Bacterial spot of pepper and tomato

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1 Website to brush up on bacterial diseases Bacterial spot of pepper and tomato Potato blackleg and soft rot Crown gall Fire blight of apple and pear Ice nucleation phenomenon Catalysis of ice crystal formation in supercooled water has been demonstrated for some plant pathogenic bacteria e.g. Pseudomonas syringae These epiphytic bacteria initiate ice formation at temperatures higher than normally required. A gene in the bacterium codes for protein in cell wall that confers ice nucleating ability. Ice damage creates wounds Droplet freezing assay: In this sample, ice- nucleating bacteria were abundant, and ice was formed In this sample, icenucleating bacteria were not present. The droplets supercool but do not freeze. The temperature of the wax covered foil is -3 Celcius 1

2 Biological control of frost injury Field application of INA-minus Pseudomonas syringae for frost protection. It was one of the first field releases of a geneticallyengineered microorganism Ice nucleation protein of Pseudomonas syringae is sold to ski areas to improve efficiency of snow making From G.L. Schumann Overwintering of bacterial pathogens There are no specialized survival structures Methods of overwintering: In host or in seed or in clonal planting stock Most common On host as epiphyte Pseudomonas spp. (bacterial canker of woody plant Free living in soil, water or debris (generally rare) Exceptions: crown gall pathogen, soft rot bacteria bacterial scab pathogens root & tuber crops 2

3 How bacteria cause disease The typical life strategy of a bacterial plant pathogen is either facultative parasite or facultative saprophyte Many survive grow and reproduce saprophytically (no disease) on plant surfaces. This growth is called epiphytic (upon the plant). Entry into plants is through wounds or natural openings: Bacterial colony growing epiphytically on a leaf surface Epiphytic growth is favored by a humid environment What does it mean to be an epiphyte? It is the ability of bacteria (and some fungi) to grow and reproduce saprophytically on plant surfaces jargon: to live in the rhizosphere (on roots) or in the phyllosphere (on a foliar surface) or within a biofilm Typically, with bacterial pathogens, an epiphytic phase occurs prior to infection Bacterial colony growing epiphytically on a leaf surface Epiphytic growth is favored by a humid environment A bacterial epiphytic phase has been shown to influence a plant host s sensitivity to frost injury ice nucleation phenomenon Epiphytic (preinfection) phases of bacterial (and fungal) pathogens are frequently susceptible to biological control 3

4 Bacterial pathogens possess the ability to grow and reproduce on or in a plant without necessarily inciting disease This growth is called: epiphytic growth (upon the plant) or endophytic (within the plant). Non-pathogenic bacterial epiphytes associate superficially Plant tissue TTS TTS Bacterial colony growing epiphytically on a leaf surface Pathogenic bacterial epiphytes associate intimately TTS Fungal Pathogens Effect of an H treatment on survival of epiphytic bacteria: 25% Wilson, Hirano, & Lindow 1999 AEM 65: % of cells in sites protected from H2O2 20% 15% 10% 5% 0% B728A 5B-530 5B-533 SG-44 Pathogen strains NPS3136 Cit7 EhWHL9 Bp1 SH1PK Non-pathogen strains Bottom line: Pathogens are specialized to associate intimately with their host 4

5 Definition: Biological control - reduction of the amount of inoculum or disease producingactivity of a pathogen by or through one or more organisms other than man Cook & Baker 1983 How is it accomplished?: - Introduce saprophytic microorganism to plant environment - Modify environment or cultural methods to favor specific saprophytic microorganisms that can accomplish biocontrol Example 1: Biocontrol experiment from lab Inoculated with Agrobacterium tumefacians strains 529 Pretreated with Agrobacterium radiobacter strain K84 followed by A. tumefacians Thus, in this example, the biocontrol agent (strain K84) protects the plant from infection by the pathogen. In the Oregon nursery industry, it is very common practice to dip the roots of plants in a suspension of K84 prior to planting. Question: What is the mechanism of protection? 5

6 Important: The mechanism of suppression influences the quality of the biocontrol interaction Mechanisms of biocontrol: Competition Pathogen Antibiosis Parasitism Biocontrol Strain K84 The importance of antibiosis to the effectiveness of K84 Control of Crown Gall of Tomato with A. radiobacter K84 Tomato Antibiosis ineffective Disease incidence Cherry Antibiosis effective Antibiosis ineffective Antibiosis effective 0.0 0E+0 1E+8 2E+8 3E+8 4E+8 5E+8 6E+8 Dose of biocontrol agent Antibiosis is ineffective either when K84 doesn t make its antibiotic or when the pathogen has become resistant to the antibiotic 6

7 Biological control Works best when: The site of infection on plant supports growth of competing saprophytes (wounds, seeds, flowers, root surfaces) when the biocontrol agent possesses multiple tools to suppress pathogen activity (superior competitor, antibiosis, ability to induce SAR) Antibiosis zone of inhibition when disease is monocyclic and the pathogen is A restricted to a short window of opportunity n t (wound healing, seed emergence, bloom period) i b i o Basidiomycetes Host resistance Epidemiology Bacteria Review 7