Transcription factor that regulates the expression of the gene clusters that mediate the biosynthesis of the host-selective toxins (HSTs) ACT-toxins responsible for brown spot of tangerine disease by the tangerine pathotype which affects tangerines and mandarins (Probable). ACT-toxins consist of three moieties, 9,10-epoxy-8-hydroxy-9-methyl-decatrienoic acid (EDA), valine and a polyketide (PubMed:22846083). ACT-toxin I is toxic to both citrus and pear; toxin II the 5''-deoxy derivative of ACT-toxin I, is highly toxic to pear and slightly toxic to citrus (PubMed:22846083). On cellular level, ACT-toxins affect plasma membrane of susceptible cells and cause a sudden increase in loss of K(+) after a few minutes of toxin treatment (PubMed:22846083). Gene clusters encoding host-selective toxins (HSTs) are localized on conditionally dispensable chromosomes (CDCs), also called supernumerary chromosomes, where they are present in multiple copies (PubMed:22846083). The CDCs are not essential for saprophytic growth but controls host-selective pathogenicity (PubMed:22846083).