How do I recognize and prevent root rot in soil?

Contributed by: LaShY and snoofer
Images archived: 2002

Root Rot/Stem Rot is a common disease that affects hydroponics, soil and soilless systems. Growers running soil and soilless systems should be aware of the environment that pythium thrives in, and actively improve their room/planting site conditions. Soil growers do not have the luxury of looking at their roots, so they should anticipate problems and recognize the early symptoms of pythium infection.

Root/Stem rot is almost impossible to treat because it is systemic (internal) in nature. Even in an advanced stage root rot may go unnoticed, with above ground growth appearing normal. Growth and yield however, can be drastically reduced. Think of pythium as flesh eating disease for plants!

Some external symptoms to look for:

Slow growth for no apparent reason

Wilting leaves.
(Narky) Often the top of growth shoots will also become limp and buckle over

Leaves turn yellow starting at the bottom-most node, from the leaf tips inwards

Reddening root collar, turning to a brown/black ring girdling the entire stem.

Eventually this black ring will turn necrotic and the external tissues will appear "eaten" away. The internal vascular tissues will still function, and allow the transpiration of water and nutrients (The plant will appear healthy). Within a week, the dissolved stem will no longer support the weight of the plant, and it will fall over!

Common causes of pythium in soil:
Root rot is also known as damping-off disease because it commonly occurs when plants are in the seedling stage, due to:

1) Inadequate Drainage - MJ requires well drained soil, so add at least 25% Perlite to all soil mixtures. Substitute sterilized pebbles or rocks (be sure to bake this in your oven for 10-15 minutes to sterilize).

tip: put an inert medium (ie. gravel/peagravel/popocks) at the bottom of the container (with your soil/sloilless mixture on top) to avoid saturation of the lowest layers of the soil and provide full draining.

2) Overwatering Growers should allow their plants to (almost) dry out before re-watering (Do the Lift test). This wet-dry watering cycle helps to minimize over watering, stagnant water conditions and low dissolved oxygen within the medium.

3) Stagnant Water Soil moisture that is not absorbed rapidly turns stagnant; the plant quickly uses up any oxygen within the water, then is unable to respire further, resulting in moisture low in o2. Pythium thrives in low-oxygen (anaerobic) conditions.

Plants should be watered at the start of their daily light cycle, so they are able to transpire much (if not most) of that supplied water, resulting in a low soil moisture content and higher oxygen content during the low-growth night cycle. A highly porous soil medium combined with a low soil moisture % provides a higher gas exchange. This is important during night cycle root respiration/flower development.

4) Low light / Low temps Plants in low light conditions does not get adequate energy for healthy growth. Low light will cause plants to stretch, grow slowly, yield poorly and causes general stress. Lowlight also usually results in lower temperatures, resulting in slow growth and stress.

Under low light/cool temp conditions, soil tends to remain cool and saturated for extended periods. Under these circumstances, the plant is unable to actively transpire moisture out of the soil. Oversaturated soils do not allow adequate gaseous exchange, and oxygen becomes depleted.
Stressed plants are more susceptible to disease.

Solution: expose your outdoor plants to more light, allow the soil to warm up by removing excess surface mulch. Plant after June 1 to avoid lower soil temps. Plant on raised burms to increase soil temps and reduced moisture.

Other tips:
Growers can add 5ml/L of No-Damp (a fungicide) to their potting mediums when mixing up. No-Damp discourages root growth however.

(Narky) Stem rot is caused by overwatering and occasionally by mulching to close to the stem while a plant is young. Remove any effected areas of your plant immediately! (Be merciless) Correcting your watering practices and spraying with a fungicide can bring a plant back to health.