Unfortunately, plant root damage isn’t visible until the damage has become so great that it affects the above-ground portion of your container plant. Root rot is one common root problem, caused by fungus that thrives in overwatered potting soil.
Root rot is a common problem with beginning gardeners who aren’t sure how to correctly water plants (just remember that it’s better to provide too little water than too much!). When plant roots sit in excess water, the roots are deprived of much-needed air, and they decay. Airborne fungus that also causes root rot can only cause damage in overwatered garden soil.
When roots are rotted, the plant can no longer take up nutrients to the top leaves. This results in yellowed plant leaves and no new growth in the container garden. When plants get root rot, they usually will not survive, so do your best to avoid overwatering your container garden plants in the first place. Also make sure all your plant containers are filled with well-draining potting soil and have adequate drainage holes. Never let your plants sit in a tray filled with water. Soggy potting soil will almost always cause problems (it will also attract mosquitoes and smell!).
There are different types of plant rots caused by fungus, including crown rot and foot rot. Certain plants are more susceptible to different rots in the container garden. Fungus in the genus Phytophthora affects rhubarb, parsnips, carrots, etc., and the genus Didymella affects tomatoes, eggplant, etc. These rots can cause other parts of the plant to rot. To prevent fungal infections, do not use old potting soil mixes (especially those that could be infected), use mulch, destroy affected plants and sterilize their pots before using them again. Most importantly, water your garden plants regularly, without splashing water or bits of potting soil up onto the leaves, and remove dead foliage from the container plant. Dead foliage at the base of the plant can promote infection near the plant's crown.