It’s hard to know, as several things can cause yellow leaves. Fusarium yellows can turn celery leaves yellow, but not sure if it attacks at seedling stage; make sure the soil that you used is sterile. Aster yellows is similar, and is spread by leafhoppers. Check for any pests, such as aphids or leafhoppers. Were your seedlings growing in nutrient rich soil? If not, perhaps it might be that they needed fertilizer. Some nutrient deficiencies can also cause yellow leaves, sometimes affecting either older or younger ones first, or between leaf veins. Make sure that the seedlings have the proper amount of light, and temperature, to make them happy. Check the water amount—curling, yellowing, and drooping limp leaves may indicate overwatering. Wilting or drooping, brittle yellowing leaves, underwatering. Hope this helps!
It’s hard to know, as several things can cause yellow leaves. Fusarium yellows can turn celery leaves yellow, but not sure if it attacks at seedling stage; make sure the soil that you used is sterile. Aster yellows is similar, and is spread by leafhoppers. Check for any pests, such as aphids or leafhoppers. Were your seedlings growing in nutrient rich soil? If not, perhaps it might be that they needed fertilizer. Some nutrient deficiencies can also cause yellow leaves, sometimes affecting either older or younger ones first, or between leaf veins. Make sure that the seedlings have the proper amount of light, and temperature, to make them happy. Check the water amount—curling, yellowing, and drooping limp leaves may indicate overwatering. Wilting or drooping, brittle yellowing leaves, underwatering. Hope this helps!