Botrytis cinerea and Sclerotium sclerotiorum

(Grey mould and collar rot)

There are other pathogens threatening the nursery. Botrytis cinerea in particular can be very damaging in float seedbed systems. 
 
This is partly due to:
- a high density of seedlings with very succulent tissues;
- excessive humidity in greenhouses;
- existence of resistant strains to certain fungicides (e.g. cyclic imides);
- the presence of cotyledons and chlorotic and senescent leaves on plants, resulting from too long conservation of plants and/or end of their nutrition. These plant tissues form real nutrient bases, they allow the fungus to move in, and then rapidly colonise the stem (figures 1 to 3) or remain latent.

Note that in the latter case, beige cankers with dry consistency will develop on the buried portions of stems after planting. One may add that Botrytis cinerea forms a specific grey mould on altered tissues in the presence of moisture, this happens whatever the situation.
 
In excessively humid greenhouses, leaf spots (figure 4a) and outbreaks of grey mould (figure 4b) may appear in the centre of the vegetation. The high density of plants facilitates the development and expansion of the fungus from plant to plant.
  
Sclerotinia sclerotiorum, another pathogen occurring in nurseries, behaves occasionally quite similarly to Botrytis cinerea. It also causes stem alterations of light brown to brown (figure 1) which are covered rather quickly with a dense, white mycelium and large, black characteristic sclerotia (figure 5).
 
 
For a better understanding of the biology of these pests, please consult the following fact sheets: Botrytis cinerea and Sclerotinia sclerotiorum.