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Aug 25, 2010

Alternaria in potatoes – Magnesium helps to prevent the disease

This year, an obvious increase of alternaria infections in potatoes was observed. The resulting ‘early blight’ is caused by a fungal pathogen and can be recognised by the well-defined, brown/black spots on the leaves which can reach up to 2 cm diameter.


High temperatures and stress factors favour the fungal infection

Infections occurring shortly after the tuber initiation negatively affect the tuber’s further development. Yield losses of 20 to 30 percent can result from of an infection of alternaria solani early on in the development of a late maturing variety. High temperatures observed in June and July 2010 offered ideal conditions for an infectio with the soil-borne fungi. Irrigation further increases the risk of the disease. Furthermore, infection depends on variety and general stress factors.


Potatoes have high requirements in potassium and magnesium

The potato has very high requirements in potassium and magnesium. But as it has a rather weak root system, it depends on a good nutrient supply. Insufficient supply generally results in the weakening of the crop and thus creates the possibility for an infestation of the plant by the soil-borne fungi.


Magnesium fertilisation strengthens the resistance against alternaria

Vascular examinations show that of all the nutrients, magnesium has the greatest influence on an infection by alternaria. Deficiencies in other nutrients led to the respective deficiency symptoms but only a deficiency in magnesium caused further infections. Only a magnesium deficiency resulted in a huge population of alternaria on the haulm as showed by the following picture:

Potato plant, ideally supplied with all nutrients (left); Potato plant with magnesium deficiency (middle); A magnesium deficiency induces alternaria disease (right). (Photos: K+S KALI GmbH)

Magnesium boosts the vigour and stability of the sensitive haulm due to its chlorophyll-forming and metabolism-regulating properties. Even when there is an adequate soil supply of magnesium, plants often suffer from magnesium deficiency. If calcium carbonate or magnesium-poor potassium fertilisers are used, a magnesium deficiency can be induced due to the antagonistic effect. This is shown by a sampling of potato leaves by the Thüringische Landesgesellschaft:

One often finds in the literature the demand for potassium-magnesium-ratios of 3:1. To prevent deficiencies or foliar diseases like alternaria in the potato, a closer ratio is beneficial. This is why the fertiliser system Patentkali (30 % K2O,10 % MgO) + EPSO Microtop (15 % MgO, 12 % S, 1 % B, 1 % Mn) is ideally suited for this crop. Low soil supply and light soils might necessitate an additional application of ESTA Kieserit gran. (25 % MgO). A deficiency in magnesium leads to early maturation during which the plant is more susceptible to alternaria. Therefore it is reasonable to add foliar fertiliser to the basic soil fertilisation by means of fast-acting, crop safe fertiliser like 5 x 10 kg EPSO Microtop ha-1 until the end of flowering (EC 79). These measures will not only promote better yields and crop quality but also help to reduce the disease pressure and negative economic effect of alternaria.


Reinhard Elfrich, Everswinkel, Germany




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