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Glossary of Plant Disease Epidemiology

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What is an epidemic (epiphytoic)?
increase in disease in a population
What factors are necessary for an epidemic to develop?
time, host, environment, pathogen
What aspects are you concerned with in epidemiology?
1) Populations of pathogens and host plants as they occur in an evolving environment
2) Population genetics of host resistance and the evolutionary potential of pathogen populations to produce pathogenic races that may be more virulent to host cultivars or more resistant to pesticides
3) other biotic and abiotic factors
Why is it important to know if your pathogen/disease is monocyclic or polycyclic?
determines the method of control and possible severity
What host factors affect development of epidemics?
Levels of host resistance and
degree of genetic uniformity(i.e. different cultivars of rice and effect on rice blast)
Crop type (i.e. annuals vs perennials)
Host age (i.e. Ontogenic resistance-plants change susceptible or resistant reaction to disease with age, Pythium only susceptible when young)
What pathogen factors affect development of epidemics?
virulence level
inoculum concentration (Verticillium wilt)
type of pathogen reproduction
ecology of pathogen (aerial parts-rusts, vascular tissue-viruses, roots-root rots)
dissemination of pathogen
What is a polyetic epidemic?
inoculum builds up one year to the next and the epidemic develops over several years (dwarf mistletoe- 5-6 years)
What environmental factors affect epidemics? How?
Moisture- promotes sporulation and multiplication, facilitates sprore release, new host growth, increase or decrease activity of vectors, absence of moisture may also cause these effects
Temperature- may reduce or eliminate partial resistance, low or high temps reduce amt of inoculum, affect diff stages of pathogenesis
What cultural control measures affect epidemics?
site selection and preparation
selection of propagative materials
cultural practices
disease control measures
introduction of new pathogens
foliage density
How do we measure plant disease and yield losses?
Incidence of disease- porportion of plants diseases compared to total number of plants
Severity of disease- proportion of diseased plants
Yield loss- proportion of yield unharvestable
What are rating scales?
a grade assigned to a set % of disease severity (or incidence)
What are the differences in soil-borne and aerial plant disease rating scales?
In aerial plant diseases, you can estimate the % of symptoms on leaves; large scale 0-7 (don't have to remove plant)
With soil-borne diseases, you have to usually remove the plant, cut in open and estimate the effect of its insides (smaller scale 0-3)
What types of disease progress curves are there?
monocyclic, polycyclic, bimodial polycyclic (blossoms then fruits)
What factors influence these disease progress curves?
Time of appearance
Inoculum level
Host susceptibility
Weather
Cultivar
How can knowledge of a disease progress curves help you?
Allows for disease forecasting and selection of best control strategies for athe particular disease and time
What are the components in the analysis of epidemics?
X = x0e^rt
X = amt of disease at any point in time
x0 = amount of initial disease in the population or the amt of initial inoculum
e = constant (base of natural log)
r = rate at which disease increases in the population
t = length of time over which this increase occurs
What causes a decrease in x0?
crop rotation
sanitation
portectant fungicides
single gene resistance
What causes a decrease in r?
multigene resistance
fungicides
cultural methods
What causes a decrease in t?
early harvest or a short season cultivar
What is the difference in epidemic rates between polycyclic and monocyclic disease cycles?
Polycylic has a higher rate
What is AUDPC and why is it important?
Area Under the Disease Progress Curve; comparisons can tell if control measures are working
What are spatial patterns and why are they important?
Progress of an epidemic in space, in terms of changes in number of lesions, amount of diseased tissue and the number of diseased plants as it spreads over distance
It shows the development of an epidemic
Why is an economic threshold important in plant disease epidemiology?
Def: the level of disease, amount of plant damage, at which control costs just equal incremental crop returns
It will determine type and amount of control
What does forecasting plant disease epidemics do?
Disease diagnosis
Evaluation of epidemic threshold
Evaluation of economic damage threshold
Assessment of initial inoculum and disease
Monitoring weather factors that affect disease development
What factors do you need to know when forecasting plant disease epidemics and why is it important?
Environmental variables (temp, precip, humidity, leaf wetness)
Host variables (crop growth stage, cultivar)
Pathogen variables (inoculum potential, spore maturity)
Important b/c it will determine the amount of control needed

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