ARTIFICIAL INFECTION OF APRICOT TREES GRAFTED ON VARIOUS ROOT-STOCKS WITH VERTICILLIUM DAHLIAE KLEB
The spontaneous infections mentioned above called attention to the specific susceptibility of the various types of apricot root-stocks to different pathogens, e.g. Verticillium. As Verticillium sp. is most likely to cause — directly or indirectly — apoplexy of apricot trees, systematic infection experiments were carried out in 1964—1966 on young apricot trees.
The investigations included different numbers (6—48 trees per treatment) of »Kécskei rózsa« scion variety grafted on selected root-stocks of 14 wild apricots, 11 myrobalans and 11 plum varieties. The inoculum was obtained partly from decaying apricot trees, partly from Cotinus coggygria and the Verticillium dahlae culture was directly transferred to fresh injuries made on the root neck of the trees. Half of the root-stock-scion combinations remained uninfected as controls.
The first two inoculations took place in the spring of 1964, but no external symptoms occured in that year. In the autumn partly without root, since the desease did not extend downward, 211 specimens of the infected plot and 171 of the control plot were selected at random. In 181 cases typical symptoms of Verticilium infection: browning of vessels and »microgummosis« were found in the xylem and the fungus could be isolated. The so called »distance effect« did not show up here. Culture was more successful in parts at some distance from the place of injection. Samples taken from the control plot were negative with one exception. A natural infection is likely to have occurred here. In 1965: one tree showed leaf symptoms only, in 138 cases tissue symptoms could be observed only, similarly to the previous year. 31 trees remained symptomless. 240 healthy and 1 infected trees were taken from the control plot, but later did not show leaf symptoms. Roots and stumps respectively left behind in 1964 started up. These too were taken out and examined but the fungus did not seem to enter the shoots.
Inoculations were repeated in 1966. Controls were not left now, because susceptibility of root-stocks was primarily examined.
Made of inoculation differed from that of the previous year in making injuries not only at the root neck, but also on the primary roots. A culture obtained freshly from verticilliosic apricot trees provided for the fungus. Dates of infection were: May 18, June 8, 9, July 7 and 26.