Secondary Blossoms Are a Fire Blight Risk

Michael Celetti, Plant Pathologist – Horticulture Crops, OMAFRA

Many apple growing regions are reporting a prolonged period of secondary or ‘rat tail’ blossoms this year. Trees with late secondary blossoms (Figure 1) are at risk of becoming infected with the fire blight pathogen since they are not protected by antibiotics applied weeks ago during the main bloom period. Apple and pear growers cannot become complacent about fire blight management after bloom period has ended and must remain vigilant in scouting for secondary or ‘rat tail’ blossoms and fire blight.

Open blossoms are the most susceptible tissue for the fire blight pathogen Erwinia amylovora to enter and infect trees. Fortunately, the application of Streptomycin and Kasumin during bloom significantly reduces the incidence of infected blooms and is an important component in an integrated disease management strategy. Three applications of Streptomycin and four applications of Kasumin are allowed during the bloom period. Correct timing and proper application of Streptomycin and Kasumin are critical for successful prevention of blossom infection.

If less than 3 applications of Streptomycin or 4 applications of Kasumin have been made during bloom, it is possible to use these products to protect late blossoms as long as it is within:

• 50 days to harvest (PHI) for apples and 30 days (PHI) for pears with Streptomycin
• 90 days (PHI) with Kasumin

However if time and labour are available, it may be best to go through the orchard and remove the rat tail blossoms altogether since the fruit produced by the late blossoms are more of a risk than they are an asset.