ABSTRACT:  Disease is a causal factor in declines of desert tortoise, Gopherus agassizii, populations at two locations in California.  In the interior of the Desert Tortoise Research Natural Area (DTNA), population densities of all sizes of tortoises declined 76% from 75/km² in 1979 to 18/km² in 1992.  Densities of adults followed the same pattern and declined 90% from 61/km² in 1979 to 6/km² in 1992.  Declines of adult tortoises are attributed primarily to an upper respiratory tract disease (URTD) caused by the pathogen Mycoplasma agassizii.  Additional disease-related mortalities are expected to occur, because 25–38% of four samples of adult tortoises from an adjacent site within the DTNA tested positive for antibodies to M. agassizii in 1992.  This disease may have been introduced to the DTNA through release of ill captive tortoises.
        A second disease, cutaneous dyskeratosis, is present at another reserve, the Chuckwalla Bench Area of Critical Environmental Concern, and is linked to population declines.  Between 1982 and 1988 the incidence of cutaneous dyskeratosis increased and lesions became more severe.  Between 1982 and 1992 the total tortoise population (all size classes) declined 54% from 153 tortoises/km² to 70 tortoises/km².  The adult population declined 61% from 87 tortoises/km² to 34 tortoises/km².  The cause(s) of cutaneous dyskeratosis remain uncertain, but possibilities include deficiency diseases or environmental toxicosis.
        Diseases, especially if introduced to a native population or if environmentally caused, can have serious consequences for threatened chelonians and will complicate conservation and recovery efforts.  Of the two diseases, URTD is of the greatest immediate concern because of its potential to harm tortoise populations on a global scale.