Comparison of disease severity and incidence at different growth stages of naturally infected canola plants under field conditions by pycnidiospores of Phoma lingam as a main source of inoculum

Abstract

Phoma stem canker, caused by Leptosphaeria maculans, is the most destructive disease of canola (Brassica napus) in the world. A study was conducted to compare mean disease severity and incidence at three different growth stages on naturally infected canola plants under field conditions. To determine the main source of inoculum over two consecutive years in Manitoba, three different approaches were employed: inspection of infested stubble pieces for the presence of pseudothecia and/or pycnidia, detection of ascospores using Burkard and rotorod spore samplers, and lastly, utilizing trap plants to study subsequent disease development. The results showed that the mean disease severity was significantly higher (P < 0.001) on plants infected at 1, 0 (cotyledon) than at 1, 3 (3-leaf) but no significant difference in disease incidence was observed between these two stages. However, these plants showed significantly (P < 0.01) higher disease severity and incidence than plants infected at 1, 6 (6-leaf) stage. A high density of pycnidia and low density of pseudothecia on stubble pieces, absence of ascospores in most weeks and high levels of disease incidence and disease severity on infected plants suggested that pycnidiospores can be considered as a main source of inoculum during these two years in Western Canada. These findings can be used to better understand the relationship between pycnidiospore inoculum and subsequent disease development on canola under field conditions.