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Fumagine linked to the presence of insects

Why does sooty mold grow on salads?
The appearance of sooty mold on salad leaves is always linked to the development of parasitic insects on plants. Of these, aphids and whiteflies are most commonly associated with the presence of this black mold on the leaf blade. The explanation is quite simple; it is linked to the eating behavior of these insects. Indeed, aphids and whiteflies have to take large quantities of sap in order to meet their protein needs. This forces them to reject excess sugar in the form of honeydew. This is therefore present where these insects develop and soils the surface of the invaded parts of the blade. This sweet honeydew is a real boon for several phylloplan fungi which use it for food and gradually cover it with a black mold ( Alternaria spp., Cladosporium spp., Capnodium sp. ...). This sooty mold certainly disrupts photosynthesis to a greater or lesser extent. To remedy this, it is necessary to control the pest populations.

The aphids ( "Aphids") and whiteflies ( "White fly") , in addition to excrete honeydew causing sooty mold and depreciate the quality of harvested salads, disrupt the functioning and growth of salads. In the case of an overgrowth of these insects, it is not uncommon to see a slowdown in the growth of plants. Several aphid species have been reported on salads in France: Nasonovia ribisnigri , Myzus persicae , Macrosiphum euphorbiae , Hyperomyzus lactucae , Aphis craccivora , Aphis gossypii , Acyrthosiphon solani ... (figure 1). With regard to whiteflies, Trialeurodes vaporariorum is certainly more common on salads than Bemisia tabaci or Bemisia argentifolii (figure 2).

These insects, and sometimes other species abroad, are effective vectors of several serious viruses for salads. The main viruses transmitted to salads in France and abroad (viruses not detected in France on salads) by these vector insects are for example:

  • Aphids

- Lettuce mosaic virus ( LMV );
- Cucumber mosaic virus ( CMV );
- Alfalfa mosaic virus ( AMV );
- Bean wilt virus ( BBWV );
- Dandelion yellow mosaic virus ( DaYMV );
- Chicory necrotic mosaic virus ( ENMV );
- Turnip mosaic virus ( TuMV );
- Western beet yellows virus ( BWYV );
- Lettuce mottle virus ( LMoV );
- Beet yellow stunt virus ( BYSV );
- Lettuce necrotic jaundice virus ( LNYV );
- Jaundice virus in sonchus network ( SYNV );
- Sow thistle yellowing virus ( SYVV ).

  • Aleurodes

- Beet pseudo-jaundice ( virus BPYV );
- Lettuce infectious jaundice virus ( LIYV );
- Lettuce chlorosis virus ( LCV ).

As a reminder, thrips ("Thrips") ( Thrips tabaci , Frankliniella occidentalis ...) can also be virus vectors:
- Tomato leaf spot virus (TSWV );
- Impatiens necrotic spot virus ( INSV ).

Other pests, sometimes vectors of viruses, develop on the leaves of lettuce and cause quite different symptoms. Their damage is presented in the links Damage due to pests (slugs, leaf miners, thrips, etc.) and Other land-based pests .

Last change : 05/26/21
  • Author :
  • D Blancard (INRAe)
Figure 1
Figure 2