The most common disease which affects mustard and turnips is white spot. Symptoms of white spot include small, circular, gray to brown or almost paper white spots, with slightly darkened margins. These spots rapidly increase in size and number, and over a period of a week or so the entire leaf may become blighted.
Leaves turn yellow and eventually die.
The key to keeping greens blight-free is to start a control program before white spot becomes widely distributed. Gardeners who have plantings of greens where white spot disease is just beginning to appear should consider application of a fungicide. Fungicides will protect greens from infection, provided spray applications are made on a preventive basis. Don't wait until disease symptoms are widespread on your greens before applying a fungicide. For best results, start a fungicide application program within two to three weeks after plants have emerged and continue through the season.