Biological control Flashcards
What is biofumigation? How does the biofumigation process work to control soilborne pathogens?
Biofumigation can be used to control diseases. Brassica is used as a rotation crop, they contain an abundance of glucosinolates, which are converted by the enzyme myrosinase into cyanogenic glycosides, isothiocyanate, nitriles and thiocyanate. These chemicals are then converted into toxins such as cyanide gas and isothiocyanates. Isothiocyanates are effective against many soil-borne fungal pathogens.
Describe the disease cycle of the take-all disease on wheat. What is a good crop rotation strategy to control take-all? Describe the process that leads to take-all decline. What causes take-all decline in farmer’s fields?
Take-all disease (Gaeumannomyces graminis):
a. Disease cycle
i. Fungus infects roots and basal stem of wheat plants;
ii. Mycelium grows from plant to plant, creating patches of infected plants;
iii. Visible symptoms include yellowish seedlings and stunted plants with few tillers;
iv. Affected plants have ears that ripen prematurely and produce heads of grain with sterile bleached spikelets (white heads);
v. Root systems of infected plants are destroyed.
b. Rye, oats and maize are non-susceptible grasses that, if planted for two years, can significantly reduce yield losses. Also of note, barley should not follow wheat in the rotation as it is tolerant but the pathogen population will increase on it.
c. Take-all decline (TAD) occurs following continuous cereal crop cultivation. Incidence and severity are reduced after one or more severe
outbreaks. TAD is a result of antagonistic microorganisms in the bulk soil, rhizosphere or rhizoplane and the subsequent build-up of
fluorescent Pseudomonas, which produce DAPG, an antibiotic.
Describe two examples of disease declines in agroecosystems. Wat is the cost to the farmer of using a long-term monoculture to generate a disease-suppressive soil?
Disease decline examples include the take-all disease of wheat (described above) and Rhizoctonia solani root rot in continuous sugar beet cultivation over 9 years. In the second case too, the decline is caused by increase in suppressive microorganisms.
Using consecutive monocultures to develop suppressive soil can, however, cause significant economic losses for the farmer, who is confronted with heavily reduce yields during soil development.
What is the difference between a disease-conducive and disease-suppressive soil?
conducive: disease occurs in presence of susceptible host and suitable environment
suppressive: environment is conducive to disease, but pathogen is not a problem in presence of a susceptible host. Presence of biological agents in soil that suppress disease development
Describe two types of experiments that can be used to show that suppressiveness has a biological basis rather than a chemical or physical basis.
Heating soil past 60 degrees for 30 minutes, caused it to lose its suppressiveness suggesting that the supressive organism was killed by the heat
Another type of experiment is a microorganism exclusion experiment, where soil samples are sterilized and some reinoculated with the organism suspected of suppression. Plants are then grown on the samples
What is the difference between general suppression and specific suppression?
General suppression: soil depresses activity of all soilborn pathogen, tends to be related to total amount of microbial activity at a time that is critical to pathogen
Specific suppresion referres to the specific effects of one or a few microbes that are antagonistic to the pathogen during an important part of the life cycle. The microbe can compete oder produce compounds that specifically antagonize the pathogen
Describe three mechanisms that can lead to a disease suppressive soil.
Nematode-suppressive soils, sometimes due to predation by nematode-trapping fungi. Competition for the same ecological niche is also a major driver of suppressiveness. Fungistasis is the third mechanism for the formation of suppressive soils and refers to a generalized restriction on fungal spore germination and growth. This may be caused by inhibitory substances of microbial origin or it may result when soils lack sufficient nutrients to support
spore germination (too much metabolic activity by other microbes)
Once a suppressive soil has been identified, why don’t farmers try to convert their soil to suppressiveness by mixing suppressive soil into their conductive soil?
It is only effective, if the suppressive microbes are identified and the requirements for successful establishment are understood.
It can be quite expensive to produce and distribute such a product, so most farmers would not be willing to do that investment. (1100$ - 3300$/ha)
Explain the mechanisms that contribute to suppressiveness in the Chateaurenard soil of France.
Fungistasis is the main mechanism of suppression due to nutrient limitations.
Fungistasis refers to a generalized restriction on fungal spore germination and fungal growth in soils.
In the Chauteaurenard soil pathogenic Fusarium spp. show a reduced growth rate ad dormant chlamdospores d not germinate in the presence of root exudates.
What are siderophores and how are they related to suppressive soils?
Siderophores are extracellular, lowmolecular-weight-transport agents, which selectively bind iron(III) with very high affinity and make iron available to the secreting microorganism. They are produced by most aerobic microorganisms in response to low-iron stress and its function is to supply iron to the cell.
What are three mechanisms known to be involved in biocontrol by Trichoderma fungi?
It is a necrotrophic parasite of many plant pathogenic fungi that attacks its hosts using a combination of fungal cell wall degrading enzymes and peptaibols
It competes with other microbes for nutrients
it is capable of inhibiting pathogen enzymes
What is a peptaibol and what role does it play in biocontrol by Trichoderma fungi?
biologically active peptid
it interacts synergistically with chitinases to breach the host cell walls and enable invasion
Describe two cases where the principle of competitive exclusion is used for biocontrol. Explain how the biocontrol is thought to work in both of these cases.
Crown gall control through competitive exclusion by Agrobacterium radiobacter K84:
The toxic antibiotic agrocin produced by K84 is effective against most A. tumefasciens strains. Some A. tumefasciens strains became resistant to agrocin by movement of a plasmid. NOGALL based on K1026 strain of A.radiobacter was first genetically engineered biocontrol agent. The strain was engineered to prevent the movement of the plasmid carrying agrocin resistance genes to sensitive strains
Competitive exclusion to reduce the production of mycotoxins:
Aflatoxin reduction using atoxigenic stains of Aspergillus flavus. Not all members of the A.flavus group produce aflatoxins.
One local strain that produced no aflatoxin but showed excellent competitive abilities was chosen.
What is AF36 and how is it used for biocontrol? Where is the AF36-based strategy likely to have the greatest impact?
AF36 is a strain of Aspergillus flavus that doesn’t produce aflatoxin but has showed excellent competitive abilities.
Displacement of toxin-producing strains appears to be the mechanism of control.
A.flavus strains interact during crop infection and this interaction greatly influences the final amount of contamination.
What is hyperparasitism? What is hypovirulence? Explain how hypovirulence is involved in control of chestnut blight (Cryphonectria parasitica), Why is the hypovirulence less effective in North America than in Europe?
Hyperparasitism is another mechanism of biological control that is the most widely used method for biological control of insect pests and it involves a parasite that infects another parasite. Hypovirulence is a condition in which a pathogenic fungus has reduced or diminished virulence and so causes less damage to the host plant compared to the fully virulent strain. Hypovirulent strains of chestnut blight
were observed when the pathogen was discovered in Italy. When cultured alongside virulent colonies, hypovirulents can render them hypovirulent too.
This, however, is limited to vegetative compatibility groups (VCGs), which are limited in Europe but more diverse in North America, where this control strategy
is in fact less successful
