Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2007
Publication Date: N/A
Interpretive Summary: Fire blight, a serious disease of apple and pear trees, limits fruit production in many areas of the world. The causal bacterium, Erwinia amylovora, colonizes the stigmas of blossoms during warm weather, and rain or dew facilitates its movement to the floral cup (hypanthium) where infection occurs through nectary openings. In the Pacific Northwest, orchard managers time protective sprays according to a system called Cougarblight, involving a moving 4-day temperature evaluation prior to wetting. The system was based on the assumption that flower stigmas support growth of bacteria for only a few days; however, later investigation of this did not produce an adequate explanation for the effectiveness of Cougarblight. Recent experiments in the laboratory with detached crab apple flowers established inoculum levels in flower hypanthia that may realistically result from typical wetting and indicated that hypanthium susceptibility to disease decreases with flower age at temperature-dependent rates. Direct inoculation of apple hypanthia in the field confirmed the relationship between infection and flower age and demonstrated a dose effect. The information was useful in proposing a conceptual model that better explains the 4-day temperature evaluation and includes aspects of both bacterial growth on stigmas and susceptibility of hypanthia to infection. The new knowledge could lead to improvements in fire blight risk assessment.
Technical Abstract: Fire blight risk in the Pacific Northwest region of the United States is widely determined with the Cougarblight model. A distinctive feature of the model at its inception was its 4-day temperature evaluation, based on the assumption that flower stigmas support growth of E. amylovora for only a few days, and on the comparison of various assessment periods applied to weather records and actual infections. Studies relating flower age to stigma receptiveness to bacteria did not offer a satisfactory or full explanation for the model; thus, the relation of flower age to hypanthial infection was investigated. Direct inoculations of flower hypanthia of various ages were performed with detached crab apple flowers in the laboratory and with Gala apple flowers in the orchard in 2005 and 2006. Both experiments demonstrated a decrease of disease incidence as flowers aged. Rate of declining susceptibility was dependent on temperature in the laboratory. Regression analysis of orchard data indicated an inoculum dose effect, with steepest decline of susceptibility occurring during the initial days after petal expansion. In 2005, when conditions were most favorable for disease, the pathogen level of 104 CFU (representing a maximum due to wetting of a few hours according to laboratory tests) caused a disease incidence that decreased from 40 to 18% for flowers 0 to 4 days old and from 12 to 9% for flowers 5 to 8 days old. Cougarblight, and other models now incorporating a blossom-age component (e.g., MaryblytTM), may be explained by multiple factors. Our general conception is that high temperatures during bloom allow E. amylovora to reach maximal levels on stigmas within a time period when hypanthia are still highly susceptible to infection upon wetting. Further study of these interrelationships could lead to improvements in fire blight risk assessment.