Fusarium wilt (Fusarium oxysporum) is one of the most damaging fungi that infects tomatoes. Fusarium wilt is a soil borne disease and infects both soil grown tomatoes and greenhouse tomatoes. During high temperatures the disease can cause significant crop damage but during low temperatures there is generally little damage. There are currently two races that cause damage to the plants. They are conveniently called race 1 and race 2 of which race 1 is the most common.
Symptoms of Fusarium wilt
The oldest leaves of young seedlings tend to wilt and droop downwards. This follows the wilting of the whole plant and finally death.
In general the mature plants can be infected at any stage during the growing season but generally infection occurs at the stage where first fruits are maturing.
First indications are the yellowing of the older leaves on one side of the stem. The leaflets also turn yellow on the one side of the petiole. The infected yellow leaves wilt and die. The disease spreads upwards until all the foliage is killed and the stem dies.
It is not uncommon for a stem of a wilted plant not to show any soft decay, but if cut lengthwise, the woody part next to the green outer cortex shows a dark –brown discolouration. The discolouration often extends upwards for some distance and is especially evident in the petioles of wilted leaves at the point where the join the stem.
Fruits are not spotted when the plant is infected with fusarium wilt. The fungus enters through the roots and passes upwards through the roots. Toxic substances are excreted by the fungus which blocks and discolours the pathways for water transport. This is the main cause for the plants wilting. Fruits that are infected decay and drop.