Monitor Streptomycin Resistance in Erwinia Amylovora Populations in New England

1.1 Fire blight is a devastating disease of apples and pears

Apples and pears represent economically important commodities in the United States and New England.  According to recent U.S. census data, apples and pears are grown on 142,000 and 24,300 hectares nationally, with cash receipt values of $2.6 billion and $372 million, respectively (USDA-NASS 2014)  .  In New England (CT, MN, MA, NH, RI and VT), cash receipt values of apple and pears exceed $54 million and $6.1 million, respectively (USDA-ERS).

Fire blight, caused by a bacterial pathogen Erwinia amylovora, is one of the most serious diseases currently limiting apple and pear production in New England and in the United States (Malnoy 2012; Zeng 2013; Zeng 2014).  Fire blight can result in both yield reduction, when infection occurs on the scion, and tree death, when infection occurs in the rootstock. Annual losses to fire blight and costs of control in the United States are estimated at over $100 million (Norelli 2003).  In recent years, severe fire blight outbreaks occurred throughout the New England region. 

1.2 Timely application of antibiotic streptomycin is an indispensable component of fire blight IPM

Most apple and pear varieties sought after by consumers, such as “Gala”, “Fuji”, and “Barlett”, are either susceptible or highly-susceptible to fire blight.  Although multiple fire blight resistant rootstocks such as “Geneva” and “Vineland” have been developed, grafting the susceptible cultivars to the resistant rootstocks only prevent rootstock infections but does not prevent scion infections.  In practice, the fire blight susceptible scions are often grafted to fire blight susceptible dwarfing root stocks for the new industry standard, high-density orchard systems, making these orchards highly vulnerable to fire blight.

The lack of effective cultural control makes chemical control an indispensable component of fire blight IPM.  The most critical step of fire blight management is to limit the multiplication of E. amylovora on stigmas of apple flowers to prevent blossom blight (Malnoy 2012).  With appropriate disease forecasting models (e.g. Maryblyt or CougarBlight), properly timed application of bactericide during bloom can effectively reduce bacterial number in flowers and control blossom blight.  Streptomycin is by far the most effective (up to 90% control) and widely-used bactericide for fire blight control in the United States .  Other control alternatives include the biological control agents Serenade MAX , BlightBan A506, and the systemic acquired resistance (SAR) inducers such as Actigard.  However, these control alternatives have not provided consistently high levels of control compared to streptomycin (Sundin 2009).

1.3 Streptomycin resistance raises critical challenges to fire blight IPM

The intensive, long-term use of streptomycin led to the evolution of streptomycin resistance in E. amylovora populations.  Since the first report in California in 1971 (Miller 1972), streptomycin resistance in E. amylovora has been commonly detected in almost all apple producing regions in the United States, such as Washington (1972), Michigan (1990), and has recently been detected in New York (2003) (Coyier 1975; Chiou and Jones 1993; Russo 2008).  Currently, the New England is the only region in the United States where streptomycin resistance in E. amylovora has not been reported.

The impact of streptomycin resistance is dramatic, both from the economic loss point of view and the environmental impact and human health point of view.  First, because streptomycin is no longer effective in regions with streptomycin resistant E. amylovora populations, thousands of acres of valuable apple and pear crops are exposed to fire blight, which has caused catastrophic losses.  In 2000, the fire blight epidemic in southwest Michigan resulted in the death of 400,000 apple trees, and over $42 million in losses.  In addition, the loss of streptomycin led the growers to adopt other antibiotics to control fire blight.  Oxytetracycline and kasugamycin have been used in fire blight control in California and Michigan due to streptomycin resistance.  Introduction of these antibiotics into agriculture raises concerns of facilitating the development of antibiotic resistance in the environment, with a potential impact to human health.