Fire blight, blossom blight

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Fire blight is caused by the bacterium Erwinia amylovora, an organism that is capable of infecting blossoms, fruits, vegetative shoots, woody tissues and rootstock crowns. There are several distinct phases of the disease including blossom blight, shoot blight and rootstock blight. The diversity of host tissues, combined with the limited number of management tools available to control the disease, has made it difficult to stop or slow the progress of fire blight epidemics. Fire blight is particularly difficult to manage, and the situation is exacerbated by three major problems: (1) most of the popular apple cultivars selected by growers are either rated as susceptible or highly susceptible to fire blight; (2) many of the popular dwarfing rootstocks utilized in Michigan are also highly susceptible to fire blight; and (3) the few chemical control options available are further limited by the development of streptomycin resistance in some areas of Michigan.

The development of fire blight is heavily tied to weather conditions. For example, pathogen cells grow well at temperatures above 70ºF and do not grow well when temperatures are below 60ºF. The availability of moisture is important for growth—this moisture can come from rain or heavy dew. Rain also disseminates pathogen cells among blossoms. Initial fire blight inoculum comes from cankers on trees in the form of bacterial ooze; this usually orange- or amber-colored goo contains pathogen cells in a sugar matrix. It is important to note that viable bacteria can also be present in cankers that are not oozing. The pathogen is disseminated by wind, rain and insects such as ants and flies, to blossoms where growth occurs on stigmas. Honeybees do not visit the ooze and are not responsible for the primary dispersal of the pathogen. Rain is particularly important in spread, for example, in the downward spread of inoculum in trees with cankers in the upper branches.

The fire blight pathogen is exceptionally adapted to growing on the stigmatic surface of blossom pistils. Populations can readily reach 1 million to 10 million cells per blossom. Each open blossom is only susceptible to infection for a few days; however, the pathogen is adapted to successfully infect blossoms during this window of time. After explosive growth on stigmas, rain or dew facilitates the movement of pathogen cells to the hypanthium (floral cup) where infection occurs through nectarthodes.

The blossom phase of fire blight is the only phase where the pathogen is located on the surface of trees and can be targeted by chemical sprays. Blossom blight can be controlled by killing pathogen cells on blossoms or by limiting pathogen growth on blossoms. The antibiotic streptomycin is the most effective control compound available for blossom blight. Reducing blossom populations and thereby reducing fire blight inoculum is critical to the control of the blossom blight phase of the disease and also in reducing disease pressure for the shoot blight phase of the disease. Streptomycin is so effective because this antibiotic kills pathogen cells on blossoms. The antibiotic oxytetracycline (Mycoshield) is also effective for blossom blight control. A Section 18 specific exemptions for the use of Mycoshield was granted by the EPA for the 2007 season. Mycoshield is bacteriostatic and does not kill pathogen cells but does inhibit cell growth. This growth inhibition is sufficient to keep pathogen populations low providing blossom blight control. However, inoculum remaining in trees can cause shoot blight later in the season. The biological control material Serenade Max provides some blossom blight control under low to moderate pressure conditions. This material can be used as a resistance management tool, but will be less efficacious under high pressure conditions. Finally, a Section 18 specific exemption for the use of Agry-Gent was granted by the EPA for Berrien, Van Buren, Kent and Ottawa counties. Please see the article in the April 10 issue of the CAT Alert for information on the specific requirements for use of Agry-Gent.

The MARYBLYT model developed by Dr. Paul Steiner is an important tool for growers to assess the potential for fire blight infection. This model, used in conjunction with accurate weather information (available at the Enviroweather site) continually assesses the upcoming risk of infection. The MARYBLYT model sets a requirement of four factors that must be reached for a fire blight infection:

  1. Blossoms open with stigmas and petals intact.
  2. Current average daily temperature above 60ºF.
  3. Accumulation of at least 110 cumulative degree-hours above 65ºF from the first open bloom.
  4. Wetting event of > 0.01” or heavy dew or fog sufficient to wet foliage or rain of ≥ 0.1” the previous day.

In addition, other major factors important for the occurrence of blossom blight include:

  • Previous history of fire blight (active cankers in orchard).
  • Varietal susceptibility.
  • Trauma event, including hail, sustained winds, or heavy rain.

With bloom upon us in many areas of the state and relatively high temperatures (conducive for fire blightpathogen growth) forecast for later this week and next week, it will be critical for apple blossoms to be protected in advance of these potential infections.

Dr. Sundin’s work is funded in part by MSU‘s AgBioResearch.

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