Cylindrocarpon Species in Pacific Northwest Douglas-fir Nurseries: Diversity and Effects of Temperature and Fungicides on Mycelial Growth

Douglas-fir nurseries play an important economic role in the Pacific Northwest timber industry, but the early death of seedlings due to disease influences regeneration success. A destructive root disease caused by fungus Cylindrocarpon results in the loss of seedlings in early stages of their growth. The objectives of this study were to: (1) identify and determine the diversity of species of Cylindrocarpon occurring in three different nurseries in the Pacific Northwest, (2) investigate the effect of temperature on the growth rate of the mycelia of these pathogens in vitro, and (3) determine the influence of four common fungicides on the growth of these pathogens in vitro. The identity of the species of Cylindrocarpon in three different nurseries (two in western Oregon and one in western Washington) was determined using sequences of the ITS region of rDNA. Three species of Cylindrocarpon were found; C. destructans, C. liriodendri and C. pauciseptatum. Cylindrocarpon destructans was the dominant species in all three nurseries, making up 61.4% of the isolates, while C. liriodendri was only found in one nursery sample. Cylindrocarpon liriodendri grew significantly faster in culture at warmer temperatures than C. destructans. The optimum growth temperature for C. liriodendri was 25° C, while C. destructans grew fastest between 18° and 22° C. There was little growth of C. destructans at 30° C. All four fungicides (Cleary 3336F, Dithane 75DF, Heritage and Iprodione E-Pro) tested at different concentrations) (10, 25, 50 and 75% of label active ingredient—i.e., 37, 94, 187, and 281 ppm) reduced the growth of both species of Cylindrocarpon mycelia in culture. However, C. destructans generally had greater growth reduction than C. liriodendri. Cleary and Dithane reduced growth more than Heritage and Iprodione. Dithane at 75% active ingredient concentration had the greatest effect in reducing the growth of both C. destructans and C. liriodendri. Dithane at 75% concentration was significantly more effective than Cleary in reducing the growth of C. destructans, but there was no significant difference between Cleary and Dithane in reducing the growth of C. liriodendri. The greater inhibition of C. destructans than C. liriodendri by the fungicides could have been related to a temperature effect since the study was conducted at 25° C. Cylindrocarpon liriodendri grew significantly faster at 25° C than C. destructans. Results of this study are useful in identifying and managing Cylindrocarpon spp. in Douglas-fir nurseries in the Pacific Northwest.