Recognize and treat potato rot

the essentials in brief

• The disease occurs not only in potatoes, but also in tomatoes and other nightshades.
• It is caused by the harmful fungus Phytophthora infestans and occurs mainly after damp weather.
• The dreaded tuber rot does not necessarily occur together with late blight, as only the above-ground parts of the plant can be affected.
• Choose potato varieties that are as resistant as possible for a crop, whereby early potatoes are generally less susceptible.

Causes and Spread

In warm, humid weather, potatoes are mainly infected by the dreaded late blight (also called potato rot, even if the disease affects not only potatoes) between the beginning of July and the end of August - as well as into late autumn. The disease usually only occurs in connection with rain, in dry weather there is usually no danger. The causative agent of this fungal disease is the harmful fungus Phytophthora infestans, which is usually transmitted by wind, unclean garden tools, infected seeds or poor hygiene. It sticks to the damp leaves and can penetrate these surfaces - because they are swollen and soft - more easily into the plant.

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Phytophthora infestans often overwinters in potatoes left behind or stored in the field, which in turn - used as seed potatoes in the following year - represent a renewed source of infection. The fungal spores form at temperatures of around 16 ° C and in damp weather for more than ten hours. They are then transmitted by the wind, for example to neighboring plants. Incidentally, the fungus can also survive in the ground after its spores have infiltrated there with the rainwater. For this reason, a permanently moist surface can also lead to an outbreak of the disease.

Damage

"A single infected seed potato is enough to destroy the harvest of an entire acreage."

Potato rot infection is manifested by the following symptoms:

• Yellowing of the leaves at the beginning of the infection
• then formation of gray or brown leaf spots
• Formation of white-gray fungal lawns on the underside of the leaves
• All potatoes die off within a few days
• Infestation of the tubers does not always occur at the same time as late blight
• infected tubers have gray, sunken spots
• Browning of the tubers under the skin

The insidious thing about tuber rot is that infected potatoes look healthy for a long time and only start to rot when stored for a long time. For this reason, you should not set up earth rents and the like near the cultivation areas, and no tubers should remain in the field after the harvest.

Fight potato rot

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If the potato rot has attacked the plants, cut the leaves immediately and discard them. Under no circumstances should you throw the infected plant parts on the compost, as fungal spores can stay there for a very long time. In the next year, these may cause a new infection. It is better to dispose of the leaves in the general waste. The tubers themselves can remain in the ground for at least three weeks after the leaves have been removed.

Digression

Plant potatoes in a different place every year

Since the fungal spores persist in the soil and in plant residues for a very long time, you should change the cultivation areas for nightshade plants such as tomatoes and potatoes every year. This is especially true if an infection occurred in the previous year. Instead, plant marigolds (marigolds) in the bed, as these clean the soil.

Effectively prevent potato rot

However, the most important measure to combat potato rot is effective prevention. The following tips will help you contain the disease:

• Be sure to adhere to the recommended planting distances and do not plant the potatoes too close together.
• Do not plant potatoes and tomatoes (or any other nightshade family) in close proximity to one another.
• Change the area under cultivation annually and keep the crop rotation.
• Never pour from above, always pour directly onto the ground.
• If possible, water in the early morning so that the moisture can dry off quickly afterwards.
• Watering in the evening, however, promotes fungal infestation.
• Fertilize moderately and especially avoid over-fertilization with nitrogen.
• Support the defense system of the plants with self-made plant manure (e.g. field horsetail or garlic tea) and rock flour. (€ 14.95 at Amazon *)

By the way, the pre-germination of the tubers, which you can position in flat bowls or egg cartons in a light and frost-free place in early spring, also has a good preventive effect against potato rot. Under the influence of light, short, green germs sprout from the bulbs' eyes after a few weeks. This pre-germination gives the plants a head start in terms of growth, so that they mature earlier and the infestation with the fungal disease can be prevented at an early stage.

Digression

These potato varieties are resistant to potato rot

There are no varieties that are really resistant to potato rot due to the diversity of the pathogen. Instead, you can use strains that are tolerant or insensitive to the disease and are less likely to be infected. This particularly applies to potatoes of the 'Annabelle', 'Bettina', 'Caprice', 'Donella', 'Granola', 'Lolita' and 'Mariola' varieties, but also to some commercial varieties.

frequently asked Questions

Can tubers infected with potato rot still be eaten?

Since the fungus Phytophthora infestans, which causes the disease, produces carcinogenic toxins in its metabolism, it is not advisable to use even slightly infected potato tubers. Cutting out brown areas is also of no use in this case, as invisible threads of fungus still pull through the potatoes. Therefore remove and dispose of infected tubers and do not eat them. These should not be fed to animals either.

Is potato rot and late blight and brown rot in tomatoes the same disease?

In fact, late blight and brown rot in tomatoes and potato rot are caused by the same fungal pathogen Phytophthora infestans. This means that it is the same disease, which is why you should not grow nightshade plants in close proximity to one another or in the same bed one after the other.

Tip Piling up the potatoes also protects a certain amount of weeds from rotting. It is best to do this step in early summer.