1. Plant Pathology 101 Master Gardener Training
Carrie Harmon, Phil Harmon
Plant Pathology Dept., UF Gainesville and the Southern Plant Diagnostic Network
UNIVERSITY OF FLORIDA
IFAS EXTENSION

2. Environment
Disease Pyramid
Disease
Time
Pathogen
Host

3. Diseases and disorders
Disease occurs when
Pathogen
Host
Environment
Disorders lack pathogens
Plant
Put it in human terms… radiation, salmonella

4. Diagnosis is critical
Incorrect changes to cultural management can make problems worse.
Fungicides only work on fungal diseases.
The applicator needs to know if plant damage is disease, if the disease is caused by a fungus, and which fungus is involved.

5. Disease diagnosis Diagnosis: changes in time/space Questions to ask:
What pathogen structures (signs) occur when and where?
What symptoms develop over time?
What is the pattern of dispersal?
 field signature or distribution
abiotic vs. biotic (images helpful!)
What are the known diseases associated with this host?

6. Disease diagnosis Injury and disorders often: occur “suddenly”
may affect all or many plant species
may have regular, uniform pattern
follow equipment patterns, boundaries
look at pattern of problem in relation to other items in the area – driveways, construction activities, etc.

7. Symptoms and Signs
Symptoms: Changes in growth or appearance of a plant in response to a damaging factor.
Sign: Evidence of the damaging factor.

8. Main types of pathogens
Fungi
Bacteria
Viruses
Others
Fungi are the most common cause of plant disease. The image on the left is a petri plate with a fungus growing in it (this fungus causes gray leaf spot of St. Augustinegrass. The center image shows fungal growth on a diseased plant. This is the pathogen. Parts of the pathogen on a diseased plant are called a “sign” of disease, mushrooms are another sign.

9. Armillaria produces white fungal growth under the bark of diseased plants This sign is diagnostic. Removing the bark allows you to see the fungus.
Armillaria produces white fungal growth under the bark of diseased plants This sign is diagnostic. Removing the bark allows you to see the fungus.

10. Sclerotinia sclerotiorum produces two distinct signs
Sclerotinia sclerotiorum produces two distinct signs: large black survival structures in hollow stems of diseased plants are sclerotia; and white fluffy cottony growth is also produced by this fungus.

11. The black bumps in the blighted area of this holly stem contain small black fruiting bodies of the pathogen, Sphaeropsis tumefaciens.
The black bumps in the blighted area of this holly stem contain small black fruiting bodies of the pathogen, Sphaeropsis tumefaciens.

12. Spores are usually too small to see
Spores are usually too small to see. In this case many spores are released from the “puffball” mushroom together and they look like smoke rising from the fruiting body of this fairy ring causing fungus.
Spores are usually too small to see. In this case many spores are released from the “puffball” mushroom together and they look like smoke rising from the fruiting body of this fairy ring causing fungus.

13. In this case, many spores of Colletoctrichum form on this rotting blueberry and appear orange in color.
In this case, many spores of Colletoctrichum form on this rotting blueberry and appear orange in color.

14. Daylily rust spores are also orange and can rub off the leaves onto your hands and clothes but are harmless to humans.

15. Powdery mildews generally produce white flat growth on diseases plants
Powdery mildews generally produce white flat growth on diseases plants. The white growth is another sign that consists of hyphae and spores.

16. Main types of pathogens
Fungi
Bacteria
Viruses
Others
Bacteria are not often seen. Occasionally bacterial ooze can be seen leaking from bradford pear shoots wilted from fire blight. Oozing plant sap can be caused by a number of other things including insect feeding. Bacterial diseases produce symptoms, but usually no signs.
Photo by Mark Longstroth
Photo by Hank Dankers

17. Main types of pathogens
Fungi
Bacteria
Viruses
Others
Viruses are too small to see with anything except the best microscopes. Viral diseases produce symptoms but no signs of the pathogen that are visible to the naked eye.
Micrograph by Carl Wetter

18. Main types of pathogens
Fungi
Bacteria
Viruses
Others
Mycoplasma-Like-Organism (MLO)
Ex. Coconut Palm Lethal Yellowing MLO
MLO’s are also too small to see and do not produce visible signs. ?

19. Types of disease symptoms What does the disease do to the plant?
Underdevelopment of tissues or organs. Examples include such symptoms as stunting of plants, shortened internodes, inadequate development of roots, malformation of leaves, inadequate production of chlorophyll and other pigments, and failure of fruits and flowers to develop.
Overdevelopment of tissues or organs. Examples include: galls on roots, stems, or leaves, witches' brooms, and profuse flowering.
Necrosis or death of plant parts. These may be some of the most noticeable symptoms, especially when they affect the entire plant, such as wilts or diebacks. Other examples include shoot or leaf blights, leaf spots, and fruit rots.
Alteration of normal appearance. Examples include mosaic patterns of light and dark green on leaves, and altered coloration in leaves and flowers.
From: Riley, M.B., M.R. Williamson, and O. Maloy Plant disease diagnosis. The Plant Health Instructor. DOI: /PHI-I

20. Incidence VS. Severity Incidence: Percent of the crop affected
Severity: a measure of impact on a plant or the crop

21. Common disease symptoms
Courtesy R.L. Forester
Courtesy J.W. Pscheidt
Peach leaf curl, caused by Taphrina deformans.
Crown gall of uonymous by Agrobacterium tumefaciens.
Overgrowth of leaf tissue causes thickening and distortion.
Galls form on all plant parts, caused by many pathogens.

22. Common disease symptoms
Wilting
Biotic factors
Root, crown or stem rots
Vascular wilts
Root crown or stem damage from insects or animals
Mainly fungal and bacterial causes
Used by permission of M.Williamson
Stem wilt of Exacum from INSV infection

23. Common disease symptoms
Leaf spots
Cankers
Foliar blights
Root rots
Black spot of rose on right; sycamore anthracnose on left; These are symptoms of the disease. You cannot see the pathogen in these photos.(no signs)

24. Cercospora leaf spot of crinum lily (symptoms no signs)

25. Common disease symptoms
Leaf spots
Cankers and diebacks
Foliar blights
Root rots
Citrus canker on left; Nectria canker on apple center; sycamore anthracnose canker symptom right
Photo by A. L. Jones
Photo by Dean Gabriel

26. Severe dieback resulting from stem and bole cankers caused by Botryosphaeria dothidea

27. Stem canker caused by Botryosphaeria dothidea

29. Needle blight on Leyland cypress

30. Entomosporium leaf spot on Dwarf Indian Hawthorne
There are resistant cultivars

31. Image credit: Celeste White
Sphaeropsis tumefaciens symptoms include galling and shoot proliferation on holly and Ixora (left and center) and cankers also on holly noticeable by cracked bark and discoloration of vasculature underneath.
Image credit: Celeste White
Sphaeropsis tumefaciens

32. Main types of diseases Leaf spots Cankers Foliar blights Root rots
Foliar blights affect leaves and stems. Ramorum blight left and white mold (Sclerotinia sclerotiorum) right

33. Main types of diseases Leaf spots Cankers Vascular wilts Root rots
Soil borne organisms, usually fungi cuase root rot. Abiotic factors also cause root rot—water logged soil.
Soil borne organisms, usually fungi cuase root rot. Abiotic factors also cause root rot—water logged soil.

34. Main types of diseases Leaf spots Cankers Vascular wilts Root rots
Photo by R.O. Hampton
Leaf spots
Cankers
Vascular wilts
Root rots
Viral diseases
Photo by W. Witcher

35. Unknown ringspot virus symptoms on phalenopsis orchids

36. Rose mosaic symptoms

37. How do we approach a plant problem?
A Five Step Process…
1. Determine that a ‘REAL’ problem exists.
2. Look for PATTERNS, in the community, on an individual plant and on an individual plant part.
3. Determine the TIME development of the damage pattern.
4. Ask QUESTIONS.
5. SYNTHESIZE the information.

38. 1. Determine that a ‘REAL’ problem exists…
Identify the plant.
Learn about it’s normal characteristics.
Determine normal vs abnormal characteristics.
Look for symptoms and signs.
Symptoms: Changes in growth or appearance of a plant in response to a damaging factor.
Sign: Evidence of the damaging factor.

41. 2. Look for Patterns… Look for patterns in the plant community.
Is the damage on more than one plant?
Is the damage on more than one plant species?
Look for patterns on an individual plant.
Is the damage on the entire plant or certain parts?
Is the damage on certain age of growth?
Look for patterns on an individual plant part.

42. Patterns of damage…
Non-uniform, expanding damage patterns are usually caused by living factors, because of movement of feeding sites, life cycles, and population increases and decreases.
Uniform, non-expanding damage patterns are usually caused by non-living factors such as chemical injuries, temperature changes, and mechanical damage.

43. Damage patterns in the plant community…
Used by permission of R. Billings
Loblolly pine killed by southern pine beetle

44. Used by permission of M. Williamson
Cotton field with chemical damage

45. Rhododendron with a Phytophthora sp. infection.

46. Woody container plants with Chemical injury .

47. Turfgrass displaying Fairy Ring symptom.

48. Damage patterns on an individual plant…
Used by permission of W. Sinclair
American Elm displaying symptoms of Dutch Elm Disease.

49. Rhododendron displaying symptoms of Nitrogen Deficiency.

50. Damage patterns on an individual plant part…
Douglas Fir with Rhabdocline Needlecast.

51. Fir with Freeze Injury.

52. Used by permission of M. Williamson
Downy mildew of Buddleia caused by Peronospora harotii.

53. Kalmia with a viral infection.

54. 3. Determine the TIME development of the damage pattern…
Progressive spread with time to other areas is characteristic of living factors.
Intensification of symptoms where damage first occurred but no spread to new sites is characteristic of non-living factors.

58. 4. Ask QUESTIONS… Get a history of the problem.
Get a history of all pesticides and fertilizers that have been applied.
Find out the history of the site.
Could environmental conditions explain the problem?
Look for obvious symptoms and signs…
Don’t ignore the roots…
Beware of secondary insects and pathogens…
Be patient and avoid jumping to conclusions…

59. 5. SYNTHESIZE the information…
Refer to literature…

60. Send a sample to the Plant Disease Clinic
UF Plant Disease Clinic
Building 78 Mowry Road
University of Florida
Gainesville, FL 32611

61. Samples must contain the right material: an entire plant or several plants if practical.
Diseases may show up on any part of the plant.
Foliage diseases
Keep most roots and soil intact if possible
Check for injuries, disease on the main stem/trunk

62. Dead Plants Tell no Tales
Avoid dead plants
Choose plants which show a range of symptoms: moderate to severe

63. Sample Quality: Packaging & Shipping
Keep soil on roots
No extra water
Wrap in dry paper then double bag in plastic
Disinfest exterior of bags
Strong crush-proof box, tape all seams

64. Packaging & Shipping
Good Intentions

65. Actual Results

66. Packaging and Shipping blunders
Soil on foliage during shipping creates “diseases” that were not there when the sample was collected.

67. Packaging and shipping blunders
Sample Soup
Don’t add water or wrap in wet paper towels

68. Good Packaging Plastic bag to keep soil on roots
Dry paper towels to protect leaves from contact with plastic bag

69. Disease management Watch for problems Rule out abiotic factors
Look for symptoms and signs
Consider the recent weather
Consult references, agent
Send a sample to the disease clinic
Follow progress
Keep records