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The downy mildew of the potato


Potato blight is one of the most common cryptogamic diseases among those affecting this tuber, and it is particularly fearful because if it is not prevented or controlled in time it can ruinously compromise the harvest.

Growing potatoes in itself is not difficult, only since it is one long cycle culture, the likelihood of some problems arising during all the months of stay in the field increases, and downy mildew is certainly one of the worst threats.

So let's see how to learn a recognize the symptoms, understand what are the environmental conditions that favor it, so as to be prepared for its arrival and able, if possible, to mitigate the consequences, and how to act with the means with the lowest possible environmental impact. We can defend potatoes with the principles and treatments of organic farming, even with success, as long as we act on time and correctly.

The pathogen of downy mildew Phytophtora infestans

The pathogen of downy mildew is the mushroom Phytophtora infestans, which in addition to the potato also affects the tomato (tomato blight is one of the most feared diseases in the vegetable garden, but has differentiated numerous physiological breeds, one of which in particular attacks the potato.

The triggering conditions

The conditions most favorable to infection by the fungus Phytophtora infestans I'm:

  • Rain lasting at least 6 hours.
  • Heavy dews that keep vegetation wet for at least 12 hours with very high relative humidity for several hours.
  • Ambient temperatures between 10 and 24 ° C, with a peak at 20-23 ° C.

The frequent recurrence of weather situations of this type makes the course of the disease even more negative, also because in repeated episodes of rain, the infecting elements of downy mildew already present on the leaves are carried by the rains into contact with the tubers, contaminating the potatoes.

The spores of the fungus they originate from the mycelium that survived throughout the winter in infected crop residues left on the ground, or from infected seed potatoes, and also from single-grown potato plants from the previous crop, if infected.

Symptoms of downy mildew on the potato

Early symptoms of downy mildew can be recognized on the basal leaves, then the infection begins to affect even those above.

One can therefore note one pale coloring of the leaves, is brown spots surrounded by yellowish halos, which then necrotize and dry out.

The infection is also borne by the stems, on which marked browning appears which easily degenerates into bottlenecks, with consequent loss of functionality and collapse of the overlying plant parts.

Surely damaged leaves and stems already lead to a decrease in production, as the underground tubers develop well only if the aerial part of the plant is healthy, but the disease does not limit itself to doing this damage: it also directly affects the tubers themselves. On the latter we can see depressed lesions, varying in color from blue-greyish to brown and with a soft and smelly consistency.

The problem is that we only notice it by taking them off the ground, with a nasty surprise, or even in the post-harvest, during storage.

How to prevent potato blight

Definitely setting up a good preventive strategy against downy mildew is an excellent base from which to start to get a rich harvest of potatoes from healthy plants. All the more reason for the fact that the measures useful for the prevention of downy mildew help us to avoid all the other diseases of the potato.

In particular, the following measures must always be given importance:

  • Rotations: they must always be well planned and applied, even in an amateur cultivation carried out on limited spaces. In this specific case it is essential not to put the potatoes where they had already been grown in the previous two or three years, and also to avoid placing them in the space previously dedicated to tomatoes.
  • Choice of resistant or at least tolerant varieties. This information can be read on any supplier catalog or requested expressly before purchase. We can certainly find good compromises of good potato varieties, suitable for the culinary preparations that interest us, and at the same time resistant to downy mildew.
  • Carefully eliminate all infected crop residues, to prevent infections in the following year.
  • Pay attention to the health of the seed. In the case of self-reproduction of seed potatoes, pay the utmost attention to their state of health, and in case of even minimal doubt, avoid using them for the propagation of the crop. We should not use seed potatoes from potato crops affected by downy mildew, even partially.
  • Respect the distances. When planting, adopt large distances between the rows to allow good air circulation between the plants.
  • Do not overdo the fertilizer. Avoid fertilizing too much, because excesses, especially nitrogen, make plants more susceptible to fungal attacks. Even with organic fertilizers it is easy to overdo it, especially with pelleted ones and with manure, which is much more concentrated than cow manure. You can read the in-depth information on potato fertilization to know how to behave.
  • Beware of irrigation, avoid wetting the aerial part of the plants and always prefer a system that wets the soil, such as drip micro-irrigation.
  • Tamping, which help to protect the tubers with a good layer of soil (see how to make the potato heap).

Bio defense: anti downy mildew treatments

Defense from mildew can be implemented avoiding systemic fungicides, not allowed in organic farming, and opting for a copper-based product which, although not biodegradable, remains a cover product that leaves no residue in the crop.

The use of copper treatments against downy mildew must be limited to need and strictly recommended doses on the labels of commercial products, avoiding unnecessary and harmful excesses.

Alternatives to copper

Wanting to avoid the use of cupric products, here are the alternatives.

We can choose corroborating products of organic or mineral origin, these are plant defense enhancers, not fungicides.

  • Soy lecithin
  • Macerates and decoctions of horsetail
  • Propolis-based products
  • Zeolite or other rock powders, as corroborants of natural mineral origin, create a veil capable of absorbing stagnant moisture on the foliage due to rain or dew, preventing fungus.

We can finally deal with products based on Bacillus subtilis which in fact for the potato are registered for treatments against rhizottoniosis. If it is necessary to treat against this other rather common pathology, the positive effect of the treatment can also occur against any mildew present.

To conclude, it is essential prevent downy mildew by all possible means and always keep an eye on the plants to intervene as soon as possible with the means available to the ecological grower.


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