Main symptoms

Salads affected by Pythium tracheiphilum show reduced growth (Figure 1); during very early attacks, plants can be completely blocked and remain dwarf. Their small size contrasts with that of the apparently healthy surrounding plants. The leaves of some salads may turn yellow and wilt. Initially, these wilts occur at the hottest times of the day and the plants recover at night. Subsequently, they sometimes take an irreversible character leading to the drying out and death of some plants.

The taproot may occasionally be deformed, superficially rough, and greyish in color; secondary roots are few (Figure 2).

A longitudinal section taken in the taproot of several diseased lettuce shows that the vessels are more or less brown (Figure 3). In some cases, contiguous tissues are also affected (Figure 4). Longitudinal section of several secondary roots confirms that the vascular system is fully affected, given its marked browning.

In addition, some authors report the presence of necrotic spots on leaves salad attacked by Pythium tracheiphilum .