Population density and internal distribution range of Erwinia amylovora in apple tree branches

Fire blight in apple and pear orchards, caused by Erwinia amylovora, is a global problem. Ongoing outbreaks have occurred since 2015. In 2020, 744 orchards were infected compared with 43 orchards in 2015 in Korea. When are insufficient. In Korea, all host plants in infected orchards are buried deeply with lime to eradicate the E. amylovora outbreak within a few days. Apple trees with infected trunks and branches and twigs with infected leaves and infected blooms were collected from an apple orchard in Chungju, Chungbuk province, where fire blight occurred in 2020. We used these samples to investigate the population density and internal distribution of E. amylovora on infected branches and twigs during early season infections. Infected branches and twigs were cut at 10 cm intervals from the infected site, and E. amylovora was isolated from tissue lysates to measure population density (colony-forming unit [CFU]·mL^-1). The polymerase chain reaction was performed on genomic DNA using E. amylovora specific primers. Real-time polymerase chain reaction (PCR) was performed to detect E. amylovora in asymptomatic tissue. The objective of these assays was to collect data relevant to the removal of branches from infected trees during early season infection. In infected branches, high densities of greater than 10⁶ CFU·mL^-1 E. amylovora were detected within 20 cm of the infected sites. Low densities ranging from 10² to 10⁶ CFU·mL^-1 E. amylovora were found in asymptomatic tissues at distances of 40 - 75 cm from an infection site.