Root rots of floricultural and woody ornamental crops are one of the most important causes of crop loss. In addition to killing plants and thereby reducing the quantity of salable crop, root rots can also slow or stop plant growth and thus suppress plant quality. Root-rotted plants are usually smaller, less vigorous, produce fewer and/or smaller leaves, flowers, and fruit than healthy plants of equal age. Flowering may be delayed when the plant's roots are rotted. As a result, the crop quality is very uneven. Root rots must be managed early in the disease if the losses are to be avoided.

Management in established plants is based on:

• Proper diagnosis of the cause of the root rot.
• Proper use of fungicides.

Symptoms

• Growth of infected plants slows as compared to healthy plants.
• Older leaves yellow and fall.
• Margins of leaves die.
• Roots appear dark brown or black and few or no white roots or root tips can be found when the root ball is washed free of soil. *Note that although some plants naturally have brown dark-colored roots (azaleas, rhododendrons), the last two symptoms below are not characteristic of those plants if their roots are healthy.
• Roots are limp and not brittle and crisp as is found in healthy plants of all types.
• When plants are pulled from the potting mix, the outer layer of cells strips off the roots leaving only the central strand of water conducting tissue.

The above symptoms can be caused by:

• Root rotting fungi
• Overfertilization
• Too much or too little water
• Root exposure to chilling or freezing temperatures
• Phytotoxicity due to the mishandling of pesticides when used as soil drenches
• Damage as a result of a plant becoming potbound (primarily to roots next to pot)

Before any action is taken, a diagnosis must be made of the actual cause of the symptoms. If the damage is due to the activity of fungi, chemicals are sometimes available that can check the fungus and allow the plant to grow. The dead roots do not recover. New roots must grow. The fungus is usually not completely eradicated (killed) by chemicals. Some fungus usually remains alive although its growth is greatly slowed as long as the fungicide is in high enough concentration. Therefore, repeated applications of fungicides are necessary.

Diagnosis

Many fungi can cause root rots. Often, it is possible to identify which fungus is responsible either by observing the structure of the fungus in the roots using a microscope or by placing infected roots on artificial media or baits (apple, carrot, or potato pieces) and allowing the fungus to grow out where it can be detected and then identified. This is done at the Plant Disease Clinic.

Management Using Fungicides

No single fungicide will kill or inhibit the growth of all fungi. In fact, some chemicals will allow certain fungi to cause more damage than usual because the chemical eliminates fungi that compete with the disease-causing fungus. However, fungicides are effective against "groups" of fungi and therefore a different chemical is not needed for each and every fungus.

• Obtain a diagnosis of which fungus or fungal group is the cause of the root rot.
• Obtain a recommendation of which fungicide is effective in managing that particular fungus or fungal group. The chemical label of each fungicide notes the fungi against which the chemical is effective.
• Be certain the name of the crop to be treated is on the label of the fungicide to be used.
• Note the rate at which the chemical is to be used when applied as a soil drench. Read the label.
• Wear goggles, waterproof gloves, boots, coveralls (water resistant or waterproof), a respirator, and other recommended safety equipment when mixing and applying the chemical.
• If two different fungicides are to be applied, check the chemical labels of each, any available compatibility charts, or check with your cooperative extension agent before mixing them together. Otherwise, apply each separately.
• It is recommended that the proper concentration be prepared and applied rather than using a proportioner to dilute the chemical from a bucket of chemical concentrate because a) the wettable powder fungicides must be agitated constantly to prevent settling out b) proportioners are not designed to dispense suspensions c) fluctuations in water pressure and hose length or obstructions to water flow in the hose results in inaccurate dilution of the chemical. If too much chemical is injected, phytotoxicity may result. If too little chemical is injected, the concentration of fungicide needed to stop fungus development will not be delivered.
• Repeat the application of the chemical at the time interval recommended on the label. Additional treatments are often needed because the fungi are seldom totally eliminated by a fungicide. Check the label for the proper dose to use on repeat applications.

Fungicides Used to Manage Root Rots in Established Plants


For Pythium and Phytophthora:

Aliette* (fosetyl-Al or phosethyl-Al)

Subdue* (metalaxyl)

Subdue Maxx* (mefenoxam)

Terrazole*, Truban*, (etridiazole)

Banol* (propamocarb)

Banrot* (etridiazole + thiophanate methyl)

For Fusarium, Rhizoctonia, Thielaviopsis, and Cylindrocladium:

Banrot* (etridiazole + thiophanate methyl)

Clearys 3336*, SysTec 1998, Domain FL* (thiophanate methyl)

Chipco 26019* (iprodione)

Medallion* (fludioxonil) for Rhizoctonia

Terraclor* (PCNB) for Rhizoctonia

Terraguard* (triflumizole) for Cylindrocladium

*Trade name