Plant Pathogens and Biocontrol Agents

1. Plant Pathogens and Biocontrol Agents

2. Plant Pests Pathogens Predators Weeds

3. Symptoms of Microbial Diseases in Plants Necrosis - death of plant cells; may appear as spots in localized areas Canker - localized necrosis resulting in lesion, usually on stem Wilt - droopiness due to loss of turgor Blight - Loss of foliage Chlorosis - loss of photosynthetic capability due to bleaching of chlorophyll Hypoplasia - stunted growth Hyperplasia - excessive growth Gall - tumor

4. Anton de Bary German botanist Investigate Irish potato blight in 1861 Proved experimentally that Phytophthora infestans was actually the cause of the disease

5. Birth of Plant Pathology de Bary gave birth to the science of plant pathology Soon other plant pathogenic fungi were described Pathogenic bacteria and viruses were identified later in the 19th century

6. Dispersal of Pathogens Necessary for repeated cycles of infection and multiplication and, therefore, the spread of an epidemic Understanding of dispersal phase is necessary for predicting the onset and severity of disease Within the realm of dispersal are the processes of release, transport, and deposition Pathogens are dispersed by wind, rain, soil water, insects, nematodes and humans

7. Scope of Dispersal Spread of plant disease can proceed over short distances through focal spread as well as over long distances Terminology of disease spread Microscale Mesoscale Synoptic scale (macroscale

8. An infection focus Area of a crop with a contagious disease Foci often circular If strongly affected by wind, may be comet-shaped or v-shaped generally have a constant radial expansion a few cm per day for a localized infection hundreds of km per year for a pandemic

9. Dispersal Mechanisms Airborne Dispersal Passive discharge Active discharge Rain splash

10. Long Distance Transport Explains the introduction of a pathogen to a new area Also explains the yearly reintroduction to areas where overwintering cannot occur Most of the well studied examples of long distance transport involve fungal spores Due to environmental hazards only a small percent of spores are able to survive long range transport

11. Boundary layer Before spores reach the free air above the crop, they must pass through the boundary layer surrounding the crop Possibly as much as 90% of the spores are deposited within the crop itself The percent that escape from the canopy depends on the balance between deposition and turbulence with greater escape during more turbulent winds Position in canopy also important

12. Pathogens : Viruses Transmission of viruses Insect vectors - especially aphids, whiteflies, leafhoppers, mealybugs, ants Nematodes Seeds from infected parent plants Airborne transmission Infected plant parts Aphids Pollen

13. Pathogens: Bacteria Generally Gram-negative bacilli: species of Erwinia, Pseudomonas, Xanthomonas, Agrobacterium, and Corynebacterium Dispersal from plant generally passive by water, wind-blown water, animals, agricultural workers In warm, humid climates, where dew and rain are common, dispersal of bacteria by rain-splash is the major means of disease spread Airborne spread on rafts of plant material

14. Pathogens: Fungi Over 70% of all major crop diseases are caused by fungi Thousands of fungal species recognized as plant pathogens Fungal diseases cost more than $3.5 billion to US farmers alone In general spores of most fungal pathogens are adapted for airborne transport

15. Late blight of potato (and tomato) Caused by oomycete Phytophthora infestans Occurs wherever potatoes are grown All potato cultivars are susceptible Without fungicidal protection, a blighted field can be destroyed within a couple of days

17. Cool, wet weather promotes disease spread Method of germination is dependent on meteorological conditions Cool, wet weather promotes zoospores Warmer, drier conditions promote germination of the sporangium itself

19. Potato late blight forecasting 40 year history in many potato growing regions When meteorological conditions indicate that Phytophthora spread is likely to occur warnings are issued to apply fungicides Forecasting models have been successful in reducing the number of fungicide applications.

20. Long distance dispersal ofPhytophthora infestans Intercontinental migration has been associated with the transport of infected plants or tubers by humans This occurred in the 1840s and again before the 1980 outbreak of the A2 mating type Long distance dispersal over tens of kilometers is attributed to wind-blown sporangia Maps showing the rapid progress of blight epidemics in the 1840s suggest that a Second-Order Epidemic of late blight could possibly occur during a single growing season.

23. Tobacco Blue mold Caused by oomycete Peronospora tabacina Unpredictable disease of both wild and cultivated tobacco causing devastation some years and not appearing at all during other years

24. Blue mold First described in Australia during 19th century In North America the disease was confined to seedbeds until 1979 1979 the first serious epidemic occurred

25. North American Epidemics The infection rate was especially severe in both 1979 and 1980 Second-Order Epidemic advancing at rates of 10-32 km/day northward in the eastern United States to southern Canada Crop losses in the U.S. and Canada during these two years were estimated at approximately $350 million

26. Source areas Both host plants and pathogen exist year-round in tropical areas such as the Mediterranean and Caribbean basins. In temperate regions, tobacco is grown as an annual P. tabacina is not able to overwinter in temperate zones As a result, the long distance transport of inocula from tropical regions

28. Disease Cycle Infection can occur within four hours after a sporangium lands on the leaf Symptom-free incubation period 5-7 days Appearance of yellow lesions and the development of new sporangia

29. Conditions for Dispersal Cool, wet, overcast weather, favors the rapid advance of the fungus Clear, hot, dry weather stops disease spread

30. Dispersal Each spring in U.S. weather conditions are favorable for the northward transport of Peronospora sporangia from southern sources Case studies of epidemics occurring from 1979 to 1986 suggest at least two likely pathways of disease spread Northward from Florida and Georgia, Cuba North and Eastward from south Texas and Mexico Forecasting systems can potentially provide time for tobacco farmers to apply fungicides

31. Forecasting Blue Mold Predictive models make use of extant disease outbreaks, weather fronts, and weather forecasts HY-SPLIT trajectory model successfully used since the spring and summer of 1995 to predict outbreaks of blue mold The model used to plot trajectories of inoculum-laden parcels of air

32. The Daily Blue Mold Forecast Forecast produced by HY-SPLIT describes future weather conditions at the source and along the anticipated pathway Emphasis given to atmospheric conditions that favor sporulation at the source, survival during transport, and deposition Overall outlook describes the likelihood of blue mold spread over the next 48 hours Available on-line at www.ces.ncsu.edu/depts/pp/bluemold/

33. Rust Fungi About 6000 species Attack a wide range of host Cause some of the most destructive plant diseases Basidiomycetes but no fruiting body

34. Coffee Rust Destroyed coffee plantation in Ceylon in 1870s and 1880s Today threatens coffee wherever it is grown 1966 outbreak in Angola produced spores that were carried across the Atlantic Ocean and washed out by rain in Brazil 5 to 7 days later

35. Wheat rust fungi Stem rust (black rust) caused by Puccinia graminis f.sp. tritici Leaf rust (brown rust) caused by Puccinia triticina (syn. P. recondita f.sp. tritici) Stripe rust (yellow rust) caused by Puccinia striiformis.

36. Puccinia graminis f.sp. tritici Globally the most serious rust pathogen Complex life cycle with 5 spore stages Major method of disease spread is by the uredospores which are easily carried by wind for hundreds or thousands of kilometers

37. Puccinia graminis Life Cycle

40. Uredium with uredospores

41. Telial Stage

42. Source Strength and Viability Mature uredium produces ~ 10,000 uredospores/day over several weeks A 5% disease severity would produce ~ 50 uredia so 500,000 uredospores/day Field of wheat with moderate infection 4 x 1012 uredospores/day/hectare Nagarajan and Singh reported that spores had to be deposited within 120 hrs after takeoff to be infectious

43. Survival of Spores Eversmeyer and Kramer studied survival of uredospores in field At subfreezing temperatures during winter no spores viable after 96 hrs During spring 10 to 20% viable after 120 hrs; 1% survived for 456 hrs In growth chamber spores viable for up to 864 hrs at temp between 10 and 30 C Although atmosphere conditions harsh, small percent will survive

44. US Source Areas In south Texas and Mexico uredospore stage can survive all winter on winter wheat Gives rise to spring infection Spores carried north by southerly winds to northern states where spring wheat grown In late summer and fall uredospores can be carried back to southern areas Puccinia Pathway studied since 1920s � Stakman first described the aerial dispersal

45. Puccinia Pathway

46. In some years the pathogen is spread gradually by anticyclones making a succession of short jumps with stops along the way where the inoculum multiples (Stakman and Harrar, 1957; Isard and Gage, 2001)

47. In other years uredospores are transported by extratropical cyclones over a distance of 1000 km or more in 1 or 2 days In early June 1925 a huge spore cloud move 1,000 km northward Spores were caught in traps throughout the area (previously rust-free) Field observations indicated that infection throughout the region was almost simultaneous (Stakman and Harrar, 1957; Isard and Gage, 2001)

48. LDT of wheat rust in Europe Two pathways studied: An east European path originating in Turkey and Romania A western path from Morocco and Spain Both paths converge in Scandinavian countries

50. Other Areas LDT of uredospores between eastern and western wheat growing areas of Australia Overseas LDT also studied - two strain in Australia identical to strains found in Africa in terms of pathogenicity and isozyme patterns LDT of uredospores in India LDT of uredospores of P. striiformis in China

53. Asian Soybean Rust Caused by Phakopsora pachyrhizi Most destructive foliar pathogen on soybean and reports of loss range from 10 to 80%. Yield losses over 50% are common when meteorological conditions favor disease development

54. Disease effects The fungus causes numerous uredial lesions on the leaves Reduces photosynthetic capacity of the host and subsequently reduces the yield of soybeans Simple life cycle with uredospores and teliospores on same host Phakopsora pachyrhizi can infect more than 95 species of plants including other edible legumes and also weeds as kudzu

58. Phakopsora pachyrhizi history First described in Japan in 1902 By the mid-1930s the pathogen reported from several other countries in Asia and in Australia India in the early 1950s. By late 1990s pathogen had spread to several countries in Africa 2001 reported from Paraguay and Brazil Over the next few years, the pathogen spread through much of the soybean growing areas of South America causing significant yield losses First detected in the continental United States in November 2004

63. Plant Pests and Their Control by Fungi and Bacteria

64. Plant Pathogenic Nematodes Obligate parasites Feed on roots of plants - may cause malformations Many have a sharp stylet that pierces plant cells Some never live in soil, they survive in host and are spread by insect vectors Reduces crop yield and increases risk of infection through wounds

65. Insects Our greatest competitors for food Damage or destroy crops before and after harvest Larval stage often most destructive Injury plants directly by using plant for food or shelter and indirectly by spreading pathogens

66. Basic feeding patterns Chewing Insects Either larvae or adults Tear or bite portions of the plants May eat their way through the plant causing holes and tunnels Lvs left as skeletons by some Others eat whole plant Sucking Insects Pierce the plant and sucks up the sap Results in curling, stunting, deformed parts

67. Weeds �Unloved plant� Injurious to agricultural crops Loss is a direct result of competition for light, water, nutrients Unchecked can dominate crop plants Indirectly damages by harboring insect pests Crop losses by weeds in US ~ $14 billion

68. Control Measures We are dependent on healthy plants to feed the world�s population Chemicals widely used to control plant pests and diseases Dangers of pesticide use apparent Economic cost of pesticides may actually outweigh the value of the crop at harvest time Use other techniques to reduce pesticide use

69. Integrated Pest Management (IPM) Multifaceted approach to disease control Sanitation Crop rotation Genetic resistance Biological Control

70. Biological Control Use of living organisms to reduce disease due to competition or antagonism i.e.. ladybugs to control aphids Ultimate aim is to reduce dependence on chemicals Today emphasis on microorganisms Bacillus thuringiensis for insect control Several Pseudomonas species for control of bacterial and fungal pathogens Numerous fungi for insects, nematodes, fungal pathogens

71. Bacillus thuringiensis Common soil bacterium well known for its ability to produce crystalline proteins with insecticidal properties Since 1960s Bt available as a safe naturally occurring biopesticide sold as a dried inoculum containing endospores and crystals of insecticidal proteins used as sprays or dusts for a wide variety of insects - especially Lepidopteran

72. Bt Toxins Genes for insecticidal proteins on plasmids Many subspecies of Bt which differ in number and type of plasmids Over 1000 strains of Bt have been isolated family of toxins over 200 insecticidal proteins identified and sequenced

73. Bt Toxins Toxins activated by enzymes in insect gut Kill insects by binding to membranes in digestive system and creating pores in membrane~contents leak into body cavity Harmless to humans, natural enemies of arthropods, and non-target organisms

74. Bacillus thuringiensis B.t. subspecies kurstaki is widely used in caterpillar control in agriculture and forestry B.t. subspecies israelensis is active against mosquitoes and black flies B.t. subspecies tenebrionis is active again beetle larvae

75. Bt Uses Spray Applications Bt toxins degrade within a few days Endospores can survive for several years after spray applications Genetic Engineering with Bt genes Transfer into crop plants Transfer other bacteria

76. Pseudomonas species Pseudomonas fluorescens for control of fire blight (also may control apple blue mold) Fire blight � bacterial disease of apples and pears caused by Erwinia amylovora Pseudomonas out competes Erwinia on stigma surface Reduces use of streptomycin which has been helpful since many Erwinia strains resistant