Main symptoms

In addition to seed rot (lack of emergence), Pythium spp. , such as Thanatephorus cucumeris , are the cause of wet and soft alterations that begin on the stem of the seedlings at ground level. In this case, the collar looks like it has been pinched. The affected tissues gradually turn brown. The roots are sometimes affected; they take on a light brown hue and their cortex disintegrates more or less (Figure 2). Les plantules it tardent a s'effondrer ( Pythium aphanidermatum , P. ultimum , P. slabs , P. dissotocum , P. uncinulatum , P. violae ...; " Damping of- f").

Some species cause root browning sometimes localized at the end of the roots. We also note the disappearance of nourishing roots and the rotting of the cortex of the roots and the taproot. These symptoms sometimes appear in the soil, more frequently outside the soil. They are accompanied by leaf yellowing and wilting more or less reversible. In some cases, the salads show poor growth and their size is reduced ( Pythium aphanidermatum , P. uncinulatum , P. myriotylum , P. irregulare , P. dissotocum , P. polymastum ...; " Pythium root rot " and " Wilt ").

Some Pythium spp. are responsible for leaf damage . Moist and brown spots of varying extent (figure 3), basal rots, reflect the effects of these fungi. ( Pythium aphanidermatum , P. uncinulatum ...; " Pythium bottom rot " and " Leaf blight ").

Whatever the symptoms observed, one often observes, thanks to a light microscope, the presence of oospores (figure 4) and / or chlamydospores in the damaged tissues.

Phytophthora cryptogea (Pethybr. & Lafferty, 1919) causes browning and root rots in lettuce grown soilless. Phytophthora porri (Foister, 1931) rather attacks the stem causing a firm and dark rot that begins at soil level.