Thirty-one clonal isolates of U. necator
were taken from diseased grapes from three different vineyards
in France and Germany.
Samples were collected in April from typical
‘flagshoot’ symptoms, and in July and August from
the same plants. The mating-type
and the sensitivity to the fungicide triadimenol were
determined for all isolates. Techniques for obtaining and
characterizing isolates
are described. Two isolates in the first sampling and eight
in the second were resistant to triadimenol, with resistance
factors ranging from 3·4 to 11·8. All isolates
in the first sampling and nine out of the 21 isolates in
the second were mating-type (+), the
remaining isolates were mating-type (−). Genetic
variation between all isolates was assessed using the RAPD
technique. Phenetic analysis based on the 364 RAPD fragments
obtained revealed two very distinct groups, one group
containing nine out of the 10
isolates from the first sampling, the second group containing
all the remaining isolates. Isolates clustered in the
first group displayed
58 RAPD fragments specific to them. These isolates did not
exhibit 60 RAPD fragments present in all other isolates.
Molecular and biological data suggested that isolates clustering
in the two groups represent two different biotypes of
U. necator, which are likely to
be genetically isolated.