Reaction of four Musa genotypes at three temperatures to isolates of Mycosphaerella fijiensis from different geographical regions.
Two tetraploid banana hybrids, FHIA1 and FHIA2, with resistance to black Sigatoka (caused by M. fijiensis), and 2 highly susceptible, naturally occurring triploids, Grand Naine and False Horn, were evaluated at 3 temperatures for their resistance to isolates of M. fijiensis from 5 geographical regions. The youngest open leaf of young plants was inoculated and plants were incubated at 22, 26 and 30°C in growth chambers. Duration of the incubation period and disease severity were used to evaluate the reactions of the genotypes. The incubation period was shortest at 26°C. Disease severity was greatest at 26°C on Grand Naine and False Horn but there was no clear temperature effect for the FHIA genotypes. The incubation period was longer on both FHIA genotypes than on Grand Naine and False Horn. With few exceptions, isolates with the shortest incubation periods caused greater disease severity than those with longer incubation periods. The level of resistance between the 2 FHIA genotypes was similar and both expressed high resistance across temperatures and isolates of M. fijiensis, indicating that no physiological races of the pathogen were detected. There were differences in durations of the incubation periods and disease severities associated with the geographical origin of the isolates. Isolates that originated in Honduras, Colombia and Costa Rica produced more disease on Grand Naine and False Horn than did isolates from Cameroon and Asia. However, no differences associated with the geographical origin of the isolates were observed for both FHIA genotypes. There were also no differences in disease severities within isolates that originated from Honduras, Colombia and Costa Rica.