Fusarium oxysporum f. sp. lycopersici , a fungus widely distributed throughout the world and much feared at one time, is no longer a threat in production areas where cultivated varieties are resistant. Indeed, all intensively cultivated varieties are currently resistant to Fusarium wilt.
 

This vascular fusarium wilt ( Fusarium wilt ) can affect very young plants as well as adult plants. On the former, it causes a slowdown in growth, wilting and yellowing of the lower leaves; the vascular tissues are strongly browned. Often the seedlings eventually wither and die.
 

On adult plants, again, it is the old leaves that express the first symptoms. Some of the leaflets show sectoral chlorosis (Figure 1). Subsequently, the yellowing spreads over the entire blade and spreads to other leaflets on the same side of the leaves (Figure 2). In addition to turning yellow, the leaves eventually wilt for much of the day.
 

The petioles and the stem also show longitudinal yellowing (Figure 3) which gradually amplifies and gives rise to a necrotic lesion affecting one side of the stem for several centimeters (Figures 4 and 5). In some cases, rough roots of adventitious roots may appear. A cut made in the stem allows the vessels to be released and to see that they have a very marked brown tint (figure 6). The marrow does not appear to be affected.
 

As it progresses, the disease causes more and more marked wilting of the plants, which ends up drying out entirely.
 

Note that such symptoms can appear on resistant varieties. In this case, several hypotheses are possible:

- it is indeed fusarium; a new breed was able to appear, overcoming the genetic resistance expressed by the variety (contact a diagnostic laboratory quickly, this type of observation may be of interest);

- you are indeed in the presence of a fusarium, but it could develop in the plant because its root system has been altered. Indeed, during severe attacks by nematodes or in the case of root asphyxiation, we have repeatedly observed attacks by F. oxysporum f. sp . lycopersici on normally resistant plants;

- a few seeds of a variety susceptible to Fusarium wilt may have been mixed with your seeds;

- you have confused certain symptoms and your diagnosis is wrong, start your observations over and pay much more attention.
 

If you have the slightest doubt, we advise you to bring or send samples to a specialized laboratory which will carry out essential microbiological isolations.

For additional information on this fungus, you can consult the sheet F. oxysporum f. sp . lycopersici .