An improved growth medium for enhanced inoculum production and in vitro cultivability of slow growing apple leaf blotch fungus Marssonina coronaria

Apple leaf blotch disease caused by the fungus Marssonina coronaria is a serious foliar disease that threatens the successful cultivation of apples in Himachal Pradesh. In vitro culture studies are important for studying host plant growth and taxonomy. The apple orchard survey in the state of Himachal Pradesh revealed that the disease was widespread. The highest disease incidence range (85.56–88.78%) was recorded in the orchards situated at Kinnaur, followed by Shimla district (73.44–76.56%). The effects of different liquid and solid media, temperature and subculturing time were evaluated on mycelial growth and conidial production of M. coronaria. Among all the media tested, a maximum fungal dry weight of 407.05 mg/50 ml and the highest (5.43) Log10 unit of conidia/ml were recorded for the culture grown on PPDB (Potato peptone dextrose broth) at 25 ℃. Maximum colony growth of 12 mm was produced within 15 d of culturing on PPDA which confirms its suitability. We noticed that colony diameter and conidiation were further increased to 15.67 mm and 5.30 Log10 units of conidia/ml, respectively when the fungus was subcultured regularly after 15 d of interval on PPDA (Potato peptone dextrose agar) at 25 ℃. This study will help to gain insight on valuable information related to disease management, deciphering the role of virulence genes and the molecular basis of pathogen- host interaction by gearing up the routine culturing of M. coronaria.