Of the many pests and disease effecting potato crops, late-blight remains the most destructive by far. Traditionally, blight has been kept at bay by chemical control. Teagasc estimate that around €5 million is spent on fungicides annually to prevent blight in Ireland alone. This is dished out over 15-20 applications per season – a worrying amount. Unfortunately, this historical dependence on fungicides meant that a narrow range of potato varieties with little or no blight resistance could be grown successfully. As a result, not much effort was made to breed resistant varieties and genetic diversity was lost.
Copper based sprays are still very effective at warding off blight. In organic production, the use of traditional copper based fungicide is still permitted but at a strictly reduced rate. Only 6kg of copper can be used per hectare in a given year. Currently there is only one product which is licensed for this use. Sold in 5kg quantities it is not suitable for home growers who may still choose to use Bordeaux mixture judiciously if not growing resistant varieties.
There is nothing inherently wrong with Copper which is an essential plant nutrient found in soil naturally. If copper levels are inadequate in the soil, plants will suffer from deficiency. However, repeated and excessive use of copper based fungicides will cause too much copper to build up in the soil and plants can then suffer from toxicity. It will also have a negative affect on soil biology such as beneficial soil fungi and has been shown to inhibit earthworm activity. Copper toxicity is a difficult problem to reverse as copper is not very soluble meaning it will persist in the soil for a long time. This is one of the reasons why it is now apparent that the best way to reduce disease pressure is to grow resistant varieties.
Breeding for Blight Resistance
Breeding for blight resistance is difficult unfortunately. To understand the problem, we have to keep in mind that late-blight, brought about by the fungus like pathogen Phytophtora infestans, is not a single fixed organism, but rather exists in many different strains which can cross and evolve rapidly. It can complete its life cycle in just 5 days! This allows for speedy adaptation and in recent years has led to the creation of aggressive strains which are even resistant to fungicides.
On the other hand, potatoes are nearly always propagated vegetatively meaning that successive generations are genetically identical. This does not give them any chance to evolve resistance to the disease. When efforts are made to breed new potato varieties, it is a very slow enterprise. This is because breeders generally have to cross plants with wild varieties to get resistant genes. The edible qualities are lost in the mix at this stage. It then takes decades of breeding and testing to develop a potato which maintains the resistant genes and has good culinary qualities too.
Blight resistance is not a straightforward 'is' or 'isn't' characteristic of a potato. It is considered on a spectrum. A range of genes and gene combinations are involved in conferring resistance. In certain cases, particular resistant genes will only be effective against certain strains of blight. So if a variety had just one or two of these genes it could still be very prone to infection depending on which strain of blight is present in a given year. Potatoes with very high levels of blight resistance will have a selection of resistance giving genes. These potatoes are described as having a 'broad' resistance. They are far more robust and likely to remain resistant for longer.
Fortunately, thanks to the perseverance of potato breeders, some excellent highly blight resistant varieties have become available in recent years. We are fortunate enough to have a supply of these and are always seeking out new resistant varieties when they become available. You will find blight resistant seed potatoes in the links below as well as advice on other cultivation tips that will decrease the effects of blight.
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