If an unappetizing, gray mold covers strawberries or grapes, it is Botrytis cinerea (Latin for gray mold rot). This fungal disease occurs primarily in wet weather and can destroy entire crops. Here's how to fight them.
What helps against Botrytis in plants?
Botrytis, also known as gray mold rot, can be treated through regular inspections, timely removal of infected plant parts, reducing air humidity and using horsetail broth or primary rock powder. Preventive measures such as good plant hygiene and plant spacing are also important.
- 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 large host range, almost all garden and house plants can be infected.
- However, fruit vegetables, berries and ornamental perennials, mainly tomatoes, cucumbers and strawberries, are particularly at risk.
- Effective protection consists of targeted preventive measures, especially the humidity should not become too high.
What is Botrytis?
Botrytis is particularly feared in winegrowing
Gray mold or gray rot is caused by the common and very adaptable ascomycete Botrytis cinerea. The harmful fungus has a wide range of hosts and therefore affects not just a few plant species, but well over 200 different ones. The occurrence of gray mold in viticulture is particularly problematic, as an infection can destroy a previously promising harvest in the blink of an eye. Here the plant disease is also known as noble rot or raw rot because it often affects unripe grapes.
Botrytis cinerea feels most comfortable at warm temperatures from 22 degrees Celsius, and the disease develops rapidly, especially when combined with high humidity. Since greenhouses are usually both warm and humid, gray mold is very common here - to prevent this, daily ventilation is essential. But the fungus can also cause infections in the garden and in indoor cultivation 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.
Major symptoms: Detect Botrytis in good time
When Botrytis is infested, flowers - which is why it is sometimes referred to as blossom rot - and fruits are primarily affected. In principle, however, all parts of the plant, including woody branches and young shoots, can be affected. This damage pattern is characteristic:
- Formation of a gray fungal lawn on affected plant parts
- Mushroom lawn creates a strong dust when touched as spores come out
- Death of plant tissue
- Formation of brown spots on affected parts of the plant
- Fruits in particular look glassy at the beginning of the illness
- Appearance of ghost spots on tomatoes
The ghost spots in botrytis are a special phenomenon on tomatoes. A bright center develops on the affected fruits, which in turn is surrounded by an equally bright ring.
Where does the disease come from and how does it develop?
The initial infection with Botrytis often occurs in favorable weather conditions - humidity over 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 wounds and injuries, and infection is generally possible at any stage of development and at any time of 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 bushes) 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 form primarily in spring, which is why infection is most likely at this time of year. Weakened plants that can no longer defend themselves against the invasion of the pathogen are particularly at risk.
This is especially true through
- unsuitable growing conditions
- an unsuitable location
- planting too close together
- a cold or wet winter
- or due to incorrect fertilization (especially over-fertilization)
pre-damaged plants.
Excursus
Which plants are particularly at risk from Botrytis?
Strawberries are often affected by gray mold
In the home garden, crops that are particularly at risk from botrytis are - in addition to grapes - many berries such as strawberries, raspberries, blackberries and gooseberries. Furthermore, it primarily affects vegetable plants grown in greenhouses, especially tomatoes, cucumbers, lettuce and onions. But even ornamental plants are not immune to gray mold, as roses, hydrangeas, rhododendrons and tulips are also at risk. In principle, however, the fungal disease can occur on almost all garden and house plants.
Prevent Botrytis effectively
Since the Botrytis fungus needs sufficient moisture to infect and spread, you should take preventative measures to ensure that all parts of the plant dry quickly after a rainstorm and to generally ensure lower air humidity. The following table gives you an overview of effective plant hygiene measures.
| Measure | Implementation |
|---|---|
| Reduce humidity | Avoid locations with high humidity, ventilate the greenhouse, winter garden and apartment (for houseplants) frequently |
| Proper watering and watering | When watering, always only pour on the ground, do not wet the leaves, shoots and flowers, prefer appropriate watering systems (drip irrigation instead of sprinkler systems) |
| Proper fertilization | Fertilize moderately and, above all, avoid over-fertilizing with nitrogen, prefer organic fertilizers |
| Thinning and other editing measures | Keep plants loose and airy, cut back growth that is too close, immediately cut back infected or injured/damaged shoots |
| Mulching | Mulch soil to retain moisture in the soil and reduce evaporation |
| Hygiene measures | Removal of fallen leaves, fallen fruit and fruit mummies, disinfecting 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 planting distance | Make sure there is a generous planting distance when planting, 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 are less likely to get sick because they resist the pathogen better. A suitable method for this is to spray with self-made, silicate-containing plant manure made from field horsetail, which you should carry out regularly at intervals of 10 to 14 days from the time it sprouts and during the growing season. A spray (€8.00 at Amazon), made on the basis of sulphate of clay, also shows good preventative success.
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Similar clinical pictures
The plant diseases 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, and downy mildew in particular also occurs in damp weather. It is often difficult for the layperson to differentiate, 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
Infected plant parts should be removed promptly with great care so that the spores do not spread any further
Check your plants regularly for signs of botrytis and other plant diseases so that you can quickly take countermeasures in the event of an infection. It is important to act quickly to prevent the fungus from spreading further - otherwise other plants will also be affected in no time. How to combat Botrytis gray rot:
- immediately remove all diseased parts of the plant
- Cut branches and shoots down to the he althy parts
- Reduce humidity (if possible, e.g. in the greenhouse)
- isolate affected plants (if possible, e.g. houseplants)
- Inject horsetail broth when infestation begins
- alternatively, dust plants with primary rock powder
These measures also help against botrytis (and other plant diseases):
- Always plant plants in mixed culture
- especially with garlic, as this keeps pathogens away
- Remove strawberries and other crops every three years at the latest
- Observe crop rotation
Before planting, add some finely chopped garlic to the planting hole (e.g. for strawberries), and only spread straw after flowering. Both can effectively prevent a Botrytis infection.
Sprays approved for home gardens
“Chemical agents must remain the exception and only be used when nothing else helps - the undesirable side effects are enormous.”
Many gardeners fear for their harvest when Botrytis appears and want to kill the fungus with chemical weapons. The intention is understandable – who likes to nurture and care for their tomatoes only to then throw them in the trash? – However, the use of chemical fungicides should be viewed critically for various reasons.
An important reason against using fungicides is the great adaptability of Botrytis - the fungus develops resistance very quickly, which is why you only use the chemical club if the infestation is very severe and can no longer be controlled in any other way should. Always take the control measures already described first and urgently focus on prevention.
If nothing else helps, products 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 action of the soil fungus Gliocladium catenulatum.
When choosing the fungicide, make sure that it is bee-friendly. Neudorff AF Pilzfrei, for example, is very effective against botrytis, but is also dangerous to bees - the active insects are also killed by the agent, which is very problematic given the current bee decline.
Frequently asked questions
Is botrytis also dangerous for humans?
Gray mold rot 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, safety glasses, gloves, long clothing) as there are a lot of spores in the air.
Can I dispose of infected clippings and infected fruits in the compost?
No, we strongly advise you not to do this. The spores as infected plant parts find ideal living conditions in compost, multiply explosively there and can infect other plants when the finished compost is distributed. 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 areas?
Eating infected fruit is also not advisable, on the one hand because the fungus has a strong impact on the taste of the fruit and on the other hand because it produces toxic substances that you would eat. In particular, unripe infected fruits should be thrown away. During winemaking, grapes infected with Botrytis occasionally get into it, but this is not a problem due to the process of pressing and fermentation.
Tip
Furthermore, you should immediately cool freshly harvested berries in particular to temperatures between one and three degrees Celsius in order to kill any fungal spores that may be present. This also keeps the fruit fresher for longer.
