Fusarium Yellows

Fusarium yellows is a disease of the cabbage family.
 
Closely related to Fusarium wilt, Fusarium yellows sometimes goes by that name. More commonly, it is simply called yellows.
 
This disease is caused by Fusarium oxysporum f. sp. Conglutinans fungi. It occurs most often in crops grown during warm weather. The disease is at its worst when temperatures are between 80°F and 90°F (27°C and 32°C).
 
The pathogen generally enters damaged rootlets at transplanting time, so be gentle with those babies.
 
Symptoms of Fusarium yellows are easy to overlook at first. Infected plants become a paler version of their healthier selves. They develop a yellowish tint, rather than vibrant green. As the disease progresses, that discoloration becomes more obvious on one side of the plant. The leaves and stems on that side start to curve. Lower leaves, midribs, and petioles eventually wilt and die.
 
Symptoms then move to the upper portion of the plant. If you look at a cross-section of an infected leaf or stem, you can see that the sap has turned dark brown, much like black rot, which turns sap black. The other difference is that symptoms appear in the upper portions of the plant first when black rot is the culprit.

Infected plants should be removed and thrown in the trash.
 
These fungi can live in the soil for many years. The best preventive methods include crop rotation, removing plant debris at the end of the growing season, and growing your brassicas during cooler weather.