1. Potato Science Lecture 7 Potato diseases – foliar, soilborne, viruses, and tuber rots.

2. What is plant disease? Anything that causes disfunction Many (most?) are caused by “infectious biological agents” that are parasitic Others are physiological or “abiotic” (Lecture 15) We will discuss several important infectious diseases of potato

3. Causal agents of disease Viruses Viroids Prions? Bacteria Actinomycetes phytoplasmas fastidious bacteria Fungi Nematodes

4. Host Pathogen Environment Disease Triangle ! Disease !

5. Disease Triangle Plant Pathology is about interactions Host can be altered by choice of variety Use of certified seed reduces pathogen Alter irrigation or drainage changes environment Usually these factors can’t be controlled very well Disease is the result

6. Epidemic Susceptible host Large population of virulent pathogen Very favorable environment Over a large area Usually wind-borne Can be very expensive Even deadly

7. Important Terms Primary inoculum Secondary inoculum Symptom Sign Incubation period Latent period

8. Symptom Sign

9. Infection Many pathogens require a wound Some use “natural openings” Stomates Lenticels Some penetrate directly Mechanical Enzymatic Combination

10. Infection Some require a “vector” Provides mobility Provides wound Insects (aphid, thrip, leafhopper) Fungi (powdery scab vectors PMTV) Nematode (Stubby root vectors TRV)

11. Causal agents of disease Viruses Viroids Prions? Bacteria Actinomycetes phytoplasmas fastidious bacteria Fungi Nematodes

12. Plant Pathology G. Agrios

13. Bacterial Diseases

14. Bacteria: Characteristics Very small Require microscopic techniques to see Biochemical techniques also needed Rapid generation times (20 minutes!?) Prokaryotic Rigid cell walls Phytoplasmas = no cell wall = ameoboid

15. Bacteria: Characteristics Damage often due to enzymatic activity Some may clog vascular system

16. Specific Bacterial Diseases

17. Disease : Soft rot / Blackleg Organism: Pectobacterium carotovorum (=Erwinia carotovora) Symptoms: Foliar blackleg appears as a black stem rot starting at the seed that kills the stem. Soft rotted tissues appear creamy and very soft, can become discolored and odiferous Source and Spread: Inoculum originates from multiple sources and spreads during handling, field spread is in water

19. Key Features Are everywhere Very opportunistic secondary invader Thrives with or without O2 Facultative anerobe Storage rot – major player Seed piece decay – major player Aerial stem rot Soft rot bacteria

21. Disease: Bacterial Ringrot (BRR) Organism: Clavibacter michiganensis subsp. sepedonicus Symptoms: Plants can show wilting and leaf rolling, tubers show a slimy yellow exudate in the vascular ring Source and Spread: Seed tubers provide inoculum which spreads during seed cutting and handling

22. Bacterial Ring Rot “BRR”

23. Key Features Almost exclusively seed borne Zero tolerance in seed Confined to vascular tissues Can spread during seed cutting Survives on equipment and in storages Bacterial Ring Rot

24. Disease: Common scab Organism: Streptomyces scabies Symptoms: Corklike scabby areas or pitted depressions on the tuber surface Source and Spread: Endemic to many soils or introduced on seed, infects upon contact with tuber skin

26. Key Features Superficial only Only develops while tuber is growing Progress stops in storage “Cosmetic” disease Common Scab

27. Fungal Diseases

28. Plant Pathology G. Agrios

29. Fungi: Characteristics Most plant diseases caused by fungi Larger, some can be seen with naked eye Also require microscopic techniques Biochemical techniques also needed Complex life cycles in some

30. Fungi: Characteristics Eukaryotic Multicellular Main body is thread-like “hypha” Many “hyphae” = “mycelium” Many form spores and other structures Spores sexual or asexual Overwintering and dispersal structures

31. Fungi: Characteristics Wind, soil, water, seed, equipment Single or multicycle Many ways of entering plant Foliar, tuber or both affected

32. Fungi: Characteristics Wind, soil, water, seed, equipment Single or multicycle Many ways of entering plant Foliar, tuber or both affected

33. Specific Fungal Diseases

34. Disease: Late Blight Organism: Phytophthora infestans Symptoms: Leaf and stems lesions, foliage destruction, tuber rot Source and Spread: Seed, cull piles and volunteer potatoes provide inoculum, sporangia move with wind and water

37. Key Features Most important disease of potatoes Responsible for Irish famine Very rapid disease development Spores windborne Must have wet conditions Effects foliage and tubers Recent changes in capabilities Late Blight

38. Disease : Early blight Organism: Alternaria solani Symptoms: Brown to black leaf lesions (bullseye) appear first on the older leaves, leaf death and defoliation, sunken surface tuber lesions Source and Spread: Inoculum in soil overwinters on debris, moves onto the plants is from splashing water, additional spore movement in air and water, tubers infected during harvest

39. Early blight

41. Late blight

42. Key Features Attacks senescing tissues May show up on lower leaves first Favored by alternating wet and dry Mostly foliage but tubers can be affected Early Blight

43. Disease: Verticillium wilt Organism: Verticillium dahliae or albo-atrum Symptoms: Wilt of stems and leaves, early death of foliage, necrotic stem vascular streaking Source and Spread: Inoculum occurs naturally in the field and overwinters on refuse, disease moves with seed and soil, only plants in infested fields are infected

44. Verticillium wilt

46. Key Features Soil borne disease One major reason for fumigation Some varieties worse than others Mostly wilt but some SED is possible Verticillium wilt

47. Disease: Rhizoctonia Canker Organism: Rhizoctonia solani Symptoms: Reddish brown lesions on underground stems and stolons that occasionally result in girdling or “damping off”, black “scurf” on the surface of mature tubers Source and Spread: Overwinters in soil or on seed tubers as sclerotia which invade developing sprouts or stolons in the spring.

48. Rhizoctonia

50. Key Features Seed and soil borne Cankers girdle new shoots More susceptible before emergence Yield unchanged, quality affected Cosmetic = “dirt that won’t wash off” Rhizoctonia

51. Disease: Fusarium tuber rot “dry rot” Organism: Fusarium coeruleum and sambucinum Symptoms: After a period in storage brown lesions form under the tuber periderm, infection area enlarges and becomes sunken, tuber eventually mummifies Source and Spread: Inoculum source is primarily seed tubers and is spread during seed handling, hyphae invade wounds

52. Fusarium sambucinum

53. Dry rot Fusarium coeruleum

54. Key Features Seed and soil borne? Must have a wound to infect tubers Cut seed tubers = huge wounds Seed piece decay Storage rot Fusarium dry rot

55. Disease: Pink rot Organism: Phytophthora erythroseptica Symptoms: Field infection can cause wilting but is primarily a tuber rot, spreads quickly and uniformly through the tuber, flesh rubbery but intact and turns pink upon exposure to air Source and Spread: Endemic to some soils, spores infect tubers through stolons, lenticels, or buds

56. Pink rot

57. Key Features Tissues remain relatively firm Rubbery texture Rot usually progress in straight line Tissues turn pink upon exposure to air Can be seen in field and storage Pink rot

58. Disease: Leak, Pythium Watery wound rot Organism: Pythium ultimum Symptoms: Solely a tuber rot, starts as a discolored area around a wound, rot moves quickly through the flesh with a dark line demarcating line between infected and healthy tissue Source and Spread: Endemic to most soils, enters tubers at wound sites during harvest and handling

60. Key Features Must have a wound Associated with high pulp temperatures Very rapid decay Often rots center of tuber, leaving shell Tissues very soft Readily “leaks” clear fluid Pythium leak

61. Diseases not discussed Powdery scab White mold Black dot Phoma Southern bacterial wilt Compendium of Potato Diseases

62. Disease Management Principles

63. Management Usually aimed at populations Single individuals not important EXCEPT Trees, other perennial s

64. Management Diseases difficult to cure Most management aims at protecting

65. Control Strategies Regulatory Cultural Biological Physical Chemical

66. Regulatory Aimed at excluding a pathogen from host or geographic area

67. Cultural Avoiding contact between plant and pathogen Create unfavorable environment or avoid favorable conditions Eradication or reduction of inoculum

68. Biological Host resistance Microorganisms antagonistic to pathogen

69. Physical and Chemical Protection from inoculum Curing an infection

70. Control Methods Exclusion Eradication Resistance Direct protection Integrated control

71. Guidelines for making disease management decisions

72. Before Planting Use only certified seed Fall fumigation? Spring not as good

73. At Planting Avoid unfavorable conditions Use a seed piece treatment Fusarium dry rot Rhizoctonia stem canker/black scurf Late blight? Single drop or healed seed In furrow fungicide applications

74. Growing season Fertility and water management Scouting, forecasting Fungicide applications may be needed for: Early blight Late blight White mold Insecticide for PLRV (virus - insect vector)

75. Vine Kill and Harvest Adequate time for skin set before harvest Wound-obligate pathogens Post harvest fungicides for some diseases Phosphorus acid (late blight, pink rot) Biologicals (bio-save) (dry rot New post harvest under development

76. Storage Wound healing period Pesticides applied in storage? Tools for the storage manager Air flow Temperature Humidity

77. Virus Diseases

78. Plant Pathology G. Agrios