Microbes really are the most remarkable little organisms. The coronavirus pandemic is a sharp reminder of their power – a miniscule nucleic acid molecule snuggled in a protein coat is wreaking havoc on the planet, bringing fear to humans, causing economies to stall and leading to an illogical urge to hoard toilet rolls.
At a more local level, plants deal with this sort of thing all the time – viruses, bacteria and fungi are constantly in search of hosts which allow them to replicate and create an epidemic. We pit our intellect against these little menaces and create chemicals to kill them and genetics to resist them. But microbes have no respect for IQ, they just find a way around the problem in order to achieve the only thing that matters – to multiply.
As crop production has intensified, it’s become a faster and faster race to keep up with the evolution of plant pathogens. Our two main wheat diseases have become experts at keeping a step ahead. Septoria is the master at disabling fungicides and yellow rust is coming up with ever more ingenious ways of eluding the combinations of resistance genes painstakingly bred into varieties.
The UK Cereal Pathogen Virulence Survey is always a fascinating insight into the latest developments in the yellow rust population. Equally it’s a challenge to dip into a world where the language of genetics is spoken and then interpret what this actually means for crops in the ground. For yellow rust, there’s the peculiar phenomena of adult plant resistance and seedling resistance where varieties with good yellow rust resistance scores on the AHDB Recommended List may still succumb to yellow rust early in the season.
Other varieties have resistance that is season long (ie at the seedling stage and adult plant). The only varieties which currently have this season long resistance are Costello, KWS Crispin, KWS Parkin, KWS Siskin, Theodore, RGT Saki and KWS Firefly (all rated 9 on the RL for yellow rust). Yellow rust on any of these varieties may mean the pathogen has another trick up its sleeve and it’d be a good idea to send samples off to UKCPVS for testing.
An interesting fact that did come out of the meeting was that the mechanism for adult plant resistance isn’t really understood and it may kick in much later in the season than most of us would think – anytime up to GS39. It’s not predictable and it was suggested that this may be because the resistance could be influenced by the environment or it may require enough plant tissue in order to be effective. That means the onset of adult plant resistance will likely differ between varieties and across seasons. All that’s known is that it will definitely be active once the flag leaf emerges, of course by then serious damage could have already been done.
In practice that really means that whatever variety is in the ground then yellow rust needs to be treated because you can’t be sure when adult plant resistance will kick in and definitely can’t know whether a new pathotype has emerged that has virulence for the resistance genes found in that variety.
Yellow rust has no respect for the IQ of the humans trying to defeat it.
In brown rust, it was KWS Firefly that appeared to suffer most last season, dropping two resistance points on the current RL because of this. The UKCPVS revealed two new pathotypes were identified for carrying virulence to Lr28 and it was virulence to this resistance gene that had the biggest increase in frequency last year in Firefly samples received for testing.
Stem rust was also discussed – a disease which is no longer problematic in the UK but in the past has caused devastating yield losses around the world and sporadic outbreaks are currently being experienced in parts of Europe. Stem rust needs the barberry plant to complete the sexual phase of its lifecycle and in recent years UK hedgerows containing barberry have been monitored for the disease. Last year it was discovered that plants in the barley crop next to one of these hedgerows had become infected with stem rust in late July-early Aug – too late to cause any harm.
The concern is that stem rust inoculum is present in the UK and climate change could bring infection forward in the season. The mood in the room had changed from previous years, when it was more or less dismissed as a disease that would be easily controlled by current fungicide programmes. The impending loss of a number of triazoles may just mean stem rust will become another disease to reckon with.
Based in Ludlow, Shrops, CPM technical editor Lucy de la Pasture has worked as an agronomist. @Lucy_delaP
