Ramularia collo‐cygni – An Enemy in Waiting

Abstract

Ramularia collo‐cygni , the agent responsible for Ramularia leaf spot disease of barley, possesses many mechanisms for avoiding detection by the host, which enables it to survive as an endophyte for much of the plant's life cycle before eventually turning pathogenic. The fungus has also eluded farmers and scientists for decades as it is a late season disease that often displays symptoms after the last possible fungicide intervention when crop scouting for disease has been completed. Visual symptoms are also hard to distinguish from other pathogens, which has resulted in misdiagnoses. The ability of the fungus to spread not only via infected seed but also on infected straw and wind dispersal, coupled with its ability to rapidly evolve resistance to fungicides, has resulted in an escalation in the perceived threat it presents to barley production worldwide. A greater understanding of the plant–fungus interaction is required if future control strategies are to be successful. Key Concepts Plant–fungi interactions can be complex and highly variable, including mutualism, parasitism and commensalism. Global movement of seed can significantly influence the spread of seed‐borne diseases. Plant breeding for disease control requires identification of a suitable breeding target, which may be challenging. Breeding for resistance to one pathogen may lead to control issues for another pathogen. Integrated pest management practices must consider nonchemical control measures and, when using pesticides, antiresistance strategies. Single‐site fungicides are vulnerable to losses in efficacy due to the evolution of resistance in the pathogen population.

Detection of Ramularia collo‐cygni from barley in Australia using triplex quantitative and droplet digital PCR

“…Assessment of Australian R. collo-cygni population structure in a global context is required to gain an understanding of the pathways for pathogen introduction. 1,40 The emergence of R. collocygni in Australia has revealed a range of research questions which must be addressed, with the impact of RLS on barley crops in the Australian environment and the potential consequences of climate change at the forefront. These investigations will rely on accurate diagnosis of R. collo-cygni in the environment, with the assays described here potentially allowing management options to be implemented, such as those in the UK, where seeds that contain less than 1 pg μL −1 of R. collo-cygni DNA are recommended for sowing.…”

“…17,20,21 Alternative hosts, such as wheat and perennial grasses, 10 and colonized plant debris 6 may also play a role in harbouring inoculum. Dussart et al 1 provide a thorough review of the current understanding of R. collo-cygni epidemiology. RLS can be managed using R. collo-cygni free seeds, fungicides, resistant varieties, crop rotation and stubble reduction.…”

“…Ramularia collo‐cygni infection of barley ( Hordeum vulgare L.) has gained increasing attention over the past 30 years 1,2 . The resulting disease, Ramularia leaf spot (RLS), has been reported in many barley growing regions of the world including Chile, Colombia, Europe, Mexico, New Zealand, South Africa and the United States 1,3–6 .…”

Fungicide Resistance Evolving in Ramularia collo-cygni Population in Estonia

“…The first report came from Austria and a year later from UK and Germany [ 35 ]. As a result, DMIs are regarded as being ineffective against R. collo-cygni in these countries [ 48 ]. Intensive monitoring of barley fields by Syngenta demonstrated that DMI field efficacy against RLS decreased from nearly 100% in 2014 to around 30% in 2019 [ 49 ].…”

Biosynthesis of Rubellins in Ramularia collo-cygni—Genetic Basis and Pathway Proposition

“…Furthermore, the disease can cause quality losses as grain size, a trait used in the malting industry to assess grain quality, can be reduced by up to 4% [ 6 ]. Controlling RLS outbreaks has become increasingly difficult because of the withdrawal in many countries of multisite chemicals for environmental reasons [ 7 ]. In addition, the fungus has now developed resistance to all major classes of single-site fungicides such as quinone outside inhibitors (QoI) [ 8 ], succinate dehydrogenase inhibitors (SDHI), and sterol demethylation inhibitors (DMI) [ 9 , 10 , 11 , 12 ].…”