This animation illustrates the biology of fire blight infections in apple and pear. Video produced by Tianna DuPont, Washington State University. Animation by Dani Abram. Funded by USDA SCRI grant #2020-51181-32158. Thank you to reviewers George Sundin, Michigan State University and Ken Johnson, Oregon State University.
| Audio | Visual |
|---|---|
| The fire blight bacteria Erwinia amylovora overwinters in the diseased tissue of overwintering cankers. | Image of a fire blight canker. |
| When humidity is high in the spring the fire blight bacteria ooze out of cankers. | A droplet of orange ooze comes out of the canker. |
| Insects and wind-blown rain can move the bacteria to flowers and tender shoots. | Insects and rain drops move from the canker to flower. |
| Bees can move the bacteria from flower to flower in the orchard. | Bees fly from flower to flower. |
| Once initial blossoms are infested, insects and rain can move the pathogen to additional flowers. | Image of flies and raindrops moving the bacterial cells from the infected plant to new flowers. |
| Fire blight bacteria can also infect young shoot tissue through | Image of a young tender shoot. |
| wounds caused by wind, blowing sand, hail, and insects. | A brown streak representing wind with sand goes across the screen hitting the leaf and leaving an abrasion. |
| As well as natural openings including the hydathodes on leaves | A close up of a leaf shows an irregular circular opening on a hydathode. Bacterial cells are shown entering the hydathodes. |
| And wounds caused when the tiny hairs on plant leaves fall off naturally. | A magnifications of a leaf shows a hairlike structure of a trichome. The trichome breaks in half and bacterial cells swim into the opening. |
| Pathogen cells multiply quickly on nutrient rich floral stigmas when the temperatures are warm. | Image of pathogen cells multiplying on the floral stigma. More and more cells appear. |
| Floral stigmas are moist nutritious environments for the fire blight bacteria. | An image of the T3SS with two rings of proteins and a needle like structure. |
| Erwinia amylovora has a complex of virulence proteins called the Type III secretion system which are essential for the bacterium to cause disease. The T3 secretion system injects proteins into plant cells which interfere with plant defense and disrupt plant cells. | A bacterial cells moves back and forth between the cells and the needle like structure of the T3SS goes into a cell. |
| When humidity is high the fire blight bacterial cells multiply more quickly and the Type III secretion system increases killing more plant cells which leak more nutrients that aid growth of the pathogen. | Image zooms in on the floral stigma to the point where you can see the bacteria. |
| Rain or heavy dew can wash fire blight bacteria down the style into the floral cup where they can further multiply and where they can invade the flowers through the natural openings called nectarthodes. Once inside the nectarthodes, the pathogen again uses its T3 secretion system to initiate a primary infection in tissue around the ovary. | The cells move down the style to the base and enter the holes representing the nectarthodes. |
| This infection can progress down the petiole, into the base of the flower cluster. | Apple blossoms become necrotic and wilt as the pathogen infects. |
| Once inside the plant pathogen cells migrate in the tree well ahead of visible symptoms. | Image of a cross section of a leaf. |
| The fire blight bacteria move in the plant several ways. | Purple bacterial cells moving through intercellular spaces in the leaf. |
| One way the bacteria move is through the intercellular spaces of the parenchyma using a system called sliding motility. Long chains of carbohydrates and sugars called amylovoran and levan protect the bacteria in biofilms and allow it to slide through intercellular spaces. | Image of plant cells bursting and more bacterial cells moving through the plant. |
| At the same time the bacteria produce a virulence protein (DspE) which is inserted via the T3SS to suppress plant defense and facilitate plant cell death adding water and nutrients to the plant intercellular spaces which promotes sliding motility. | Image of bacterial cells clustered inside xylem tissue. |
| As the bacteria get to xylem vessels they start to multiply and colonize the xylem. | The round openings of the xylem become more and more filled up as the purple bacterial cells multiply filling the area. |
| In the xylem at the shoot tip fire blight bacteria form biofilms from long chains of carbohydrates, sugars and cellulose. | Image of a branch coming out from the central leader of a tree. |
| The biofilms allow the bacteria to aggregate | A circle representing magnification starts at the end of the branch where there is a flower. |
| and protect the bacteria from the force of sap flow and host defenses. | Day 1 to 5 show in the left part of the screen. The circle representing magnifications shows a mass of bacteria cells moving down the branch 4 to 5 cm per day. |
| The bacteria can be moving 2 inches per day and are moving far faster than the symptoms. | Day 5 shows on the screen. The mass of cells reaches 22.5 cm (8.8 in) from the central leader. |
| Wilting symptoms in the plant are the result of this bacterial invasion and the buildup of biofilms of carbohydrates and sugars by the bacteria that plug xylem tubes restricting water flow as well as by the parenchema collapsing. | Day 7 shows on the screen. The mass of cells reaches 31.5 cm (12.4 in) away from shoot tip towards central leader. The shoot tip starts to show symptoms of purple necrotic areas. |
| As pathogen cells continue to reproduce in the parenchyma vessels, pressure builds up and bacteria can break back out into the intercellular spaces of the parenchyma. | Day 10 shows on the screen. The shoot tip starts to bend and wilt. |
| When the pressure builds sufficiently, a mound forms and ooze exudes. Each droplet of ooze may have 1 billion bacterial cells. | Day 15 shows on the screen. The flower under magnification has bacterial cells ooze out of it. The branch shows wilting 5 inches or so in length. The first five leaves from shoot tip starting to collapse. |
| As the bacteria moves through the tree cankers are formed. If not removed these are overwintering sites for the bacteria. | Image of a fire blight canker shows again. |
