Damping Off: If seedlings fail to emerge or starts die quickly, damping off is a likely cause. It’s caused by a variety soilborne fungi and fungus-like pathogens, most commonly Pythium, and occurs most often in cool, compacted, wet soils. Typical symptoms of infection include seeds that rot before they germinate, young sprouts that are mushy or decayed, and starts that die rapidly after they emerge. Start seeds in well-drained soil at a time when temperatures are warm, or under whatever conditions are needed to encourage rapid germination and initial growth of seedlings. If problems persist, sprout seeds indoors in a covered glass dish before planting, or sterilize soil mixes prior to starting seeds.
Powdery Mildew: Powdery mildew appears as white or whitish circles or whitish layers on the upper or lower surfaces of leaves. Leaves slowly yellow and/or develop dead spots, which may cause powdery mildew to be mistaken for other problems, such as nutrient deficiency. Powdery mildew tends to become a problem in warm-to-hot, dry weather. Powdery mildew is far more common on vegetables in your zone than downy mildew, which is uncommon to rare on all plants in most areas in your zone. How do you distinguish powdery mildew from downy mildew? Damage from downy mildew tends to appear initially as yellowing spots that are often angular and limited by leaf veins. Some mildews can be discouraged using a nontoxic homemade spray of one tablespoon each of baking soda and tea tree oil per quart of water, shaken vigorously and regularly as applied via spray bottle. Mildews are often difficult to control and are best managed through prevention and good cultural practices, such as planting resistant varieties, planting in areas with good air circulation, allowing extra space between plants, keeping foliage dry, less-frequent irrigation, encouraging biologically active “living soil” that will support strong plant immune systems, and rotating crops. Mildews are often not fatal; plants may produce reasonable yields of vegetables and greens may produce fresh uninfected leaves despite ongoing infections on older leaves.
Root Rot: Various bacteria and fungi may cause rot and kill plants, especially in wet soils. Dry out soils, encourage a healthy soil ecosystem, and avoid replanting the vegetable and its relatives in the same area for at least 1 to 3 years after difficulties with disease.