Grey mildew

Botrytis Cinerea

Botryotinia Fuckeliana
Conidial Form:
Botrytis Cinerea

This is a pathogen which affects a large number of plant species, among which the Gerbera unfortunately is one of the most susceptible, especially in periods or very damp cultivation conditions. From this point of view it is undoubtedly (together with root rot) one of the diseases that limits te cultivation of the Gerbera in many areas.

Grey mildew on a plantlet Grey mildew on a plantlet
The disease, commonly known as Grey Mildew, typically causes a “soft rottenness” on all the leaves that are covered (in severe cases) with large amounts of grey mildew. This grey mildew consists of the multiplication or conidial forms of the fungus. The mycelium and sclerotium are their resting structures; these live in the foliage remains on the soil, or on wild plants (Amarantus Genus). Botrytis spores (conidia) contain very little water so they depend on the dampness of the environment.
They germinate extremely well even in a thin layer of free water, especially if it contains sugars. Thus, condensation on leaves is perfect. A short dry period (about 2 hours) does not harm the germinating spores, but longer periods (several hours) of dry conditions will dry out and kill them. When the leaves are wet and the temperature is 15-25°C, the spores germinate and grow.

Grey mildew under a microscope Grey mildew under a microscope

They form mycelium threads that penetrate the leaves, which marks the start of infection. Once the disease is established it forms new spores, especially under humid conditions. Spores are released when the spore-bearing plantmaterial is disturbed or when the relative humidity (R.H.) changes rapidly (in the morning and late afternoon).

Under normal conditions (low R.H.), unless an abundant innoculum source is available, the fungus is incapable of attacking vital organs and in active development. These, in fact, are damaged only if they are come into contact with an organ already infected with abundant grey mildew.

The pathogen develops in abundance on old organs, producing a big quantity of enzymes that damage the adjacent vegetable tissue and facilitate the invasion of the fungin mycelium. On the young Gerbera plants, the rottenness produced at the base can cause their death with notable economic repercussions.

Fungus attack When fungus attacks young flowers it causes their death, and thus reducing production

On the flowers, the most characteristic symptom is the appearance of brown sunken spots of various dimensions; when climatic conditions are not too negative, they are not very visible to the naked eye during the harvest. However, these spots grow bigger after the harvest, during the storage and packaging becoming so noticable as to make them unmarketable.

The fungus is so virulent that it can also cause damage in high summer if at nightime the relative humidity in the greenhouse reaches values of about 95-100%. The infection is encouraged by temperatures between 4 and 32°C, but occurs especially when R.H. levels in the greenhouse are higher than 85-90%. The spread of the pathogen takes place through water and air.

Typical Botrytis “spots” on a flower Typical Botrytis “spots” on a flower

Control of Botrytis

To fight this dangerous pathogen we should adopt all those agronomic practices which reduce the occurence of the conditions of predisposition for the disease, such as: the reduction of films of water on the plants; reduced high humidity, and the reduction of especially luxuriant and etiolate plants.

To prevent Botrytis damage we must make use of:

  • Heating with pipes of warm water placed at the base of the plants
  • Irrigated systems that do not wet the foliage, especially during the autumn-winter period
  • Openings at the top of the greenhouse
  • Heating systems with pipes of warm water positioned between the plants and the openings at the top

Source: p. 131 & 132 of “Gerbera Cultivation in Greenhouse” by Gerardo Mercurio

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