Caught out by blight

This article was first published in Potato Review, May 2009, p24 (http://www.potatoreview.com/ )

In the spring of 2007 blight took many people by surprise and new lessons had to be learnt. On June 14, I was asked to look at several outbreaks, around the Cambridgeshire – Suffolk border, which appeared to have common features. Four outbreaks occurred in the same district in crops grown from a single seed stock. Three of these were a mere 5 weeks from emergence and of particular interest because at that stage the outbreaks could be easily examined. The infections could be readily spotted by the collapsed state of the foliage. Each patch consisted of about 4 to 5 plants in a row. The infection appeared to have initially taken hold around the first or second leaf axil above ground and spread along the stems, thereby, as it were, snapping the stem. Because the canopy had barely closed, spread was along the row but not across rows. I estimated that the infection was at least 2 to 3 weeks old. From the website http://www.blightwatch.co.uk/ it was possible to estimate that the most likely time of first infection had been about a fortnight after emergence, when a Smith period had been registered and that during the week preceding the identification of outbreak there had been a second prolonged period of high risk, enabling the infection to build up to such damaging proportions.

The question in my mind and that of the agronomist who was looking after the crop was whether this could be a seed borne outbreak? If it had been carried on the wind from outside the crop you might have expected some patches to predominate in one part of a field and not another, especially if the source was nearby. The pattern in which blight had developed in several crops from seed of the same origin was very similar. The patches of infected plants appeared at irregular intervals across each of the fields but with no particular bias to one or other part of each field. If the infection had originated from a seed tuber you might have expected to find a heavily infected single stem in each patch which could be traced to an infected seed tuber but there were none. However, it is not always appreciated that seed borne infection can also be transmitted directly through the soil to stems and leaves above ground. The difficulty is that seed tubers may carry latent (without symptoms) infection and the same goes for underground developing sprouts and stems. These may be activated underground to produce infective spores. On balance, but not with absolute certainty, we concluded that this was most likely seed borne transmission.

What about oospores, the thick walled resting spores we have been hearing so much about? These have a longer lifespan but the evidence to date suggests that these become a problem with short rotations of less than 4 years. This is the experience in the starch potato region of the Netherlands. In this country, progressive growers of premium quality potatoes look for longer rotations, often of 6 to 7 years and this was also the case here. This helps with a number of disease problems but also makes infection from blight oospores unlikely.
What I found instructive about this case was the possibility of looking at the diary of risk periods and fungicide sprays. The risk periods are defined by the blightwatch.co.uk website as Smith periods. The diary page shows not only the half (24h) and full (48h) Smith periods but also the humidity (hours above 90%) and minimum temperature data for the other days on the calendar. This is important because whilst it is clear from the diary that many days appear as low risk, it is not the case that blight stops abruptly when the minimum temperature drops a few degrees below the official threshold of 10ºC. So from the data presented on the calendar it is possible to consider days when there may have been some risk. In our case there had been a number of days which were not classed as Smith periods but during which there may have been some expansion of blight from the already established infections. The spray programme had been robust but unable to cope with the early onset of the outbreak. Subsequently we learned that each of the outbreaks we looked at was caused by the aggressive "blue thriteen" strain. We now know that more aggressive strains of blight have spread in the UK making extra vigilance a necessity.

In detailed surveys in the Netherlands over a 6 year period, 36% of outbreaks could be attributed to infected seed. It would be wrong to jump to conclusions that the figure would be similar here. Indeed the Dutch figures show quite a bit of variation with most seed borne outbreaks in the South West and least in the East of the country. It would be useful to know what the figure would be for different parts of the UK. It would also be useful to know the impact of variety resistance and fungicide choice by the seed grower on this figure.
Up to about 1980 blight sprays were not considered necessary until the canopy closed across the rows and this was then modified to along the rows. More recently this was again modified to when plants are as big as “saucers”. Recent experience suggests that this may even be too late.