Microscopic and biochemical evidence of differentially virulent field isolates of Diplocarpon rosae causing black spot disease of roses

Black spot disease caused by Diplocarpon rosae is one of the most widespread diseases of roses that are very difficult to control due to the generative reproduction and complex genetic constitution of roses and up to now the control of black spot still requires intensive use of systemic fungicides. Here we report for the first time evidence of differentially virulent field isolates of D. rosae. Using a combination of fungal structures, disease symptoms and host cells protein expression pattern analysis we here provide direct biochemical evidence that tropical field isolates of D. rosae are more virulent and caused disease symptoms earlier than their temperate counterparts. The tropical fungal field isolates strongly induced an excessive accumulation of ROS and repressed activity of pathogenesis-related proteins such as peroxidases, chitinase and phenylalanine ammonia lyase compared to their temperate counterparts. These findings bring insights into a hidden pathogenic characteristic of tropical D. rosae field isolates compared to their temperate counterparts and open a novel dimension of parameters to be considered when controlling black spot disease of roses by fungicides in tropical versus temperate regions. Interestingly, we found that treatment of rose leaves with ROS (H2O2) prior to fungal inoculation promoted plant defense response regardless of the isolate virulence based on protein expression pattern analysis, suggesting that ROS (H2O2) can be efficiently incorporated into black spot disease management.