Heat and dryness may be at the heart of the current secondary growth problems in many potato crops and perhaps they are a result of a combination of both. Speaking with people in the sector, many believe that heat has a part to play either in breaking dormancy through stop-start growth patterns or through accumulated degree days in the tubers. No one seems to know for sure because these issues cannot be easily predicted or researched because of their very infrequent occurrence.

What we know

Some level of sprouting can now be found in most potato crops up and down the country, but there is a suggestion that the problem may be more severe towards the south. Crops in the northwest which have enjoyed a more favourable growing season appear to be less affected but there are still some reports of sprouting. It would also appear that the problem is less evident in irrigated crops but these are not totally free of secondary growth issues either.

Secondary growth ranges from the appearance of sprouts on the daughter tubers, the development of new or granddaughter tubers on these sprouts (also known as chain tuberisation), irregular or un-uniform regrowth in the daughter tubers and a level of secondary foliage growth.

We also know that these problems are also evident in Britain but reports suggest that they are much more prevalent in England than in Scotland. This is important because we depend on Scotland for a lot of our seed imports but some seed does originate from England too. In 2017, for example, Ireland imported over 4,900t of seed from the UK, with over 3,500t of this coming from Scotland and 700t each from Northern Ireland and England.

The level to which secondary growth problems are evident seems to have been influenced to some degree by variety, a bit by location and a lot by the presence or absence of irrigation. It would appear that Rooster is affected in most areas, but being the most widely grown variety it is the most common one to find. Kerr’s Pink appear to be particularly prone to sprouting as some of the most severe problems were found in this variety. Sprouting was even evident in crops which had been irrigated in areas where there had been significant pressure from heat.

We also know that most stolons are longer than normal this year and this is attributed to the warmer conditions. We have also seen that the production of stolons and the formation of tubers appear to be ongoing in many varieties. In most instances, the mother tubers were still solid in the crops we visited and some were even still producing new sprouts, which would be very untypical for the time of year.

It would appear that tuber numbers are particularly high in some crops, adding to the challenge to get them filled beyond the 45mm size needed to make ware grade. But some crops of Rooster have low tuber numbers.

What we do not know

The solution needed for a drought-related problem is rainfall. But the question is will a return to rapid growth merely exacerbate all forms of secondary growth or will we see a drive to bulking in the granddaughters? It is generally accepted that sprouting itself runs the risk of the destruction of the starch structure in the tuber close to where the growth is happening. This causes a phenomenon known as glassy texture which can affect the eating quality of the tuber.

Given that so many daughter tubers have already broken dormancy to produce sprouts, it is less than clear as to whether these tubers can revert to bulking at the return of rapid growth. Could these tubers effectively freeze and give hormonal preference to bulking up the new granddaughters? Some believe that if the crop was to suffer a severe (induced) stress now the tubers might abort their sprouts and return to bulking up. But this is not known for certain and neither is it certain as to how any such stress could be initiated.

As of last week, most crops still have a lot of growing and bulking up to do to produce even a modest ware yield. This means that all crops will need careful and close monitoring to assess what is happening in each one. It may be that the option for some crops could be to burn off now or in the coming week(s). For others there may be no need to do anything, either because the problem goes away or it can be largely stopped by some form of imposed stress.

The options here may be different for a ware versus a seed crop. There is additional concern around seed crops in that it may be very difficult to hold or stop sprouting once dormancy has broken, which it has. There is also concern that tubers destined for seed which have produced glassy tissue could rot, even post planting. It is not that anyone knows that they will or even might – the concern is that we do not know either way.