Causes of Root Rot in Potted Chinese Cymbidium (Cymbidium sinense) and How to Treat It

They say rainwater is the best for watering Chinese Cymbidium, so you go to great lengths to collect rainwater to nourish it. Yet unexpectedly, the roots still rot and the leaves still turn yellow. Have you run into this same frustration? Root rot in Chinese Cymbidium mostly occurs in winter, largely because of poor soil drainage and overwatering. Many orchid growers do not know what to do about root rot in Chinese Cymbidium, nor do they know the harm it causes.

Chinese Cymbidium has poor cold tolerance. When grown in pots north of the Yangtze River basin, it is usually overwintered indoors. Because of the low winter temperatures, compacted soil, overwatering and fertilizer burn, the roots of Chinese Cymbidium are often prone to rotting. In mild cases this affects normal growth and flower bud formation and blooming; in severe cases it can kill the entire plant. So why does potted Chinese Cymbidium develop root rot? How can you tell whether a potted Chinese Cymbidium has root rot? And how do you rescue a plant that has already developed it?

Why does potted Chinese Cymbidium develop root rot? The main causes of root rot in Chinese Cymbidium are the following three aspects:

1. Poor soil permeability

Poor soil permeability is linked to watering. If the soil is fairly clayey, it easily becomes compacted, water sits on the soil surface for a long time, and the roots can hardly absorb any water.

2. Overwatering

The main cause of root rot is overwatering: the orchid plant does not absorb the water in time, and the potting soil stays wet or even waterlogged for a long time. When the soil is not aerated for a long period, the roots cannot get enough nutrients and begin to rot.

3. Fungal infection

Root rot in Chinese Cymbidium is generally a plant disease caused by fungi, nematodes and bacteria. Root rot is a soil-borne disease, spread mainly through soil moisture, soil-dwelling insects and nematodes.

The simple and direct treatment methods are:

1. Cut away the rotted parts

When you find that a Chinese Cymbidium has root rot, take the plant out of the pot. Depending on the degree of root rot, cut off the mouldy, blackened and hollow rhizomes, keeping the parts that have not rotted. At the same time, cut off the diseased leaves that have spots and have turned yellow. As long as the bulbs have not completely rotted, the plant can still survive.

2. Change the soil

Break old red bricks into pieces about the size of a hickory nut and mix them with bark at a ratio of 1:1. If you use only soil, add crushed red brick as well: put the larger pieces at the bottom, and the smaller pieces with the soil on top. However, so that the plant is not infected again by pathogens, when replanting, change to a clean plain clay pot; you can also use uncontaminated plain sand for planting.

3. Spray with chemicals

If a plant has root rot, some kind of pathogen is certainly present, so apply a fungicide promptly; use carbendazim as a root drench, 2 to 3 times.

4. Control water and fertilizer

In the care that follows, control water and fertilizer well. Water by the rule of "water when dry, stop when wet": stop once water flows out from the bottom of the pot, and water promptly when the pot surface dries out or even turns white. For orchid plants that have just been repotted, do not fertilize before they resume growth.

How do you judge whether a potted Chinese Cymbidium has developed root rot?

To tell whether a Chinese Cymbidium has root rot, mainly look at the leaf colour and the potting soil. When spring returns and the weather warms, if the leaves of the Chinese Cymbidium gradually lose their glossy, oil-green sheen, and feel dry, hard and wrinkled to the touch, you can make a preliminary judgment that the plant has developed root rot. Secondly, when the weather is fine, if the potting soil does not turn white and dry for several days, this also indicates that the plant's fleshy roots have lost their absorbing function. Of course, to judge the degree of root rot accurately, you must take the plant out of the pot to make a final determination.

What should you do about root rot in Chinese Cymbidium? The methods for rescuing a Chinese Cymbidium with root rot are:

1. Repot and prune away the rotted roots; use plain sand to encourage new root growth. When you have made a preliminary judgment from the leaves and potting soil that the root system of a potted Chinese Cymbidium has rotted, you can tip the orchid plant out of the pot and directly see the rotted parts of the root system and the extent of the damage. At this point, promptly cut off the mouldy, blackened and hollow roots, keep the parts that have not yet rotted, and thin out some leaves with disease spots. As long as the corm-like pseudobulbs have not rotted away, the plant can generally be treated and revived.

2. So that the plant is not infected again by pathogens, when replanting, change to a clean plain clay pot and use uncontaminated plain sand for planting. When planting, cover with plain sand only up to the base of the pseudobulbs; be sure not to bury the pseudobulbs. Finally, add a layer of clean culture soil or moss on top of the sand, then immediately place the repotted orchid in a cool, ventilated place with moderate shading.

3. During the process of the Chinese Cymbidium's root system recovering and regenerating, do not fertilize in general, and be sure to protect it from heavy rain. Only when the basal leaves regain their green colour and gloss and the leaves stand upright, and after small buds emerge from the base of the bulbs, can you resume normal water and fertilizer management. This process takes about 3—4 months.