What can be done against the cucumber diseases mildew and botrytis?

Garden Experts
For years, I have been struggling against cucumber diseases in my (hobby) greenhouse – without success. The leaves become diseased and “dry up”, in spite of regular watering, and the developing cucumbers rot at an early stage. I have read up on the subject and found botrytis and/or mildew to be the cause of the problem. But what is this exactly, and how can I control it without having chemical residues in the soil and fruits in my greenhouse? I am already using grafted varieties. Bell peppers, too, do not do very well, whereas my tomatoes grow and produce fruit beautifully. I replaced the soil in spring, and yet the symptoms are already coming back again.

The GARDENA gardening expert

Dear Mr. H., that is of course a nuisance, as going to all the effort of operating a small greenhouse means that one does expect a return on one's work! 

If you have found mildew and botrytis to be the cause of the damage, I have the following tips:
Both of these diseases are considered subsequent damage to climate problems, in particular grey mould (stale air) but also mildew (dew formation on leaves when the dew point is underrun). This can be combatted through accurate watering directly onto the roots, for example using a drip system. Never water from the top, over the leaves (overhead watering) 

Also crucial is ventilation: 25 percent of the greenhouse floor area should be a freely ventilatable area in the gable. It is important to reduce excessive humidity in the air to combat the cultivation of botrytis and mildew spores. When fertilising, observe the following: Nitrogen should be provided in appropriate quantities; too much of it will make the plant tissue too soft and therefore less resistant to fungal infestation. Ensure sufficient supplies of potash to strengthen the cell maturity on leaves and fruits. The enclosed photo appears to me to indicate a patchy leaf disease – this would accord with the suspected excess of humidity in your small greenhouse. 

Initially, the aim is always to work preventatively against fungal infestation. However, in serious cases, fungi can also be combatted chemically. Preparations in harmony with nature such as Fungisan Rosen- und Gemüse-Pilzfrei (fungicidal agent for roses and vegetables) or Netz-Schwefel WG (both by Neudorff) can even be used without having to adhere to waiting periods. 

The use of grafted cucumber plants is expedient in view of the fact that you have had several years of plant protection problems in your small greenhouse. However, this grafting is only effective against certain soil-borne fungi, e.g. through Fusarium wilt, but not against fungi which occur on the leaves due to climate problems. 

As far as the soil replacement is concerned: This, too, is always an expedient measure. But the replacement cannot guarantee you spore-free soil. The problem can be re-imported through spores remaining on your tools. It is difficult to decontaminate soil in a home garden, too. What helps more is a radical change in fruit and vegetables – right up to vegetable plants from other plant families. But before you begin these measures, try out changes in air conditioning first. I wish you every success!

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