The addition of an internal control (IC) at the appropriate concentration enables recognize of false negatives in the detection of Agrobacterium tumefaciens and improves the reliability of PCR for crown call diagnosis. Co-amplification of the IC and target sequence from A. tumefaciens ensures the attainment of at least one amplification product in every PCR reaction if the DNA extracted is of high enough quality to be amplified. (C) 2002 Elsevier Science B.V. All rights reserved.