Potato Blight

A few facts about this pathogen that potentially totally wrecks potato and tomato crops.

Phytophthera infestans is an obligate fungal parasite of potatoes and tomatoes. If it ever damages other plants it is never very serious. Early in its career it drove millions of Irish people to the New World, when successive potato crops were ruined.

From time to time a new variety is brought onto the market that offers improved resistance to blight, but for the most part it is only a while before the disease adapts to overcome the resistance. This is why the advice on blight-resistance in books, and even Alan Romans’ guide to varieties, is unreliable … it is out-of-date. Let’s look at why.

The fungus is a relatively uncomplicated species that grows vast numbers of asexual spores that are carried on the wind. If you are downwind of the spores and the conditions are right, you will get blight! Warm and wet are the ideal conditions. Wetness on leaves is essential to germinate the spores after which it is only a matter of days before your crop is spreading the disease to more plants and getting ready to infect other gardens.

For over a hundred years the fungus was only able to mutate slowly and plant breeding kept a step ahead. Only one strain had arrived in Europe and it had not undergone sexual reproduction.  Let us call it the + strain. Then a – strain reached us and the fungus underwent sexual reproduction. The resulting oospores are new strains. The ones that infect potatoes best are the ones that survive and carry on asexually.

Nearly all the traditional blight strains are now extinct. Or effectively so.

The survivors are A2 Blue 13, which has over 80% of the spread, the other is A1 Pink 6 taking up most of the other percentage. It no longer needs to be that warm for these strains to thrive. So all the traditional ways of potatoes minimising the susceptibility are compromised, everything we used to know is open to question. To give an example, Verity was a great blight resister ten years ago. It is now only in collections and poorer in performance to some traditional varieties like Cara.

Copper chlorate is still effective against blight but as you will know it needs to be applied before the blight arrives and is hardly organic even if the Soil Association allows it. You only need to look at the snails it kills to see it is potent. It is relatively expensive, and there is a limit to the amount of copper you want in your soil.

So far the Sárpo (pronounced Sharpoh) varieties still show tolerance of the new blight strains. The fungus makes small lesions on leaves but they stay localised and the plants remain standing. Tuber blight is not a problem so far. These cultivars are developed by the Savari Trust near Bangor and can be purchased from the East Anglia Potato Day in February, but are generally still not stocked in retail outlets. Sárpo Mira and Sárpo Axona have been with us for many years. This year Sárpo Shona turned up. Savari varieties without the Sárpo prefix are less resistant by the way. These varieties remain a bit of a secret and are not widely grown commercially. They have obvious appeal for organic growers, but many have complained that the taste is too strong. I am not one of these critics … I only criticise potatoes for not having enough flavour.

Gardeners tend to be manic about controlling an infestation of blight.

Here is some advice.

If the above-ground parts of the plants are removed as soon as possible, no blight will be translocated to the tubers. These tops can be composted. The spores are killed by composting and the likelihood of making a mess trying to burn them as an alternative is high. Next year’s infection will come on the wind, it is not in your soil. The overwintering is done in infected tubers, so try to compost all of them too. When blighted potato tops are dry they are also no threat with spores. If you do not believe it, try putting a dried leaf into a cut tuber and seeing if it infects it. Do remove all the tops for composting however as spores washed into the soil could remain moist and infect your tubers. Most varieties have tubers less prone to blight than their leaves are however.

A starting point for more information: www.sarvari-trust.org

Julian Turner (Organiser of East Anglia Potato Day)

This article was originally published in Norfolk Organic Group‘s Newsletter and is reproduced here, with permission.