Look out for Pink Rot

This article was first published in Potato Review, September 2008, p30 (http://www.potatoreview.com/)

HOW serious is pink rot in the UK? Whenever I ask growers and agronomists this question, the impression I get is that it does not occur very frequently. However, for the individual grower who is affected the consequences can be serious and even if an outbreak has been contained it is valuable to know how to keep it that way. In the United States, pink rot and watery wound rot are considered to be of greater significance than tuber breakdown caused by blight. This appears to be associated with warmer tuber temperatures when crops are being harvested and a tendency towards shorter rotations. Pink rot is caused by Phytophthora erythroseptica, an organism that shows many similarities to P. infestans, which is responsible for late blight.

We have heard a lot about blight in recent years as a result of changes in the genetic make-up of P. infestans and the potential formation of oospores which are able to survive for long periods in soil. P. erythroseptica also has the ability to form oospores but in contrast to the blight pathogen this does not need two mating types to be present. Each strain has the ability to develop both male and female sexual organs and to produce copious amounts of oospores from infected tubers left after harvest and from infected stems and underground parts of the plant. We are not yet sure how important oospores are to the way in which blight outbreaks develop, at least in the UK, but pink rot oospores accumulate in soils which then become infective, especially in damp patches. The reason that these damp patches play a role is because they enable the oospores to germinate and form a sporangium from which zoospores are released which in turn releases zoospores. It is the zoospores that will infect the next crop of potatoes but in order to find their target they need water in which to swim. In this way tubers become infected before it is time to harvest the crop and these become a source of further spread as a result of the harvesting activity.

Pink rot is quite wet and the rotten tubers will contaminate equipment. Healthy tubers may be damaged and the wounds become points of entry for the contamination on the harvester. An understanding of the key points in the life-cycle gives us several clues about how to manage this disease and reduce its impact. Infection of the following generation of tubers depends on zoospores reaching their targets and the chances of this happening can be reduced by avoiding excessive soil moisture. In the United States potato growers have approval to use metalaxyl to prevent tuber infection. The method of fungicide application has to be adapted for this purpose and currently there is no similar approval in the United Kingdom. After many years of successful metalaxyl use there are now resistant strains of P. erythroseptica in the US though because the disease is soil borne and moves relatively slowly from field to field it is likely that the spread of these strains can be monitored and to some extent contained.

The second stage of the disease is its spread from tuber to tuber. This can be reduced or prevented by ensuring a good skin set, by minimising damage on the harvester and by ensuring that wounds have a chance to heal at moderate temperatures of around 8-10°C. Experience in the US suggests that tubers are at greatest risk when the tissue is above 18.5°C. That may seem out of normal range in many instances but it is worth bearing in mind that choosing an earlier harvest date may mean moving closer to this temperature. Lifting at temperatures higher than 18.5°C should be avoided and once they have been harvested, their temperatures should be brought down to 10°C as quickly as possible, observing the usual safeguards to prevent condensation. Longer rotations will help to reduce the build-up of soil contamination and British experience with other pests and diseases, PCN and Rhizoctonia, for example, has already convinced many growers of the wisdom of growing fewer potato crops on the same land. Temperatures may rise as a result of climate change but higher temperatures at harvest may also come about as the result of earlier lifting dates which are aimed at reducing the impact of black dot and black scurf.

• Avoid excessive moisture, especially towards the end of the season.
• Ensure good skin set prior to harvest.
• Minimise damage during harvest.
• Tuber temperatures must be below 18.5°C for harvesting.
• Prevent infected tubers from going into store.
• Ensure adequate crop curing to promote wound healing.
• Reduce tuber temperatures to below 10°C as soon as is practicable.
• Prevent condensation in store.
• Prevent movement of soil from contaminated areas to non-infected fields.

(Photograph by Blackthorne Arable)