Pondering planting pear trees for a profit? You should know all cultivars are not created equal – at least when it comes to fire blight resistance.

Emma T. Alstott, Suzanne M. Slack and Xiaochen Yuan, researchers from Iowa State, have been studying the fire blight susceptibility of European (Pyrus communis) and Asian (P. pyrifolia) pears. The European cultivars are grown extensively throughout the U.S.; the Asian cultivars are more often planted in the Pacific Northwest.

“Fire blight disease, caused by the bacterium Erwinia amylovora, is the limiting factor for pear production worldwide,” the research team stated. However, in recent years, new cultivars of both European and Asian pears have been introduced claiming decreased susceptibility to the disease.

To test these claims, in 2023 and 2024 they compared fire blight infections on fruits, shoots and blossoms. A total of 18 cultivars of European and Asian pears were evaluated for disease incidence against various strains of the fire blight-causing bacteria:

  • Flowers were sprayed at 80% to 100% bloom with E. amylovora strain Ea110
  • The youngest leaf on an actively growing shoot was cut with scissors dipped in the inoculum
  • Immature fruits were stab wounded and then inoculated with strains Ea110, Ea1189, Ea88, CA11, MR1, FC01 or IA01, representing isolates from different American regions

Specific pear cultivars were highlighted. ‘Harrow Delight,’ ‘Keiffer’ and ‘Moonglow’ are European; ‘20th Century,’ ‘Atago,’ ‘Chojuro,’ ‘Niitaka’ and ‘Olympic’ are Asian.

First, the percentage of diseased flower clusters was recorded. ‘Moonglow,’ ‘Kieffer,’ ‘Olympic’ and ‘Atago’ had no symptoms after inoculation – good news! Bad news: the worst performer was ‘Niitaka,’ with closer to 40% diseased clusters.

The Iowa State team found that susceptibility to shoot blight caused by Ea110 does not correspond to blossom blight. The lesion ratio for shoots was also noted. ‘Moonglow’ and ‘Harrow Delight’ had no symptoms after shoot inoculation. However, ‘Niitaka’ saw about 80% lesions, and ‘Atago’ had almost 100% lesions.

Four additional European cultivars – ‘Comice,’ ‘Harrow Sweet,’ ‘Shenandoah’ and ‘Sunrise’ – and four Asian cultivars – ‘Kosui,’ ‘Shinko,’ ‘Ya Li’ and ‘Yoinashi’ – displayed low susceptibility to shoot blight (more good news!).

Fire blight symptoms did vary in severity between different strains of the bacteria. In immature ‘Moonglow’ pears, strain Ea88 (from the Pacific Northwest) caused no lesions. Ea110 (from the Midwest) and Ea1189 (from Europe) resulted in smaller lesions compared to other Midwest strains (IA01 and FC01).

The overall susceptibility of the various cultivars to fire blight was ranked at the end of the trial, with either low, moderate or high risk. European varieties ‘Moonglow’ and ‘Harrow Delight’ ranked low in the percentage of diseased flowers, lesion ratios and area of necrotic tissue on fruit.

Asian cultivar ‘20th Century’ was low in fruit and shoot damage and moderate in blossom disease; other Asian cultivars had mixed results. For example, ‘Chojuro’ was also low in fruit and shoot damage but high in diseased flowers while ‘Olympic’ was high in fruit damage but low in blossom and shoot issues. ‘Niitaka’ was the worst performer, with high shoot and blossom issues and moderate fruit damage.

Across the board, smaller lesion areas were seen in European cultivars than in Asian cultivars following inoculation with the Ea110 strain (from Michigan).

The research team concluded that susceptibility to blossom blight is comparable between European and Asian cultivars.

For those looking to add pear trees to their orchards, note that in this trial, European cultivars bred to be less susceptible to fire blight exhibited overall lower incidence of fruit lesions, shoot blight and blossom blight – but results may vary by regional strain.