decay located in the roots; root decay is usually developed from the bottom up. and crown symptoms may or may not be visible.

Merriam-Webster Online Dictionary
root rot (noun)
any of various plant diseases characterized by decay of the roots and caused especially by fungi
root rot (Wikipedia)

Root rot is a condition in which anoxic conditions in the soil or potting media around the roots of a plant cause them to rot. This occurs due to excessive standing water around the roots. It is found in both indoor and outdoor plants, although it is more common in indoor plants due to overwatering, heavy potting media, or containers with poor drainage. The leaves of plants experiencing root rot often yellow and die, and if allowed to continue, the condition can be fatal.

Chickpea with root rot
Chickpea plant (Cicer arietinum) with root rot. Note the symptomatic discolouration in some of its leaves.

To avoid root rot, it is best to only water plants when the soil becomes dry, and to put the plant in a well-drained pot. Using a dense potting media such as one dug up from outdoors can also cause root rot. Plants from different environments have different tolerances for soil moisture: plants evolved for desert conditions will experience root rot at lower moisture levels than plants evolved for tropical conditions. In both indoor and outdoor plants, it can be lethal and there is no effective treatment, though some plants can be propagated so they will not be lost completely.

Many cases of root rot are caused by members of the water mold genus Phytophthora; perhaps the most aggressive is P. cinnamomi. Spores from root rot causing agents do contaminate other plants, but the rot cannot take hold unless there is adequate moisture. Spores are not only airborne, but are also carried by insects and other arthropods in the soil. It can be controlled by drenching carbendazim.[citation needed]

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