Epidemiology

1. Epidemiology Charcoal rot in pine seedlings Root knot nematode in bermuda grass Dogwood liners with powdery mildew

2. What is Epidemiology? Epidemiology is the science of disease populations. Epidemiologyis the study of disease development, spread of disease and factors affecting these processes.

3. Definitions • Epidemic – increase in disease (in a population) in time and space • Pandemic – an epidemic occurring over a large area

4. Disease Triangle Susceptible host Favorable environment Disease causing agent

5. Disease Pyramid Susceptible host Time Favorable environment Disease causing agent

6. Disease Triangle + + Host Host Disease causing agent Favorable environment Time Disease

7. Factors that affect epidemicsHost • Seedling disease – once plants are beyond 2-4 leaf stage they are no longer susceptible. • Botrytis, Penicillium, Monilinia, Septoria – plants are very resistant while young and become more susceptible as they age

8. Factors that affect epidemicsPathogen • Level of virulence – increase in virulence means fast reproduction • Quantity of inoculum – high quantity means more chances to overwinter and more chances for initial infections

9. Types of Inoculum • For some fungi, nematodes and parasitic seed plants – relative low levels of reproduction means epidemics take long time to get going • For some fungi and bacteria – short reproductive cycle means many generations in one growing cycle

10. Mode of Spread Wind – propagules spread quickly Insects – can spread quickly; many km Splashing rain – severe, but local epidemic

11. Mode of Spread Seed transmission – can be spread anywhere but epidemics depend on secondary spread or transmission Soil – usually does not cause sudden and widespread epidemics; usually local, slow spreading, but considerable severity

12. Measurements of Disease Incidence - # or proportion of plant units diseased Severity – proportion of area or amount of tissue diseased Yield loss – proportion of yield that growers will not be able to harvest

13. Estimation of Incidence and Yield Loss In some diseases such as cereal smuts, brown rot of stone fruit, vascular wilts of annuals, disease incidence has direct relationship on yield loss (loss of whole plant or direct loss of yield products – for example with brown rot, the peaches are destroyed) Many diseases (rust, leafspots, root lesions) – plants vary in level of disease severity and the affect of disease on yield

14. Disease Distribution Clumped Random Regular

15. The pattern of an epidemic can be expressed as the number of lesions, amount of diseased tissue or diseased plants. These numbers can be expressed as a curve that that shows the progress of thedisease over time. This curve is called a disease progress curve.

16. Disease Progress Curve

17. Type of Disease Progress Curves

18. Simple Interest or Monomolecular Disease curve (Curve A) • Inoculum does not contribute to the epidemic during the growing season in which it was formed • Often the pathogen is a soil-borne pathogen • Example is southern blight (Sclerotiumrolfsii, brown patch)

19. Compound Interest or Logistic Disease (Curve B) • Inoculum produced during the current epidemic is responsible for starting secondary disease cycle • Epidemics are “explosive” with logistic growth • Examples are: gray mold (Botrytis), dogwood anthracnose, stem rust of wheat

20. Bimodal or Sigmoid Disease (Curve C) • Epidemic often affected by resistance of plant (often aging cotyledons early in season and older leaves late in season are susceptible whereas expanding leaves are resistant • Example: brown spot of soybeans (Septoria gylcines)