Light intensity, photoperiod, relative humidity, and temperature during fern growth and fernage at timeof inoculation, and wetnessduration, light intensity, and temperature during infection period were examined to establish their effect on severity ofStemphyliumleaf spot on asparagus. Plants were inoculatedwith a conidialsuspension of two isolates ofStemphyliumsp. from asparagus. Disease assessments based on percentage stem area necrosis,percentagediebackof stem,and percentage infection of cladode tissue were made 10–14 days after inoculation. Asparagus fern grown under conditions of low light (particularly a reduced photoperiod), high relative humidity, and temperature became severely infected. Disease severity decreased with increasing age of fern at time of inoculation. Two days of wetness following inoculation were sufficientfor maximum diseasedevelopment; plants held at low temperature (14°C) had higher levels of infection than those held at 20 or 26°C. Light intensityduring the infection period had no effect on disease severity levels. Of the three methods of disease assessment, cladode infection provided the most reliable measure of disease severity levels. The method of assessing percentage of cladode infectionon a group of seedlingsin a pot was more reliable than the assessments made on individual seedlings.