Blossom blight management for upcoming warm conditions

The combination of bloom and warm temperatures significantly raises the risk of fire blight infection in Michigan apple and pear orchards.

Fire blight ooze on apple fruit.
Fire blight ooze on apple fruit. Photo by MSU Extension.

The combination of bloom and warm temperatures in apple and pear orchards significantly raises the risk of fire blight infection. The fire blight pathogen grows on flower stigmas and can grow to populations of one million cells per flower under optimal conditions. Infection occurs with rain or a heavy dew as the free moisture enables pathogen cells to swim down the style to the base of the flower where infection occurs through the nectaries. Preventing flower infection is critical to successful fire blight management, as blossom blight infection leads to two dangerous conditions:

  • Pathogen cells from infected flower clusters will migrate through trees and eventually kill the tree if left unchecked.
  • Pathogen cells can also ooze out of infected flower clusters; the ooze droplets provide further inoculum for shoot blight infections.

If shoot blight infections occur soon after bloom, the risk of an epidemic is high as secondary cycles of shoot infection, ooze emergence and new infection events can occur very quickly.

The MaryBlyt disease model predicts the potential for disease occurrence and the magnitude of the risk. If the Epiphytic Infection Potential (EIP) number moves above 70, the significance of the risk has increased to the point that only our best materials for blossom blight management should be used: streptomycin (if no streptomycin-resistant pathogen present) and Kasumin. Summary information on blossom blight antibiotics is available in the Michigan State University Extension article, “A primer for Streptomycin, Kasumin and Oxytetracycline use for fire blight management.”

As temperatures warm later this week, EIP numbers will continue to increase. The main reason for this is that the warm temperatures are more conducive to pathogen growth on flower stigmas. We have found that a particular risk factor favoring pathogen growth on flowers is the occurrence of warm, humid nights (evening low temperatures in the 50s and 60s). Thus, as you are examining weather forecasts, play particular attention to days with highs in the 70s-80s with lows in the 50s-60s; these types of days are highly conducive to pathogen growth and provide the highest risk of infection.

Antibiotics should be applied prior to the first warm evening, thus, before the buildup of pathogen cells. It is much easier to control pathogen populations when they are low, i.e., get the material (streptomycin or Kasumin) on the flowers before the warm night that will trigger growth. This strategy offers the best chances to reduce populations.

The effectiveness of streptomycin and Kasumin is about two days when the EIP is 100 or less. However, the length of time before the next spray is needed may be shorter than two days and is affected by many factors, but the three most important factors are:

  • The occurrence of newly-opening flowers that are unprotected by the previous spray application and can be colonized.
  • The occurrence of a rain event that can distribute pathogen cells throughout the orchard.
  • EIPs are above 100. Also, shorten the spray interval if the previous spray was applied under conditions that would have prevented optimal coverage.

Finally, these upcoming high-risk conditions for blossom blight signify a need to actively control for shoot blight as well. Using prohexadione-Calcium (ProCa) is our best material for shoot blight management. King bloom petal fall is the optimal first timing for shoot blight management, a time when initial shoot growth is around 1-2 inches and there is plenty of available foliage for uptake of the material

In addition, we have been working with strategies for using lower rates of ProCa in combination with host resistance inducers for shoot blight management in high density situations where growers are hesitant to impact tree growth. Our current best bet on rates is a combination of 2 ounces Apogee (BASF) plus 1 ounce Actigard (Syngenta) per acre. This rate has provided effective shoot blight management while not significantly impacting shoot growth.

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