Vegetable Seedlings or Transplant Leaves Yellowing, Turning White, or are Spotted or Scorched

Leaf yellowing

Environmental stress 

• Leaf yellowing and discoloration of seedlings and transplants can be caused by a wide variety of factors. Some of the environmental stressors that contribute to leaf yellowing are temperature extremes; wide temperature swings; cold, cloddy, compacted, or waterlogged soil; drought; high winds; and poor quality seeds or transplants. Providing optimum conditions for good growth at this early stage will help ensure healthy growth and good yields through the season. Follow these cultural recommendations
• Plant in well-drained soil high in organic matter.
• Use high-quality seed and transplants. Check transplants prior to purchase. Avoid plants with roots that are brown and growing around the bottom of the container.
• Keep soil evenly moist and fertilize with a balanced soluble fertilizer after seedlings emerge or after transplanting.
• Protect plants from wind and cold with row cover material, a cold frame, or a cloche (e.g. an empty 1-gallon plastic milk jug with the bottom removed) over each individual plant. 
• Avoid damaging plant roots through cultivation, tilling, or walking on the soil.
• Low yields and reduced eating quality can be expected if plant growth is checked significantly at any point in the life cycle from seedling to fruit maturation.

Nutrient deficiency

• Leaves may yellow and die if sufficient nutrients are not available to plant roots.

Spider mites

• Spider mites suck chlorophyll from leaf tissues, creating fine white spots referred to as stipples and also resulting in leaf yellowing.

Whiteflies

• Whitefly feeding produces stippling of leaves, followed by yellowing; drying and distortion, and premature leaf drop in heavy infestations

Whitened leaves

• Frost and cold injury cause leaves to turn white.
• Frost injury is commonly seen in the early spring when warm-season vegetable crops experience night temperatures below 32° F.
• Affected leaves out-grow the injury if plants are healthy and not stressed further by low temperatures.

Spotted or Scorched 

• Spotted or scorched leaves are often caused by sunburn -- direct sunlight striking leaves. This occurs more frequently in early spring on plants with poorly established root systems or plants that are not properly hardened-off. 
• Other causes are fertilizer burn from foliar applied fertilizers, cold water sprayed on plants, or pesticide spray injury.
• Healthy plants will outgrow the injury.