“Without resistant potato strains you can't do anything against Phytophthora in extreme rainfall”

Every once in a while potato disease Phytophthora crops up again, and this year is no exception; the first reports have already started trickling in. Especially potato cultivators from the south of the Netherlands are worried. “Due to the inimitable weather with its extreme rainfall this month, you cannot do much against this disease. With organic cultivation you can, at most, hope to avoid an outbreak by starting the cultivation process as early as possible. Or by switching to Phytophthora resistant strains,” says Christoffel den Herder of Delphy. “But then the entire chain needs to cooperate, even more than they do now.”

By now, Twitter is being flooded with reports from cultivators who found Phytophthora in the green of their potato plants. This also did not escape Jan van Hoogen, general manager of Agrico. “Our fieldwork employees already established an outbreak of Phytophthora on 14 June. An alert was sent out right away to notify our member cultivators of this, so that they, where possible, could take measures to limit a consequential loss.”

Christoffel den Herder, who works as senior advisor organic agriculture for Delphy, says excrescences of this outbreak can mostly be felt in the south of the Netherlands. “South of Flevoland, where the flowering season just finished, the disease has already reached an advanced stage.”

“The changeable weather circumstance of recent weeks are not very helpful either,” Den Herder continues. You might expect conventional cultivation in whatever circumstances could better withstand Phytophthora than organic cultivation, because of the availability of pesticides, but that is not always the case. “Pesticides can only be effective when the weather stays dry for three weeks after using them. In the absence of stable weather the fungus had free reign in the south. You cannot do anything about it. Just as when onions get mildew, the air is full of spores.” Phytophthora clings to every plant that is susceptible to it and as the wind blows, any crop that crosses its path. 

Because you cannot place your hopes in better weather circumstance as an organic cultivator – dry weather can curb further growth of the fungus but never completely remedy it – the consequences for this product group are much more rigorous. Van Hoogen: “Once the potato plant is affected, pretty much your only option is to burn off the green. The potatoes stop growing when you do that, but it is the only way to exterminate the fungus.” 

Den Herder: “If the disease is still in an early stage and only the leaves are affected, it might not yet be the end of the story. It is a different story when the fungus enters the ground because of persistent rainfall, it then affects the tubers. Once the tubers are affected, chances are the batch will be very difficult to store after harvesting.”

You cannot fall back on pesticides in the organic cultivation. Den Herder: “You can therefore at most try to stay ahead of the outbreak by starting the cultivation process as early in the season as possible. Using preventive cultivation measures you can hope your plants have developed as much as possible when Phytophthora arrives. The pressure of the disease increases over time. It is advisable to germinate the potatoes beforehand, to plant them early, and also to regulate manuring well. When potatoes are then planted, they grow much faster, and you could possibly stay ahead of the outbreak as much as possible.”

“Because hardly any methods exist to combat the disease in organic cultivation, it is necessary to use Phytophthora resistant strains more often,” according to Den Herder. Van Hoogen agrees with him. “Organic cultivators need those kinds of strains most of all. They are at their wit’s end with each new outbreak, because up till now they were powerless to do anything. Agrico is the first party to market a Phytophthora resistant strain to take away that uncertainty. That process started 60 years ago already, so it truly is a long-winded process. And we still can not sit back, because the resistance might be broken. As with antibiotics for medical purposes, the strains of the fungus are continuously changing, and as agriculturalist you end up in a rat race. You have to keep developing.”

“We have the know-how, but the consumer needs to accept the product,” says Den Herder. “By now, cultivators are convinced of this pragmatic approach, but the chain itself is still a bit reserved. Potato strains, colours and cooking characteristics are unique for each variety, and wholesalers, supermarkets and consumers always have a certain image in mind. It is completely different with onions, to many, an onion is just an onion. People have got so used to potatoes having a specific combination of product characteristics, such as flavour, look, firmness and texture of the skin for example, and Phytophthora resistant strains cannot always completely facilitate these. Perhaps it is just a matter of time. Besides, new resistant strains are being developed all the time to fulfil those wishes.”

The Netherlands
W: www.agrico.nl
E: hoogen@agrico.nl
T: +31 (0)527 639 911
F: +31 (0)527 639 880

Christoffel den Herder
Senior Advisor Organic Agriculture
Delphy
W: www.delphy.nl
E: c.denherder@delphy.nl
M: +31 (0)6 1215 5131
T: +31 (0)317 491 578
F: +31 (0)317 460 400