1. Plant Pathology

2. Plant Diseases
Plant diseases can be divided into two categories: infectious (caused by pathogens) and non-infectious (commonly called disorders)
Infectious diseases can be transmitted from plant to plant and produce signs resulting from the interactions of the pathogen, host and environment
There are three main types of infectious plant diseases: bacterial, fungal and viral
Non-infectious diseases (disorders) are caused by a range of non-living agents such as nutritional imbalances, environmental extremes, chemical toxicities, mechanical injuries and genetic problems
Disorders may produce signs that are similar to infectious diseases, and may also make plants more susceptible to infectious diseases

3. Infectious diseases in plants
Affect some or all plant functions
Disease may reduce yield and quality of harvested product
Disease is a process or a change that occurs over time
It does not occur instantly like injury

4. Symptoms and Signs of Plant Disease
Visible effects of disease on plants are called symptoms
Any detectable changes in color, shape, and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom
Signs of plant disease are physical evidence of the pathogen, for example, fungal fruiting bodies, bacterial ooze, or nematode cysts
Signs also can help with plant disease identification

5. Symptoms
Blight – A rapid discoloration and death of twigs, foliage, or flowers
Canker – Dead area on bark or stem, often sunken or raised
Chlorosis – yellowing (chlorosis is so generic that without additional details diagnosis is impossible)
Decline – Progressive decrease in plant vigor
Dieback – Progressive death of shoot, branch or root starting at the tip
Distortion  – malformed plant tissue
Gall or gall-like – Abnormal localized swelling or enlargement of plant part.  It could be caused by insects, mites, diseases, or abiotic disorders
Gummosis – Exudation of gum or sap
Leaf distortion – The leaf could be twisted, cupped, rolled, or otherwise deformed
Leaf scorch – Burning along the leaf margin and into the leaf from the margin
Leaf spot – A spot or lesion on the leaf
Mosaic – varying patterns of light and dark plant tissue
Necrosis – dead tissue (necrotic areas are also so generic that without additional details diagnosis is impossible)
Stunting – lack of growth
Wilt – General wilting of the plant or plant part
Witches broom – Abnormal broom-like growth of many weak shoots

6. Symptoms
Even though a plant has symptoms on a specific part, it does not necessarily mean the damaged tissue contains the organism causing the symptoms
For example, a root rot can cause chlorosis and wilting of stems and leaves, BUT the disease causal organism is in the roots
It is imperative to examine as much of the plant as possible to determine exactly where the problem is originating

7. Signs Signs are the actual organisms causing the disease.
Conks –  woody reproductive structures of fungi
Fruiting bodies – reproductive structures of fungi; could be in the form of mushrooms, puffballs, pycnidia, rusts, or conks
Mildew – whitish growth produced by fungi composed of mycelium
Mushrooms –  fleshy reproductive structures of fungi
Mycelium – thread-like vegetative growth of fungi
Slime Flux or Ooze – A bacterial discharge that oozes out of the plant tissues, may be gooey or a dried mass.
Spore masses – masses of spores, the “seeds” of a fungus.

8. What causes plant disease?
Infectious plant diseases are caused by living organisms that attack and obtain their nutrition from the plant they infect
The parasitic organism that causes a disease is a pathogen
Numerous fungi, bacteria, viruses, and nematodes are pathogens of major commodity crops and vegetable crops
The plant invaded by the pathogen and serving as its food source is referred to as a host

9. Types of pathogens
Fungi
Bacteria
Viruses
Nematodes

10. Role of the environment
A favorable environment is critically important for disease development
Even the most susceptible plants exposed to huge amounts of a pathogen will not develop disease unless environmental conditions are favorable

11. The Disease Triangle

12. Fungi Fungi reproduce and spread by producing spores
When a spore comes in contact with a susceptible plant, it will germinate and enter the host if the proper environmental conditions are present
Hyphae develop from the germinated spore and begin to extract nutrients from host plant cells
The hyphae secrete enzymes to aid in the breakdown of organic materials that are ultimately absorbed through their cell walls

13. Fungi Damage plants by killing cells and/or causing plant stress
Can infect plants through natural openings (e.g. stomata), via mechanical pressure or through wounds
Can survive in a wide range of habitats, including living and dead plants, and soils
Fungi can be transmitted by:
water, wind and air
soil
seed
insects and other animals
human activities such as chipping, transplanting, tractor passes and movement of soil

14. How fungi penetrate plant cells

15. Bacteria Extremely small organisms requiring microscope to be seen
Can rapidly multiply under favorable conditions
Cells clump together in masses called colonies
Obtain food from dead or decaying organic matter or living tissue

16. Bacteria
They are usually carried in infected plant material, but can survive in soil and on the surface of plants, even in dry conditions, for long periods of time.
Bacteria can infect plants through wounds and natural plant openings such as stomata
Unlike fungi, bacteria must find a natural opening for entry
Bacteria can be transmitted by:
water (rain or irrigation water)
seed
vegetative propagation material
insects
human activities such as chipping, transplanting, harvesting, tractor passes and movement of soil
Bacterial pathogens are dependent upon water
Conditions must be very wet and/or humid for them to cause significant and widespread damage

17. Bacteria
Bacteria move between plant cells and secrete substances that degrade plant cell walls so the contents can be utilized
Some produce enzymes that break down plant tissue, creating soft rots or water- soaking
Like the fungi, bacteria cause symptoms such as leaf blights and spots, galls, cankers, wilts and stem rots
Bacterial leaf spots appear different from fungal leaf spots due to their intercellular movement
Veins often limit the development of a lesion, so they appear angular or irregular, not round
Comparison of Fungal and Bacterial Leaf Spots
Symptom Description
Fungal
Bacterial
Water-soaked appearance no
yes
Texture
dry, papery
slimy, sticky
Smell
none
Pattern
circular, target-like
irregular, angular
Disintegration
Color change
red, yellow and purple halos
Structures of pathogen
mycelia, spores, fruiting structures

18. Viruses Viruses can multiply only in living cells
Some viruses are often systemic; others leave some plant tissue virus- free
Viruses cause mottling, spots, and mosaic-like patterns, crinkling, and other malformations on leaves and fruits, and may stunt plants. 
There is no cure for infected plants, so control depends on prevention
Viruses are named after the first plant on which they were studied and the most obvious sign of the disease on that plant
Viruses can be transmitted by:
living vectors such as insects, mites, nematodes, fungi, humans
mechanical vectors such as tools
leaf-to-leaf contact, seed and pollen

19. Also….nematodes Round, slender, threadlike worms
Some are parasites on animals, insects, fungi, other nematodes, and plants
Plant-parasitic nematodes have a stylet
Most live in the soil and feed in or on plant roots

20. Disease Cycle
Fungal, bacterial and viral diseases follow a cycle of the infective agent invading and infecting an appropriate host
dependent on the interaction between host, pathogen and environment
The infective agent grows and reproduces in the host, which develops signs/symptoms of disease
Specific fungi, bacterium or viruses have one or a number of methods for spreading
air, water, soil, insects, seed, cuttings and humans.
Some disease organisms need to survive in a growing host plant, but others may be able to survive for a time outside a host plant in dead host plant debris; in water, soil and/or seeds; or as spores.

21. Disorders in plants
Conditions that promote disorders in plants include:
unfavorable temperatures
unfavorable soil moisture
unfavorable atmospheric moisture
unfavorable soil structure
incorrect soil pH
nutrient deficiencies and toxicities
harmful substances
mechanical injuries
genetic abnormalities
Disorders can predispose plants to attack by pathogens

22. Disorder or disease? Look at the whole plant
Check for damage to roots, foliage and flowers
Note the distribution of signs within the plant—for example, is only the new growth affected, or only one stem?
Note the pattern of affected plants within the crop
Note the changes to plants over time—the signs produced by plant diseases often vary with weather conditions or age of plants

23. Diagnostics

24. Plant Disease Management
Cultural management
Mechanical management 
Well-adapted varieties
Disease resistant varieties
Plant certified seed or seed pieces
Place plants in the appropriate environment for optimum growth (minimize stress on the plant)
Apply fertilizer and water according to plant needs
Prune correctly, as needed, and at the correct time of year
Clean up or prune out infested plant materials to reduce the source of inoculum
Rotate crops when possible to starve pathogens
Apply mulch in gardens to create a splash barrier
Use soil solarization to reduce soil pathogens
Rogue host plants

25. Summary
Understanding the difference between a sign and a symptom is key in identifying a plant disease
A plant disease cannot develop if a susceptible host, pathogen, and favorable environment do not occur simultaneously
The major plant pathogens responsible for infectious disease development in plants are fungi, bacteria, viruses, and nematodes
The disease cycle describes the interaction of the pathogen with the host