Scientists know quite a lot about fire blight. Its basic source is in cankers from previous infections.
The bacteria overwinter there. As the canker begins to ooze in spring, bees and other insects are attracted and physically move the bacteria to blossoms. Wind and rain add to the movement.
Fire blight bacteria multiply rapidly and increase to huge numbers—“millions on flowers, even billions in ooze drops, in an area no bigger than the head of a pin,” said Michigan State University plant pathologist Dr. George Sundin.
“Temperature regulates the division time,” he said.
Bacteria can double in number in 16 to 24 hours when temperatures are in the 50s, in 6 to 12 hours when temperatures are in the 60s, and in 1 or 2 hours when temperatures are in the 70s. Higher temperatures slow it down.
“Erwinia always grows better than the competition,” he said, something to remember when using biological products that compete for space in the apple blossom.