Transcription factor that regulates the expression of the gene clusters that mediate the biosynthesis of AM-toxins, host-selective toxins (HSTs) causing Alternaria blotch on apple, a worldwide distributed disease (Probable). AM-toxins have two target sites for affecting susceptible apple cells; they cause invagination of the plasma membrane and electrolyte loss and chloroplast disorganization (PubMed:22846083). Expression is up-regulated more than 10 fold in toxin producing cultures. 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:17990954). The CDCs are not essential for saprophytic growth but controls host-selective pathogenicity (PubMed:17990954).