Main symptoms

• On seedlings

Such Pythium spp., as Thanatephorus cucumeris ( Rhizoctonia solani ), are agents damping-off well-known causing damage in nurseries. In addition to preventing seeds from germinating, they attack seedlings both in meadow and post-emergence. Quite diverse symptoms can be observed depending on the stage of development of the seedlings and the species prevailing:
- wet and brown lesions on the roots (figure 2), sometimes reaching the crown (figure 3), leading to their decomposition;
- wet and soft changes appearing on the neck and which may extend over a few centimeters. In this case, it gives the impression on very young seedlings of having been more or less pinched. The affected tissues gradually turn brown.

Regardless of the primary symptoms, the seedlings soon wilt, collapse and die. They eventually decompose and disappear completely (damping , -off damping-off ).

Examples of species concerned : P. aphanidermatum , P. de barianum , P. irregular , P. torulosum , P. ultimum ...

Some Phytophthora can cause similar symptoms in seedlings or young plants; this is particularly the case with P. capsici in some producing countries, but not in France to our knowledge.

• On adult plants

When Cucurbits have their root system attacked by Pythium spp. or Phytophthora spp., one notes especially the browning, the decomposition and the disappearance of rootlets and roots of small diameter. The main roots and taproot often show moist, orange to brown lesions (Figure 4); they sometimes end up rotting more or less locally. The collar may also be affected, showing wet, brown to blackish lesions surrounding the collar of young plants for several centimeters (Figures 5 and 6). The cortex is strongly altered, it can sometimes superficially suberise.

Root attacks of Pythium spp., Like those of Phytophthora spp., Strongly disrupt the functioning of the root system, notably reducing the absorption of water and nutrients. They are therefore accompanied by more or less marked and reversible wilting ( vine decline ). It should be noted that lightning attacks cause significant plant mortality ( sudden wilt ).

Exemples d'espèces concernées  : Pythium aphanidermatum , P. deliens e , P. myriotylum ; Phytophthora capsici , P. cryptogea , P. drechsleri , P. melonis ...

The leaves (Figure 7)  and twigs also show moist, dark lesions that gradually necrotize.

Oomycetes also affect fruits (figure 7), especially at the level of their face in contact with the ground. Large moist lesions, rather circular and slightly concave, spread rapidly and invade them. They are covered by a whitish cottony mold that can host various structures formed by these chromists.

Exemples :  P. aphanidermatum,  Pytium torulosum , ; Phytophthora capsici (figures 6 et 7), P. melonis , P. nicotiana e, P. cactorum , P. citrophthora ...

Indeed, thanks to a photonic microscope it is possible to observe, depending on the species of sporangia, chlamydospore oospores which make it possible to confirm the presence of one or more of these Oomycetes.