effects of weeds on other organisms
TRANSCRIPT
Effects of weeds and weed management on other organisms Abolfazl Hajihassani Department of Soil Science, University of Manitoba
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
Host crops
Insects Soil
inhabitants
Temperature
Water Light
Annuals
Biannual Perennials
Fungi
Virus
Nematodes
Bacteria
Interaction of weeds with other components of the biotic and abiotic community of an ecosystem
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
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
Effects of weed on microorganisms (plant pathogens)
Fungal pathogens:
Effects of weed on microorganisms (plant 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
Diseases harbored by specific weed species:
Adapted from Robert Zimdahl, 2007
Life cycle of wheat stem rust (Puccinia graminis var. tritici): an example of weed serving as a host for a pathogen
The burrs of Noogoora burr carry the Verticillium wilt pathogen, enabling wide dispersion of the pathogen
Effects of weed on fungal pathogens
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
Example:
Viral pathogens:
Effects of weed on microorganisms (plant 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
Effects of weed on microorganisms (plant pathogens)
Cucumber mosaic virus (CMV) Members of the weed families of Compositae, Cruciferae, Cucurbitaceae, Papillionaceae and Solanaceae
Alopecurus myosuroides, Sorghum vulgare, Centaurea cyanus, Convolvulus arvensis
Beet necrotic yellow vein virus (BNYVV) Beet soil-borne virus (BSBV)
Russian thistle Curly top virus (CTV) of sugar beets and tomatoes
Example:
Plant-parasitic nematodes:
Effects of weed on microorganisms (plant pathogens)
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
Nematodes harbored by specific weed species
Adapted from Robert Zimdahl, 2007
Thistledown is an important method of seed dispersal by wind
Stem and bulb nematodes: Ditylenchus dipsaci Ditylenchus weischeri
Effects of weed on microorganisms (plant pathogens)
Canada thistle that exist on the edges of crop fields serve as hosts and as sources of D. 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
Example:
Wild radish
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
Common lambsquarters, redroot pigweed, or black nightshade
lack rot bacterium of brassicas (Xanthomonas campestris pv. campestris)
Potato blackleg disease (Erwinia carotovora var. atroseptica) Potato soft rot (Erwinia carotovora var. carotovora)
Arabidobsis thaliana, Gnaphalium spp., Lamium amplexicaule and Stellaria media
Pseudomonas syringae pv. tomato
Example:
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
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
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)
Effects of weeds on Insects
Weeds are primary food resources for insects - Positive aspect - Negative aspect
Reducing weed competition with crops
Colorado potato beetle hairy nightshade
Resource and habitat-driven effects, causing crop damage
army cutworm stems of tansy mustard between rows of wheat
John Capinera (2005)
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).
Weeds affect host-finding by herbivore insects
- Modify the attractiveness of crops to the insects
Weeds affect beneficial insects
Effects of weeds on Insects
vision and odor
color
- Direct influences on the abundance and survival of beneficial insects
Predators Parasitoids Pollinators
Weeds are a source of diseases - Reservoir for insect disease
- Harboring insect vectors of plant pathogenic agents (mostly virus)
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
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
Some examples of herbicides:
Adapted from Johal and Huber, 2009
Lists of some diseases increased in glyphosate weed control programs
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
Effects of weed management (herbicides) on plant disease
Reduction of fungal population and spore production Rhizoctonia solani in soybean Suppression of initial fungal infection Sclerotinia homoeocarpa in Bentgrass
Glufosinate
Diphenylether
• Reduction of disease severity due to Increasing phytoalexin production in plants
Sclerotinia sclerotiorum
Triazine
Inhibition of mycelial development Sclerotinia sclerotiorum
Herbicides have the potential to affect plant disease by several mechanisms
No or little data is available regarding mechanism of herbicide-disease 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.
Effects of weed management (herbicides) on plant disease
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