These problems, diseases and pests are known to affect Broccoli plants:
Downy mildew: Yellow patches on leaves are usually caused by moist weather. Keep leaves as dry as possible with good air circulation. Buy resistant varieties. 4
Nitrogen deficiency: If the bottom leaves turn yellow and the problem continues toward the top of the plant, the plants need a high nitrogen (but low phosphorus) fertilizer or bloodmeal. Apply at planting, after the main head emerges, and after the main head is harvested 4.
Clubroot: Quickly wilting plants may be due to this fungus in the soil. The entire plant, including all roots and root tendrils, must be gently dug up and removed. If the roots are gnarled and misshapen, then clubroot is the problem. Act quickly to remove the plants so that the fungus doesn’t continue to live in the soil. Do not compost the plants. Raise the pH of your soil to above 7.2. You may need to sterilize your soil, too.
Hollow stem: A boron deficiency. If the plant grows very fast, and there is not enough boron available, the cells will lack elasticity and break, thus forming a hollow stem 5.
Side shoots: This is due to heat stress 5. Side shoots divert energy and resources away from the main head, but this may be desirable for some gardeners.
Bracting: The covering leaves grow through and distort the broccoli head (or “curd”), disfiguring it 5. This is caused by heat stress and/or water stress. Some varieties are more susceptible than others.
Cat eyes: Premature opening of small yellow flower buds 5. Some varieties are more susceptible than others.
Flea Beetles 4.
Aphids: Curling leaves may mean that the plant’s sap is being sucked by insects. Apply soapy water to all sides of leaves whenever you see aphids 4.
Cabbage loopers: Small holes on the leaves between the veins mean small green caterpillars are present. Look at the undersides of the leaves. Hand pick if the problem is small or control with Bacillus thuringiensis. Use a floating row cover just after planting through harvest to prevent caterpillars 4.
Cabbageworm (the larval stage of the white cabbage butterfly) and other worm pests: a common problem. Handpicking may control a light infestation. For organic control of a more serious infestation, Bacillus thuringiensis or spinosad can be used. Treat same as loopers 4.
Browning : Browning is a symptom of Boron deficiency that occurs in Broccoli.
Hollow Stem : Nutrient deficiency that affects Broccoli. Hollow Stem can be caused by excess nitrogen fertilization, especially during warm weather (no internal discoloration); or boron deficiency (dark brown internal discoloration) .
Broccoli buttoning : Broccoli "Buttoning" can be caused by nitrogen deficiency, cold temperature shock to young transplants, drought stress or other factors that markedly restrict vegetative growth
Ricing : “Ricing” may occur from excess nitrogen and high temperatures combining to favor very rapid growth.