Articles
ASSESSMENT OF THE SUSCEPTIBILITY TO ERWINIA AMYLOVORA OF 90 VARIETIES OR SELECTIONS OF PEAR
Two inoculations were carried out, one in 1984 (12 to 40 shoots by genotype) and one in 1985 (24 shoots).
The results are presented in diagrams where the severity is in absciss and the frequency in ordinate.
Some genotypes as ‘Kieffer’ or ‘HW 605’ show a high frequency but a somewhat low severity.
On the other hand ‘Beurré Hardy’ shows a low frequency but a high severity.
An other point is the stability of the classification for the varieties with a low or a high susceptibility and the variability for the varieties with medium susceptibility.
The responses were generally more severe in 1985 than in 1984.
A collection of 70 varieties of Pear and 20 U.S. or Canadian Selections was planted in the experimental orchard in Dax (South-West of France) where climatic conditions are very favourable for the bacteria. 3 replications of 3 trees were planted (see the list below).
When the trees were four year old they were artificially inoculated by needle prick and syringe just below the top of at least 30 cm long shoots.
In 1984 12 to 40 shoots were inoculated by genotype and in 1985 24 shoots (strain 2045 ; density 108).
The results take in consideration three criteria :
- The Frequency (or Infectivity) : the ratio between the number of shoots with blight and the total number of inoculated shoots.
- The Severity : the mean of percentages of extended lesion to the total length of the shoot for all blighted shoots.
- The Index of susceptibility : the product between Frequency and Severity.
On diagrams we put the Severity in absciss and the Frequency in ordinate and it is possible to draw the curves of iso-index.
A) A variety is classed as very few susceptible if Severity is less or equal to 50, Frequency less or equal to 65 and Index less or equal to 10.