METHODS OF TESTING APPLES FOR RESISTANCE TO FIRE BLIGHT
The aim of phytopathological breeding consists of a complex resistance. Checking for resistance therefore should take into consideration all economical important virus, mycoplasma, bacterial and fungous diseases. The most important diseases and pathogens of fruit trees in the German Democratic Republic have already communicated at an earlier time (Kegler u.a. 1980).
One of the pathogens most severely damaging apples under distinct conditions is the agent of fireblight, Erwinia amylovora (Burr.) Winsl. et al. It therefore was integrated into the programme of complex resistance checking as well as into the process of breeding and valuation of pome fruit varieties in our country.
Investigations on the susceptibility of apple to E. amylovora internationally are carried out in the field as well in glasshouse conditions. Good correlations or conformities of field and glasshouse tests are frequently noticed (Aldwinckle and Preczewsky, 1976; van der Zwet, 1975; van der Zwet and Keil, 1979).
Both procedures have advantages and disadvantages. The combination of testing under controlled conditions seems to be most favourable followed by checking in the field. In many countries, also in the German Democratic Republic, field investigations for quarantine reasons have not been possible so far.
In our conditions the following procedure has been followed. Up to now checking for fire blight resistance took only place in the glass-house. The source material to be tested was handgrafted on 'Bitten-felder Sämling' seedling rootstocks (in future on 'M4' rootstocks) and planted each 40 of a test variant into a container. After the variety reached a shoot length of 8–15 cm the inoculation took place with a 48 h culture of a pathogenic isolate of E. amylovora.