If an unappetizing, gray mold growth covers strawberries or grapes, it is Botrytis cinerea (Latin for gray mold rot). This fungal disease occurs mainly in humid weather and can destroy entire crops. Here's how to fight them.

Gray mold rot occurs primarily in warm, humid weather

Table of Contents

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  1. the essentials in brief
  2. What is botrytis?
  3. Damage: identify botrytis
  4. Effectively prevent botrytis
  5. fight botrytis
  6. frequently asked Questions

    7. the essentials in brief

    • Botrytis cinerea is also known as gray mold or gray rot. It is a common fungal disease.
    • The pathogen multiplies explosively, especially in warm and humid weather. It has a wide range of hosts, almost all garden and house plants can be attacked.
    • However, fruit vegetables, berries and ornamental perennials, mainly tomatoes, cucumbers and strawberries, are particularly at risk.
    • Effective protection consists of targeted preventive measures, above all the humidity should not be too high.

    What is botrytis?

    Botrytis is particularly feared in viticulture

    Gray mold or gray rot is caused by the common and very adaptable sac fungus Botrytis cinerea. The harmful fungus has a large host spectrum and therefore affects not only a few plant species, but well over 200 different ones. The occurrence of gray mold in viticulture is particularly problematic, because here an infection can destroy a previously promising harvest in no time at all. Here the plant disease is also referred to as noble or raw rot, since it often affects unripe grapes.

    Botrytis cinerea feels most comfortable in warm temperatures above 22 degrees Celsius, and the disease develops rapidly, especially in combination with high humidity. Since it is usually both warm and humid in greenhouses, gray mold is very common here - daily airing is essential to prevent this. But the fungus can also cause infections in the garden and indoor culture if the climate is right. For this reason, you should take appropriate precautions and keep a close eye on your plants during warm and humid summer weather.

    Damage: Identify botrytis in good time

    In the case of an infestation with Botrytis, flowers are primarily affected - which is why one sometimes speaks of blossom rot - and fruits. In principle, however, all parts of the plant, including woody twigs and young shoots, can be affected. This damage is characteristic:

    • Formation of a gray layer of fungi on affected parts of the plant
    • Mushroom lawn emits a lot of dust when touched because spores are released
    • death of plant tissue
    • Formation of brown spots on affected parts of plants
    • fruits in particular look glassy at the beginning of the disease
    • Appearance of ghost spots on tomatoes

    Botrytis ghost spots are a peculiarity of tomatoes. A bright center develops on the affected fruit, which in turn is surrounded by an equally bright ring.

    Where does the disease come from and how does it develop?

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    The initial infection with botrytis often occurs in favorable weather conditions - humidity above 85 percent and temperatures between 15 and 25 degrees Celsius - through wind and rain. Both transmit the mass-produced spores from already infected plants or from the soil. The fungal spores penetrate the plants and fruits through the smallest of wounds and injuries. In principle, infection is possible at any stage of development and at any time of the year - even in winter, with fruit trees in particular being at risk after winter pruning.

    The fungal spores overwinter either on infected plant parts (e.g. leaves and fruits remaining on trees and shrubs) or in the form of fungal mycelium in the soil. Here Botrytis feeds on dead plant parts and remains permanently in the soil. The spores mainly form in spring, which is why infection is most likely at this time of year. Weakened plants that can no longer defend themselves against the invading pathogen are particularly at risk.

    Above all, this hits the mark

    • unsuitable growing conditions
    • an unsuitable location
    • too narrow a planting
    • a cold or wet winter
    • or through incorrect fertilization (above all over-fertilization)

    pre-damaged growth.

    digression

    Which plants are particularly endangered by Botrytis?

    Strawberries are often attacked by gray mold

    In the home garden, crops that are particularly endangered by botrytis are - in addition to grapes - many berries such as strawberries, raspberries, blackberries and gooseberries. Vegetables cultivated in greenhouses, especially tomatoes, cucumbers, lettuce and onions, are also affected. But even ornamental plants are not immune to gray mould, as roses, hydrangeas, rhododendrons and tulips are also at risk. In principle, however, the fungal disease can occur on almost all garden and indoor plants.

    Effectively prevent botrytis

    Since the botrytis fungus needs sufficient moisture for infection and for its spread, you should take preventive measures to ensure that all parts of the plant dry off quickly after a downpour and generally ensure that the humidity is lower. The table below gives you an overview of effective plant hygiene measures.

    measure execution
    reduce humidity Avoid locations with high humidity, ventilate the greenhouse, conservatory and apartment (in the case of indoor plants) frequently
    Proper watering and watering when watering, always only pour on the ground, do not wet leaves, shoots and flowers, prefer appropriate watering systems (drip irrigation instead of sprinkler system)
    Proper fertilization fertilize moderately and above all avoid over-fertilizing with nitrogen, prefer organic fertilizers
    Thinning out and other cutting measures Keep plants loose and airy, cut back growth that is too crowded, cut back affected or injured/damaged shoots immediately
    mulching Mulch soil to retain moisture in soil and reduce evaporation
    hygiene measures Removal of fallen leaves, fallen fruit and fruit mummies, disinfect cutting tools
    pest control many pests are carriers of the disease and / or weaken the plants, which is why any infestation must be combated at an early stage
    Maintain plant spacing When planting, ensure a generous planting distance, do not place plants too close together

    You can also strengthen the immune system of endangered plants so that the fungus has no chance of becoming infected - strong plants get sick less often because they resist the pathogen better. Spraying with home-made, siliceous plant manure made from field horsetail is suitable for this purpose, which you carry out regularly at intervals of 10 to 14 days after the sprouting and during the growing season. A spray, made on the basis of sulphuric alumina, also shows good preventive success.

    digression

    Similar clinical pictures

    Plant diseases such as downy mildew and black rot, which are also caused by fungi, cause similar damage. Here, too, the affected parts of the plant turn brown and eventually die off, and downy mildew in particular also occurs in damp weather. A distinction is often difficult for the layman, but preventive and combating measures are similar for all diseases caused by fungi - so what helps against gray mold is often also effective against other fungi.

    How to fight botrytis

    Affected plant parts should be removed promptly with great care so that the spores do not spread further

    Check your plants regularly for signs of botrytis and other plant diseases so that you can take countermeasures quickly in the event of an infection. It is important to act quickly to prevent the fungus from spreading further - otherwise other plants will be affected in no time at all. How to deal with Botrytis gray rot:

    • Remove all diseased plant parts immediately
    • Cut away twigs and shoots down to the healthy parts
    • Reduce humidity (if possible, e.g. in the greenhouse)
    • Isolate affected plants (if possible, e.g. for houseplants)
    • Spray horsetail broth when the infestation begins
    • alternatively, pollinate plants with rock dust

    Furthermore, these measures help well against Botrytis (and other plant diseases):

    • Plants should always be planted in mixed cultures
    • especially with garlic, as this keeps pathogens away
    • Transplant strawberries and other crops every three years at the latest
    • Note crop rotation

    Before planting, put some finely chopped garlic in the planting hole (e.g. with strawberries), and only spread straw after flowering. Both can effectively prevent a botrytis infection.

    Sprays approved for the home garden

    "Chemical agents must remain the exception and only be used when nothing else helps - the undesirable side effects are enormous."

    Many a gardener fears for their harvest when Botrytis occurs and would like to kill the fungus with chemicals. The intention is understandable - who likes to cherish and care for their tomatoes only to then throw them in the garbage? - however, the use of chemical fungicides should be viewed critically for various reasons.

    An important reason that speaks against the use of fungicides is the great adaptability of Botrytis - the fungus develops resistance very quickly, which is why you should only use the chemical club in the case of a very severe infestation that can no longer be controlled in any other way. Always take the control measures already described first and focus on prevention as a matter of urgency.

    If nothing else helps, agents with these active ingredients are approved for the home garden:

    • cyprodinil: for example Syngenta Switch
    • fludioxonil: Syngenta Switch contains both cyprodinil and fludioxonil
    • trifloxystrobin: for example Baymat Plus AF fungus-free or Celaflor fungus-free Saprol N

    Otherwise there is the biologically active fungicide Prestop from Kwizda, which relies on the effect of the soil fungus Gliocladium catenulatum.

    When choosing a fungicide, make sure it is bee-friendly. Neudorff AF Pilzfrei, for example, is very effective against botrytis, but is also dangerous to bees - the hard-working insects are also killed by the agent, which is very problematic given the current bee mortality.

    frequently asked Questions

    Is botrytis also dangerous for humans?

    Gray mold can cause allergies in sensitive people

    Botrytis cinerea has a high allergy potential and is therefore dangerous for people with a fungal allergy. This particularly affects people with a penicillin allergy. Protect yourself with appropriate clothing (mouth and nose protection to prevent inhalation, goggles, gloves, long clothing) as there are many spores in the air.

    Can I dispose of infested clippings and fruit on the compost?

    No, we strongly advise against this. The spores, which are infested plant parts, find ideal living conditions in compost, where they multiply explosively and can infect other plants when the finished compost is distributed. Therefore, if possible, dispose of infectious plant material with household waste or burn it.

    Can you still eat slightly infected fruit if you cut away the moldy parts?

    Eating infested fruit is also not advisable, on the one hand because the fungus greatly affects the taste of the fruit and on the other hand because it forms toxic substances that you would eat with them. In particular, unripe infected fruit should be discarded. During wine production, grapes infected with botrytis get in again and again, but this is unproblematic due to the process of pressing and fermentation.

    tips

    Furthermore, you should immediately cool down freshly harvested berries to temperatures between one and three degrees Celsius in order to kill any fungal spores that may be present. In addition, the fruit stays fresh longer.

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