DETERMINING FACTORS REDUCING THE HOST SUSCEPTIBILITY OF FRUIT TREES FOR FIRE BLIGHT INFECTIONS

Prediction of fire blight infection risk is a very delicate task in which climatological risk assessment has always to be combined with host susceptibility factors on a determined phenological stage of the fruit trees.
The different warning systems determine the climatological infection risk and combine this with the phenological risk.
The role of frost damage in this climatological risk assessment is not clear.
It is generally accepted that frost in or around the flowering period reduces the overall fire blight infection risk in pear and apple orchards.
In a detailed study the fire blight infection risk was evaluated in two years, ’91 and ’97, with heavy night frost during the blossom period of pear trees cv.
Conference.
Following systems were included to determine the fire blight infection risk: Maryblyt 4.3, Parefeu and BIS. The results indicate that frost damage doesn’t exclude primary blossom infections of fire blight on pear completely.
Pear trees with overwintering cankers resulting from late flower infections on regrowth reactions at the end of the season showed primary bloom infections even after repeated presence of frost during bloom period.
This conclusion should be taken into account when judging about the fire blight risk during primary bloom of young trees.
The presence of overwintering cankers seems to be an essential condition to find primary bloom infections after frost.

Host susceptibility is a third factor which should be brought in every fire blight risk assessment.
Reduction of the vegetative shoot growth can reduce the possibility of fire blight shoot infections.
This is clearly indicated for cycocel applications on pear trees and prohexadione-Ca treatments on apple.
But there is also the risk of late regrowth reactions when the vegetative growth is blocked completely in the beginning of the season.
When this regrowth reaction is accompanied with a late secondary flower, the fire blight infection risk can be very high.
Chemical growth regulation can thus reduce or increase the fire blight host susceptibility of fruit trees.
The newest evolution in chemical management of fire blight aims to increase the host resistance to fire blight infections.
It is important for this control strategy to position the treatments with the chemical compounds some weeks before the fire blight infections occur.
For secondary bloom protection this could be a very useful tool to find an acceptable protection of these late flowers.

VIII International Workshop on Fire Blight

T. Deckers, E. Daemen

https://doi.org/10.17660/ActaHortic.1999.489.82