The data base on transmission of western X-disease agent (WXDA) by Colladonus montanus leafhoppers was expanded in studies using celery, Apium graveo/ens as the host. Transmission was affected or limited by the availability threshold, instar efficiency, and acquisition and inoculation rates, along with vector biotype and pathogen strain. Transmission also was moderated by circadian rhythms. Serial passage by injection of the pathogen into a, sequence of healthy insects was accomplished, and some evidence was obtained that [etracydine pressure during serial passage could result in selection of a tetracydineresistant strain of the pathogen. Leafhoppers did not become infectious by membrane-feeding during trials made to concentrate or purify JheWXDA. Comparative tests using C. geminatus, C. montanus, and Eusce/idius oariegatus indicated C. montanus was the most, and E. oariegatus the least, efficient experimental vector. Simultaneous feeding of the three species on a common disease source reduced vector longevity. Transmission efficiency was temperature sensitive. At 30 C, transmission by any species was rare. Comparisons at 20 and 25 C resulted in similar acquisition rates. The median latent period and longevity of both Colladonus species decreased at higher temperatures. Eusce/idius uariegatus however, apparently was not affected by the pathogen and survived longer at 25 C than at 20 C. When inoculated by injection, C. geminatus was a more efficient vector than E. uariegatus under insectary conditions, but the reverse was true under conditions of constant light and temperature. A fourth leafhopper species, Fieberiella florii was similar to C. montanus in acquisition efficiency and transmission of the WXDA, but it had a longer developmental time; thus, transmission could occur in the nymphal stage. The median latent period of the pathogen in F. florii, following inoculation by injection, was half of that when infectivity was established by feeding.
