Epiphytic Bacteria and Yeasts on Apple Blossoms and Their Potential as Antagonists of <italic>Erwinia amylovora</italic>

نویسندگان

  • P. Lawrence Pusey
  • Virginia O. Stockwell
  • Mark Mazzola
چکیده

Pusey, P. L., Stockwell, V. O., and Mazzola, M. 2009. Epiphytic bacteria and yeasts on apple blossoms and their potential as antagonists of Erwinia amylovora. Phytopathology 99:571-581. Apple blossoms were sampled for indigenous epiphytic populations of culturable microorganisms during different stages of bloom at two locations in central Washington State and one site in Corvallis, OR. Frequencies and population sizes of bacteria on stigmas of apple were lower in Washington than at Corvallis, where average relative humidity was higher and possibly favored greater colonization; however, bacteria at Corvallis were mainly pseudomonads, whereas those in Washington were diverse, composed of several genera. In Washington, yeast as well as bacteria were isolated from both stigmatic and hypanthial surfaces. Sampled blossoms were processed immediately to assess microbial populations, or after a 24-h incubation at 28°C and high relative humidity, which broadened the range of detectable taxa evaluated as potential antagonists. Identifications were based on fatty acid methyl ester profiles and rDNA sequence analyses. Yeasts or yeastlike organisms were detected at frequencies similar to or greater than bacteria, particularly in hypanthia. When microbial isolates were tested for their capacity to suppress Erwinia amylovora on stigmas of detached crab apple flowers, many were ineffective. The best antagonists were the bacteria Pantoea agglomerans and Pseudomonas spp. and a few yeasts identified as Cryptococcus spp. Further evaluation of these taxa on flowers could lead to the discovery of additional biocontrol agents for fire blight. Microbial communities on aerial plant surfaces are diverse and include many different genera of bacteria, yeasts, and filamentous fungi that affect plant health as pathogens, symbionts, and antagonists of disease organisms (28,29). On leaf surfaces, bacteria are most abundant, followed by yeasts; filamentous fungi are considered transient inhabitants present mainly as dormant spores (3). For diseases caused by pathogens with an epiphytic stage in their life cycle, such as many bacterial diseases, indigenous microbial populations can affect the outcome of plant–pathogen interactions prior to infection (6,29). Thus, many research programs are directed toward characterization of microorganisms that live epiphytically on plants, where they individually or jointly reduce plant disease. Such investigations can lead to an understanding of interactions affecting disease development. Although the microbial ecology of foliar surfaces has been studied extensively in relation to disease development (29), less is known about microbial communities on floral surfaces, which also serve as infection courts for many plant pathogens. Blossoms of apple and pear are the primary sites where Erwinia amylovora, the causal agent of fire blight, becomes established epiphytically and initiates infection (58). The pathogen grows mainly on stigmas, which have a moist and nutrientrich surface (46), and subsequent rain or heavy dew facilitates its movement to the cup-shaped hypanthium where infection occurs through nectarthodes (55). Dilution of nectar in the hypanthium by water is also thought to decrease osmotic pressure and increase the suitability of this environment for bacterial colonization (20,41,55). Management programs for fire blight have emphasized risk assessment (7) and suppression of E. amylovora on floral parts with antibiotics (39,58). Reliance on streptomycin, in particular, led to pathogen resistance in many production areas (34,39). Microbial suppression of E. amylovora on floral surfaces, particularly the stigmas (22), is a viable alternative or complementary measure to antibiotics (30,51). In 1996, Pseudomonas fluorescens strain A506, initially selected based on its inhibition of an ice-nucleating strain of Pseudomonas syringae on corn leaves (27), became the first commercially available antagonist for fire blight management (BlightBan A506; Nufarm Americas Inc., Burr Ridge, IL) (22). Bacteria present in other products now available or under development for this disease include strains of Pantoea agglomerans (15,16,59) and Bacillus subtilis (9). Yeasts or yeastlike species being evaluated in commercial preparations include Aureobasidium pullulans, Candida sake, and Metschnikowia pulcherrima (13,21). Until bacterial populations on pear blossoms in the northwestern United States were studied by Stockwell et al. (52), little was known about the population sizes, distributions, or diversities of indigenous bacteria on these floral tissues. Likewise, there has been a lack of detailed information in the literature regarding epiphytic microorganisms on apple blossoms, even though many epiphytic organisms have been obtained from aerial surfaces of apple and evaluated as antagonists of E. amylovora (22). Methods used for screening and selection of microbial antagonists of E. amylovora have evolved from the use of artificial media (5,19) to laboratory procedures with immature pear fruit (60,65) and later to bioassays involving detached flowers or detached flower-bearing branches (35,36,40). Mediaand fruit-based assays have been useful for identifying and studying antagonistic strains that produce antibiotics inhibitory to E. amylovora, but results often do not correlate with suppression of the pathogen on blossoms (14,62,63). Evaluation of microbial strains directly on flowers is more likely to identify antagonists effective on floral surfaces Corresponding author: P. L. Pusey; E-mail address: larry.pusey@ars.usda.gov doi:10.1094 / PHYTO-99-5-0571 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological

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تاریخ انتشار 2009