2015 is a big alternaria year
As we approach the end of the growing season, alternaria has been a topic of conversation across the country. In mid-August the AHDB potatoes Fight against Blight maps only showed about 20 red dots (which are used to show confirmed blight incidents) across the whole country and it commented in its August update ’blight had not materialised as expected’.
It would be interesting to compare an alternaria map if there was one, as I’m sure that would be full of red dots with large amounts of infection up and down the country. Within our business we have asked our team of potato agronomists to report alternaria infections, and it’s pretty clear this year is a big alternaria year, with symptoms being reported up and down the country from Cornwall to Scotland, East Anglia to Shropshire and all points in between.
In addition to this we have also asked for varietal info and it appears most varieties seem to be affected to some extent, not just the classics like Vivaldi and Markies. It also interesting to note where we have had the samples tested in the laboratory there has been a high level of alternaria solani which can be the more damaging of the two species especially if the infection occurs early in the crop’s life. This contrasts to 2013 where we also had high levels of infection but it was all alternaria alternata. Even the spore traps which monitored our trial site only caught A. alternata in that year.
In 2014 our trials run by John Kerr, Holbeach, did have some significant A.solani infections, but not until September, when the weather was still warm and we had significant humidity and periods of leaf wetness. Potato blight also went rampant at this time and many were caught unawares, as the risk had been very low and several crops had problems in store with tuber blight infections which came in very late.
We’ve seen in the trials materials containing difenconazole do a good job on controlling A.solani, but until we know more about the disease life cycle and varietal susceptibility it’s difficult to target the applications with great accuracy. With the increasing number of newer varieties – especially in the processing sector – it makes sense to regard them all as susceptible as currently we’ve no other information on which to judge.
Chemical control of alternaria has therefore been a struggle and even where the best products have been applied it’s difficult to say how well they’ve worked, as there are still plenty of leaves which are carrying significant infection with an overall yellowing of the canopy and signs of early senescence. In most instances this is on mature canopies on less robust soils, but not in all cases.
It’s not easy to pin down what the actual effect on yields will be, as many of the infections have come in when the crops are mature with good tuber yields already formed. But it’s certainly having some effect, as the loss of photosynthetic capacity will always reduce the yield to some extent. We’ve had some very hot weather this summer and it would appear this has been the trigger, particularly when it’s been combined with periods of leaf wetness. Potato blight needs considerable periods of leaf wetness for the spore to be able to germinate and infect the potato leaves, but we suspect alternaria needs a lot less. It may be the brief periods of heavy rain we had in July and – even irrigation – may play a significant role in A. solani development, much more so than potato blight.
There is quite a lot R&D effort going on in various places in the UK, not least our own work looking at alternaria, and this should give us more insight into this problem over the next few years.
In the meantime, until we know more it’s always going to be quite a challenge to control this increasingly problematic disease, and with the potential build-up of spores in the background it may get worse before it gets better.
Darryl Shailes is root crop technical manager for Hutchinsons, with a nationwide remit. He has been working in potato agronomy for more than 20 years.