As new late blight strains emerge and the traditional fungicide armoury becomes more limited, control programmes are having to adapt. Using elicitors to bolster plant defences could be an important part of this new approach, CPM learns why.
“By using elicitors regularly within blight programmes, we may be able to slow down or stop the selection of more aggressive strains.” LAWRENCE VERYSER
By Rob Jones
There’s little margin for error when controlling late blight (Phytophthora infestans), especially as strains evolve, become more aggressive and cycle faster. Early action to keep crops clean from the start is widely recognised as crucial, and so, elicitors could play an important role in slowing disease progression even of new, more aggressive genotypes.
Elicitors work by mimicking an attack on the plant to ‘prime’ its natural defence mechanisms, thereby boosting protection against fungal threats including late blight. Individual products achieve this in various ways (see box), and when used preventatively early in the programme, can make a genuine difference to late blight protection, suggests SRUC senior plant pathologist, Neil Havis.
Importantly, this activation of defence mechanisms applies to foliage present at the time of application and new leaves emerging afterwards, although the longevity of protection varies depending on disease pressure, he adds. “You can reapply most products to trigger the defence mechanisms again, but generally, the greatest response is from the initial treatment.”
Research in other crops suggests there could also be a trans-generational benefit from some elicitors, where enhanced disease defence is passed to progeny in seed, highlights Neil. “We’ve seen it in oilseed rape, for example, where we induced a defence response to clubroot and found the seed sown from that crop was more resistant. It’s not something we’ve looked at in potatoes yet though.”
While elicitors aren’t a replacement for conventional fungicides, they could form a useful part of integrated control strategies, especially early in the season, he says. “They should be used before the pathogen arrives to induce a response and ‘prime’ the plant ready to repel any attack.”
Doing so may, therefore, could reduce the pressure on early chemistry. However, in many cases, a partner fungicide will be included with the elicitor to control any disease present, and applications will be followed by a robust spray programme, explains Neil.
With fewer conventional fungicides available, and a desire to alternate products and modes of action to mitigate resistance risks and tackle new strains, having elicitors as an extra option could help to prolong the longevity of chemistry, adds Lawrence Veryser, Certis Belchim laboratory trialist.
Lawrence was one of the team that discovered the elicitor properties of valifenalate (available in co-formulation with cyazofamid as Areli), that help to boost blight protection, and mitigate the risks of selecting for resistant populations.
Furthermore, research by Certis Belchim supports the argument that elicitors can help control new, more aggressive blight strains. In trials comparing the effectiveness of Areli against a moderately aggressive EU36 and a very aggressive EU43, there was a notable advantage against the more aggressive strain, notes Lawrence.
“Although valifenalate is a CAA inhibitor fungicide, to which there is known resistance in EU43, we’ve seen that it’s still bringing added value compared with using cyazofamid on its own,” he adds.
The reason for this is that aggressive blight pathogens have evolved to target ‘easy wins’, where they can get into the plant quickly and multiply rapidly, explains Lawrence.
“By using an elicitor that makes it harder for the pathogen to get into the leaf in the first place, it’s more likely to be killed on the surface by other environmental conditions such as UV radiation or dryness.
“By using elicitors regularly within blight programmes, we may even be able to slow down or stop the selection of these more aggressive strains, which would be sustainable for the whole sector in the long-term.”
With an ever-changing blight population featuring resistance to some key actives, Neil believes any tools, such as elicitors, that can support and protect existing chemistry to prolong its future use, are a critical ‘building block’ of integrated management strategies.
“Because elicitors aren’t specifically targeting one chemical process like many fungicides, it’s unlikely the pathogen will completely overcome the defence mechanisms induced in the host plant.”
Although valifenalate is a CAA inhibitor, Neil says any additional benefits from its elicitor properties should be taken into consideration. “Think of the bigger picture when it comes to managing blight. IPM requires us to use as many different products and approaches as we can, so why not use elicitors as part of that?
“Chemistry is still at the top of the IPM pyramid, but we have to support it with everything else available, whether that’s elicitors, blight monitoring, or cultural controls. If we have something that can elicit a defence response, that can only be a good thing.”
The co-formulation of cyazofamid with valifenalate endorses FRAC advice to use actives in combination, and only in curative situations may any additional mix partner be required.
According to Neil, the way elicitors prime plants to resist fungal attack means their benefits aren’t exclusive to late blight. “Strengthening cell walls or producing anti-microbial compounds will help against any fungal pathogen that lands on the plant.”
Indeed, Lawrence says research suggests Areli could potentially offer some benefit against alternaria, for example. The reason for this is that valifenalate induces the expression of a particular protein (a chitinase), which binds to chitin in the cell walls of alternaria, inhibiting spore germination and hyphal growth.
“Chitin isn’t present in phytophthora, but it is in alternaria, so even though Areli isn’t registered for controlling alternaria, it does have potential for some side efficacy against these potential threats.”
Some may worry that stimulating plants to divert more energy into disease defence rather than growth and development could result in a yield penalty, however, Neil points out that research suggests this isn’t the case.
“What we did find in cereals, is a varietal effect, with some varieties responding to elicitors better than others, however we don’t yet know whether there are similar varietal differences in potatoes,” he concludes.
This article was taken from the latest issue of CPM. Read the article in full here.
For more articles like this, subscribe here.
Sign up for Crop Production Magazine’s FREE e-newsletter here.
