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Selecting for Resistance to Plant Disease: Part I

Selecting for Resistance to Plant Disease: Part I. Martha Rosemeyer Organic Seed May 20 2004. Outline. Importance of disease resistance Identifying symptoms of disease (those plants that are not resistant!) Types of pathogens Koch’s postulates The disease triangle

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Selecting for Resistance to Plant Disease: Part I

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  1. Selecting for Resistance to Plant Disease: Part I Martha Rosemeyer Organic Seed May 20 2004

  2. Outline • Importance of disease resistance • Identifying symptoms of disease (those plants that are not resistant!) • Types of pathogens • Koch’s postulates • The disease triangle • Implications for selection

  3. Most plants are resistant to most pathogens • Most plants grow relatively unhindered by diseases • Why? • One of most powerful methods for organic farmer, and coming back in importance

  4. What is a disease? • “A harmful deviation from normal functioning of physiological processes” • “Dis-Ease” • Plant diseases cause loss of 12% of food crop worldwide (Science 257: 482, 1992)

  5. What is a plant pathogen? • Any organism which can cause plant disease

  6. What are the mechanisms of resistance to disease? • Structural- pathogen cannot enter- sticky or tough • Chemical- pathogen enters but is hindered by “plant secondary chemicals” of plant used in plant defense • can inhibit insect growth, e.g. tannins are protein-binding • Organic foods higher in antioxidants because insects bite and induce formation

  7. Mechanisms of resistance to plant disease cont. • General systemic PR (pathogenesis-related) proteins act like immune system • Example- pathogen cell wall degrading • Specific antibody-like resistance genes (vertical) • Genetic engineering moving them between species • Gene cloned “osmotin” that inhibit fungal reproduction by bursting spore cases of Phytopthora infestans

  8. Other resistance mechanisms • Hypersensitive reaction- immediate death of tissue (necrosis) around the point of entry of the pathogen • Systemic induced resistance (SIR) • heightened immunity to plant pathogens

  9. Usually mechanism of resistance is not apparent • However we can still select plants that are resistant • Let’s now turn to susceptibility, so we know which plants to eliminate in our screening for disease resistance (whether vertical or horizontal)

  10. In selection the observation of symptoms is key! • Many plant problems caused by non-pathogens in PNW, like unfavorable weather and poor growing conditions or herbicide • Only 1/3 caused by insect pest or pathogen (WSU extension bull. 1247) • Non-pathogens may be identified by regular pattern, more than one crop affected

  11. General steps to diagnosis • Examine leaf or affected plant part and find the time course of symptoms • Is it environmental condition, nutritional problem, pest or disease? • Examine pattern of symptoms in plot- random or regular • Find appropriate resources • Insect references or on-line Web databases • Host Disease Index; Web: Cornell site • Take to plant disease diagnostician • Perform Koch’s postulates

  12. General bacterial vs. fungal leaf symptoms Bacterial, usually Fungal, usually

  13. Mildew vs. Virus or nutrient deficiency

  14. Non-pathogenic conditions Chemical spray injury Salt or chemical injury

  15. Nutrient symptoms can also be confused

  16. Insect damage can cause disease like symptoms Look for the insect Can cause chlorosis, leaf curl, distorted growth

  17. Blue aphid symptoms on alfalfa

  18. Red currant in OR

  19. Aphid damage can manifest in various symptoms AND most importantly they can vector virus

  20. Sooty mold Fungus complex living on aphid “honeydew” (excrement)

  21. Other: Walnut Wilt of Tomato (allelopathy caused byjuglone) MacNab et al. 1983

  22. Lightning damage MacNab et al. 1983

  23. Types of plant pathogens and their symptoms • Viruses (NA plus protein shell) • Bacteria (single cell, no nucleus) • Fungi (most multicellular, have a nucleus) • Nematodes (multicellular organisms) • Most organisms do not cause disease-- less than 0.5%

  24. Viruses Parry 1990 MacNab et al. 1983 MacNab et al. 1983

  25. Cucumber mosaic virus • Wide host range • Symptoms: • chlorosis • distortion of leaf

  26. Tulip Breaking Virus • First plant disease recorded in 1576 by • Carolus Clusius in Netherlands • Realized viral nature in 1900s • Potyvirus- forms filaments strands of • protein coat with nucleic material inside

  27. Bacteria Bacterial Spot of Tomato Xanthomonas vesicatoria MacNab et al. 1983

  28. Angular leaf spot of strawberryXanthamonas frageriae Symptoms: watersoaked lesions leading to necrosis and chlorosis Avoid contaminated plant material when planting (crowns)

  29. Fungi MacNab et al. 1983 Late blight of tomato and potato

  30. Late Blight of Tomato and Potato MacNab et al. 1983

  31. Check disease cycle to know when crop may be susceptible so select at correct time Schumann 1991

  32. Disease started plant pathology as a discipline • Potato famine of 1846 in Ireland (8 million population) caused by Late Blight of Potato, pathogen is fungus Phytopthora infestans • 1 million people die, 1.5 million emigrate to US or Canada • Observations of these organisms were previously seen as result not cause of the disease. Accepted Pasteur’s “germ theory of disease” originally proposed in 1863.

  33. Result of the Irish Potato Famine 30% of ppn died or emigrated Schumann 1991

  34. How do you know any pathogen causes your disease? • Koch’s postulates • German • 1843-1910 • Grew microrganisms on potato slices and then gelatin • Student was Petri • Isolated cholera and anthrax

  35. Koch’s postulates first demonstrated with anthrax Anthrax disease of herbivores caused by the bacteria, Bacillus anthracis Wade, N. New York Times. October 23, 2001

  36. Koch’s postulates • The disease must always be associated with the pathogen • The pathogen is isolated from the diseased tissue in vitro • The pathogen is inoculated onto susceptible host with resulting symptoms of disease as was originally found • The pathogen is re-isolated from the host tissue and found to be the same

  37. Control of Late Blight of Potato • Major problem for organic farmers • yields are about 30% lower and tuber size small • Major control is resistant varieties • problem is keeping up with the pathogen • Irrigate using drip, hill soil, kill foliage before harvest • Fungicides • Org: Copper sulfate and lime (Bordeaux) • Conv: copper, Maneb, Mancozeb, et al.

  38. Sclerotinia Rot of Tomatofungus Sclerotinia sclerotiorum Often disease of Sunflower and Pepper in PNW Symptoms vs. signs (the visible organism) MacNab et al. 1983

  39. Clubroot of Cabbage(Slime mold)Plasmodiophora brassicae • Serious disease of crucifersworld wide • First see wilt • Longevity in soil

  40. Liming may reduce or completely control the disease. The pH of the soil should be raised to 7.2 or higher with hydrated lime • Resistance in cabbage ‘Badger Shipper’ and turnips, radish and rutabaga

  41. Tulip Fire or Tulip blight Botrytis tulipae (Fungus) • Attacks all parts of plant • By far most common and serious disease • Can cause complete loss • Control- remove diseased plant and bulb • Rotate- 2 years minimum

  42. Blue mold of Tulip, Penicillium sp. Grows especially on damaged bulbs Avoid damaging bulbs

  43. Blue Mold of Onion and GarlicPenicillium sp. (Fungus) • Appears during harvesting and storage • Watersoaked lesions to green/blue powdery mold • Dry immediately store at 41°F with low relative humidity Blue mold of onion

  44. Early Blight of Potato and TomatoAlternaria solani (Fungus) • Early in season • Same disease affects tomato, potato, eggplant, pepper • Look for target like spots

  45. Can infect tuber when lifted out of the ground through infested ground • Can affect crown of stem • Control: Minimize leaf wetness • Rotate 3-4 years • Remove infected material • Resistance

  46. Tomato Varieties Resistant to Early Blight • Mountain • Mountain Fresh • Plum Dandy • Supreme • Big Beef • Floramerica • Juliet • Potato Varieties Less Susceptible to Early Blight • Kennebec

  47. Root Knot Nematodes(there are other pathogenic ones as well)technically a parasite Nodules vs. Gall? Root knot nematode of bean Cardona et al. 1982 Dindal’s, Foodweb of the Compost Pile

  48. A susceptible host plant A pathogen capable of causing disease Environmental conditions Need all 3! So if your plant does not appear susceptible, do you know if it is or not? Just because the pathogen is there will disease be present?

  49. If disease is present you can select it out of your breeding population • If no disease present you don’t know if you don’t have the right environment for expression of disease or the pathogen is not present

  50. Summary • Know your germplasm and know your potential plant disease, pest and abiotic problems, so you can select appropriately

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