Effects of weeds on other organisms


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Effects of weeds on other organisms

  1. 1. Effects of weeds and weed management on other organisms Abolfazl Hajihassani Department of Soil Science, University of Manitoba
  2. 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. 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. 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. 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. 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. 7. Diseases harbored by specific weed species: Adapted from Robert Zimdahl, 2007
  8. 8. Life cycle of wheat stem rust (Puccinia graminis var. tritici): an example of weed serving as a host for a pathogen
  9. 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. 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. 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. 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. 13. Nematodes harbored by specific weed species Adapted from Robert Zimdahl, 2007
  14. 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. 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. 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. 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. 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. 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. 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. 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. 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. 23. Lists of some diseases increased in glyphosate weed control programs Adapted from Johal and Huber, 2009
  24. 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. 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. 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. 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