When vegetables begin to develop problems -- or at the very least, begin to look odd -- more than one factor may be involved. In the case of cauliflower (Brassica oleracea spp.), for example, you may find the head a different color than the classic white, while also noticing unusual growths on the plant. If the head is purple and leaves or stems are looking fuzzy, chances are the causes of the two unusual characteristics are unrelated.
Check the Variety
The most obvious cause of a cauliflower developing a purple head is that you've unwittingly purchased a purple cauliflower variety. Technically, this type isn't a cauliflower at all, but does taste like common white cauliflower. Check the labeling information on your seedling or seed packet. Varieties such as "Purple of Sicily" cauliflower are, botanically speaking, purple broccoli types (Brassica oleracea spp), although their heads develop cauliflower-like curds. Eat them raw or cooked. When cooked, they will turn a broccoli green.
When white cauliflower plants turn purple, it's likely that they have been exposed to the sun's rays without protection. Cauliflowers do best in sunny locations, but only when planted in time to mature during cooler months. At the height of summer, high heat and direct exposure to the sun's rays sometimes result in an increase in pigmentation, especially in the vulnerable plant heads. The curds can turn a purple-red color. To keep curds creamy-white, take the protective measure known as blanching. When cauliflower heads are blanched, their heads are protected by a covering. Often the plant's leaves are large enough to cover the heads, especially if held in place with twine or even a clothespin. Paper bags fastened around the heads can also keep cauliflower curds from turning purple.
Purple on Stems
If your entire plant hasn't turned purple, but only parts of the stem, a disease may be indicated. Although cauliflower isn't as susceptible to the disease known as black leg as cabbage is, it is possible for cauliflower to contract the fungal disease. If you find brownish spots on the stems of some of your cauliflower seedlings, check to see if they have dark purple edges, which indicates black leg. Remove the affected seedlings immediately. In future years, do not grow cauliflower where other brassica crops, including broccoli and cabbage, have grown previously.
Some fungal diseases can cause a fuzzy or "cottony" look on cauliflower plants, especially the foliage. If the leaves are fuzzy underneath and have yellow-to-brown spots on top, downy mildew may be the culprit. Rainy, cool springs and too-close plantings can contribute to the problem. Powdery mildew, which gives leaves a fuzzy aspect on both sides of leaves, can appear even in dry weather, especially if the cauliflower is too closely spaced. White mold, a third fungal disease, gives a fuzzy look to both stems and leaves, and can cause wilting and rotting of plant parts. The primary cause of white mold is soil that doesn't dry off on the surface.
Planting cauliflower in raised beds, and at a generous spacing, may reduce the incidents of downy mildew in future years. The increased air circulation resulting from wider spacing is also a good control for powdery mildew, as is planting in a sunny area and using slow-release fertilizer. If your garden practices seem to be encouraging white mold, use raised beds, perhaps in combination with drip irrigation that wets soil below the surface. Spacing the plants farther apart can also cut down on white mold problems. Whatever the type of fungal "fuzziness" that seems to be causing your cauliflower problems, destroying affected plant parts or seedlings when they appear may help prevent spread. Growing cauliflower and other brassicas in a different part of your garden each year can also halt the spread of disease.