Effects of weeds on other organisms

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:


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:


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


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:


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


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


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


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


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)


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


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

Effects of weeds on other organisms

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