Top four “baddies” that ornamentals growers should watch out for in 2020
“Multiplex” is a word commonly associated with big screens and an accompanying bag of over-priced popcorn. But, now that the clapperboard for 2020 has sounded, British ornamental growers should be aware that “multiplex” has a far more ominous meaning. The havoc that Xylella has recently wrecked throughout several European countries has, in fact, been caused by three separate subspecies of this villainous bacterial pathogen. These include Xylella fastidiosa subsp pauca, Xylella fastidiosa subsp fastidiosa, and (that’s right you guessed it folks) Xylella fastidiosa subsp. multiplex.
According to Ed Birchall*, principle inspector for the Plant Health and Seed Inspectorate (part of the Animal and Plant Health Agency), this cinematic-sounding subspecies has the potential to cause the most damage in the UK. He notes: “It [multiplex] has the widest range of [host species of] all the Xylella fastidiosa subspecies and the highest risk rating. It’s important to keep it out of the UK because it’s the one that’s of the most concern to us in that it’s more cold tolerant.”
Subsp. multiplex can affect a wide range of perennial plants and broadleaved trees such as oak (Quercus robur). But, so far, British growers have been able to watch the Xylella horror movie from the safety of their cosy, reclining seats because no form of Xylella has reached the UK (yet). The stealthy subsp. multiplex, for example, has been found in Corsica and on mainland France – and is currently under eradication in North America.
Ed reminds growers that Xylella, which colonises plants’ xylem vessels like a mind-flaying monster, is spread by xylem-feeding insects like the meadow spittlebug (Philaenus spumarius) – a species commonly-found in the UK and the rest of Europe. “They feed on the xylem vessels and that introduces the bacteria into it. It’s like having a really bad cold. And the bacterium multiply within the xylem vessels – and when these become blocked the disease symptoms are produced. These include wilts, diebacks, stunts, and leaf scorches.”
To avoid their nurseries resembling a post-apocalyptic zombie-land, growers should be vigilant and watch out for any possible signs of this disease. After all, a full-blown outbreak requires plants within 100m of the hosts to be destroyed and the introduction of a 5km-wide zone banning all host plant movements for five years.
Ed says: “If we can take early action and demonstrate that the vectors and the disease haven’t moved to another nursery or the wider environment, the measures we have to take are much more mitigated and not as draconian as if you have an outbreak. So, what I’m trying to get at is – deal with it early on. Don’t let it fester.”
And other brutes
Ed warns that there are also a few other sinister characters to look out for, including rose rosette virus (RRV) – which is spread by a microscopic mite and can leave roses in a cabbage-like state. He notes that, whilst the virus is not present in the UK or [the rest of] Europe, it has caused significant damage in the USA and Canada. With this in mind, all rose plants and cut flowers imported from Canada, India, Mexico or the USA must be accompanied by an official phytosanitary certificate confirming that they have been grown in an area free from RRV.
Another “baddy” to be aware of includes Liriomyza leafminers (L huidobrensis, L sativae and L trifoli) – which reproduce faster than gremlins fed after midnight. Ed says: “Their reproduction rate is phenomenal – you get a new generation every four to five weeks. This is a particularly troublesome pest – back in the 1990s we used to see it quite a lot. And then along came imidacloprid and thiacloprid. So, we might see a resurgence of this pest because husbandry techniques may have relied too much on those pesticides.”
Historically, tobacco whitefly (Bemisia tabaci) was also controlled by imidacloprid. “Now we might see a resurgence again. There is a very wide range of host species that Bemesisa will go to, but its main ability is the transmission of viruses – about 110 of them – and so if you can control your vector then by default you are going to control the viruses as well.”
So, there we have it – the four main characters that, providing we are vigilant, will not turn the UK into a cinematic horror flick.
*Ed was speaking at this year’s Hort Science Live, held during October 2019 at New Leaf Plants, Worcestershire.