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Main symptoms

Sclerotinia sclerotiorum and S. minor are fungi which cause very similar symptoms on lettuce, after planting and especially when they are headed and near harvest.

They are responsible for damp alterations and light brown affecting the parts of plants in contact with the soil and in particular the senescent leaves, then the crown (figures 1 and 2). These alterations very quickly develop into rotting which spreads to leaf strata close to the ground (Figures 3 and 4). The petioles and main veins as well as the crown (figure 5) are invaded, which leads to chlorosis and wilting of the outer leaves (figures 6 to 8), then of the plants, sometimes in less than 2 days. Subsequently, the rot spreads to all the leaf tissues which decompose and collapse (Figures 9 and 10). Pulling up these plants offers no resistance.

Spots initially, then alterations comparable to those described above, can initiate higher up, from leaves exposed to ascospores of Sclerotinia sclerotiorum . The spread of the spots is rapid and inevitably leads to rotting of several leaves and of the apple (Figure 11).

Slow and late wilting of lettuce has also been reported in the past, following attacks by Sclerotinia minor deeper in the soil. The latter caused a brown alteration of the taproot, 5 to 10 cm below the soil surface.

Whatever the location of the attack, a mycelium more or less cottony and white forms on certain parts of the affected tissues (Figures 3 and 4). We observe structures that allow to differentiate these two Sclerotinia spp. :
- a few large black sclerotia , rather elongated, 2 to 20 mm long and 3 to 7 mm wide, for Sclerotinia sclerotiorum (figure 12);
- an aggregate of small black sclerotia , irregular, rather circular, 0.5 to 2 mm in diameter, for Sclerotinia minor (figure 13).

The teleomorphic (perfect) form of these fungi is sometimes visible on the soil surface, mainly in the case of Sclerotinia sclerotiorum . It is materialized by the formation of small "trumpets", apothecia (figure 14), especially on the largest sclerotia. These apothecia produce ascospores, at the origin of airborne contaminations.

Last change : 04/26/21
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