• Spatial distribution
• Genetic diversity
• Sizes of weed seedbank
Weeds harbor a wide range of organisms thereby increasing
opportunities for those organisms to survive and re-infest crops
in succeeding years.
A larger and more diverse weed seedbank can contribute to the
biodiversity of various groups of micro and macro organisms.
Robert Zimdahl, 2013

5. Effects of weed on microorganisms (plant pathogens)
Weeds can interact with pathogenic agents in several ways including:
 weeds can serve as reservoir (food and refuge) alternative hosts for pathogens
and their vectors
-Weeds external to crop fields as resources for mobile pathogens and their vectors
-Weeds within crop fields as overwintering resources for nonmobile pathogens
 weeds may be obligate alternate hosts for some plant pathogens
 weed seedbanks can directly serve as vectors of plant pathogens

6. Effects of weed on microorganisms (plant pathogens)
Fungal pathogens:
Initial inoculum sources for plant infection
 Serve as alternate hosts for some fungi to complete their life cycle
 Facilitate the survival of some fungi in soil for prolonged period of time
 Contribute to the dispersal of some fungi spices to other fields
 Weed canopies provide the humid and cool microclimate in which fungi infect
their host crops

7. Diseases harbored
by specific weed
species:
Adapted from Robert
Zimdahl, 2007

8. Life cycle of wheat stem rust (Puccinia graminis var. tritici):
an example of weed serving as a host for a pathogen

9. Effects of weed on fungal pathogens
Example:
The burrs of Noogoora burr carry the Verticillium wilt
pathogen, enabling wide dispersion of the pathogen
Bladder ketmia may act as an alternative host for the
pathogens that cause Verticillium wilt, Fusarium wilt
and Alternaria leaf spot of cotton
Sesbania pea is known to be symptomless hosts of the
Fusarium wilt pathogen

10. Effects of weed on microorganisms (plant pathogens)
Viral pathogens:
Means of survival for periods between cropping seasons and re-infest subsequent
crops
Weeds that exist on the edges of crop fields serve as overwintering hosts when
crops are not present
Source of virus vectors (especially insects)
- Insect vectors feed on various parts of infected weeds and acquire the virus
- Weeds can provide shelter from adverse conditions such as bad weather
Each virus situation can be different and it is very important to
determine the role of weeds in disease incidence and epidemiology

11. Effects of weed on microorganisms (plant pathogens)
Example:
Members of the weed families of
Compositae, Cruciferae, Cucurbitaceae,
Papillionaceae and Solanaceae
Alopecurus myosuroides, Sorghum
vulgare, Centaurea cyanus,
Convolvulus arvensis
Russian thistle
Cucumber mosaic virus (CMV)
Beet necrotic yellow vein virus (BNYVV)
Beet soil-borne virus (BSBV)
Curly top virus (CTV) of sugar beets and tomatoes

12. Effects of weed on microorganisms (plant pathogens)
Plant-parasitic nematodes:
Weeds support nematode reproduction and survival both in the presence
or absence of crop hosts
Nematode populations may increase or at least persist in the roots of
weeds, providing a source of inoculum for the following season
Their existing along with host crops can increase nematode pressure for
subsequent crops, potentially increasing yield losses

13. Nematodes harbored by specific weed species
Adapted from Robert Zimdahl, 2007

14. Effects of weed on microorganisms (plant pathogens)
Example:
 Stem and bulb nematodes:
Ditylenchus dipsaci
Ditylenchus weischeri
- Weed seeds and plants can act as carry-over hosts in
the epidemiology of this nematode, when no suitable
crops are grown
- High weed diversity can preserve the nematode
population and enables its juveniles to maintain longer
under different environmental conditions
Canada thistle that exist on the edges of crop fields
serve as hosts and as sources of D. weischeri
Thistledown is an important method
of seed dispersal by wind

15. Effects of weed on microorganisms (plant pathogens)
Bacteria:
 Sources for survival of the pathogen
 Act as carry-over hosts for bacterial disease, thus facilitating re-infestation
in subsequent host crops
Provide microclimates conducive to infection by bacteria
Example:
Potato blackleg disease
(Erwinia carotovora var. atroseptica)
Potato soft rot
(Erwinia carotovora var. carotovora)
Common lambsquarters, redroot
pigweed, or black nightshade
Wild radish
lack rot bacterium of brassicas
(Xanthomonas campestris pv. campestris)
Arabidobsis thaliana, Gnaphalium spp.,
Lamium amplexicaule and Stellaria media
Pseudomonas syringae pv. tomato

16. Effects of weed on macroorganisms
Earthworms:
 Feed on weed seeds
 Indirectly improve soil quality;
- expediting decomposition and mineralization of soil organic matter
- improving soil structure
- stimulating particular trophic interactions
 Increasing germination rates of seeds due to incomplete digestion in their guts

17. Effects of weed on macroorganisms
Slugs and Cutworms:
 Weed seeds as an alternative food resource
 Seedbanks as a complement to their common diet
 consuming imbibed seeds, as well as seedlings

18. Effects of weed on macroorganisms
Granivorous carabid beetles:
 Predators of weed seeds in agroecosystems
 Complete their life-cycle on weed seeds
 Regulating the weed seedbank size and composition
 Reducing pest populations (such as aphids,
coleopteran pest larvae)

19. Effects of weeds on Insects
 Weeds are primary food resources for insects
- Positive aspect
Reducing weed competition with crops
Colorado potato beetle
- Negative aspect
hairy nightshade
Resource and habitat-driven effects, causing crop damage
army cutworm
stems of tansy mustard
between rows of wheat
John Capinera (2005)

20. Effects of weeds on Insects
Weed species that remain uncontrolled in the field serve as alternate hosts
for some insect pests, thus increasing populations of insect that become
major problems in field.
The opposite of this situation is that weed control (using herbicide) may
worsen an insect problem by eliminating the weed hosts of insect and
forcing migration to the crop.
Norris and Kogan listed more than 94 insect pests that attack 45 different crops
through resource and habitat-driven interactions (Weed Science, 2000).

21. Effects of weeds on Insects
Weeds affect host-finding by herbivore insects
vision and odor
- Modify the attractiveness of crops to the insects
color
 Weeds affect beneficial insects
- Direct influences on the abundance and survival of beneficial insects
 Weeds are a source of diseases
- Reservoir for insect disease
- Harboring insect vectors of plant pathogenic agents (mostly virus)
Predators
Parasitoids
Pollinators

22. Effects of weed management (herbicides) on fungal diseases
Herbicide application:
Direct effects:
 Inhibitory and stimulatory influences on spore germination and mycelial growth of fungi
 Alteration the level of phytoalexins in plants
 Interfere with other physiological processes in plants
Indirect effect:
 Antagonistic soil microorganisms
Some examples of herbicides:
Glyphosate
• Positive effect
Inhibition of growth of several fungal disease
Leaf rust (Puccinia recondita) in wheat
Stem rust (Puccinia garamini) in wheat
Fusarium solani in green pea
• Negative effect
Increase disease incidence and severity

23. Lists of some diseases increased in glyphosate weed control programs
Adapted from Johal
and Huber, 2009

24. Increase disease severity caused by Corynespora root rot
after glyphosate application to soybean seedlings
Non-inoculated control
inoculated with fungus
Inoculated plants
sprayed with glyphosate
Adapted from Johal and Huber, 2009

25. Effects of weed management (herbicides) on plant disease
Glufosinate
 Reduction of fungal population and spore production
 Suppression of initial fungal infection
Rhizoctonia solani in soybean
Sclerotinia homoeocarpa in Bentgrass
Diphenylether
• Reduction of disease severity due to Increasing
phytoalexin production in plants
Sclerotinia sclerotiorum
Triazine
 Inhibition of mycelial development
Sclerotinia sclerotiorum

26. Effects of weed management (herbicides) on plant disease
Herbicides have the potential to affect plant disease by several
mechanisms
No or little data is available regarding mechanism of herbicidedisease interaction
Thus, more knowledge about
 type of herbicides and its efficient dose rates
 timing of herbicide application relative to infection
 species of plants and or type of cultivar
 species of pathogens and or type of strain
...
is needed.

27. Effects of weed management (herbicides) on insects
Direct effects of herbicides:
 feeding deterrent
Glufosinate application on cotton leaf worm, Spodoptera littoralis
 decreasing body weight
2,4-D application into black cutworm, Agrotis ipsilon
 negative impacts on ovipositon of beneficial insects
2,4-D application on
Ladybird beetle larvae
Indirect effects of herbicides:
 Negative influence on growth and development of insect via mediated changes in plants
 alteration the nutritive quality of plant tissues for insects
Changing in feeding behaviors