Fungal Root Rots and Chemical Fungicide Use

In addition to killing plants and thereby reducing the quantity of saleable crop, root rots can also slow or stop plant growth and thus suppress plant quality. Root-rotted plants are usually smaller, less vigorous, and 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:

1. Proper diagnosis of the cause of the root rot.
2. Proper use of fungicides.

Symptoms of Root Rot

• Growth of infected plants slows 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), symptoms nos. 5 and 6 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 rot fungi
• Over-fertilization
• 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 pot-bound (primarily to roots next to the 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 manage 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 fungi usually remain alive, although their 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.

1. Obtain a diagnosis of which fungus or fungal group is the cause of the root rot.
2. Obtain a recommendation on 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.
3. Be certain the name of the crop to be treated is on the label of the fungicide to be used.
4. Note the rate at which the chemical is to be used when applied as a soil drench. Read the label.
5. Wear the Personal Protective Equipment (PPE) required on the product label when mixing and applying the fungicide.
6. If two different fungicides are to be applied, check the chemical labels of each, and any available compatibility charts, or check with your Extension Educator before mixing them together. Otherwise, apply each separately.
7. 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.
8. 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.
9. After using a fungicide product as recommended on the label, assess whether further fungicide treatments will be needed.  The Fungicide Resistance Action Committee has developed a numbering system in which chemicals with the same FRAC Group number have the same mode of action. If additional fungicide treatments are needed, rotate to a fungicide with a different mode of action group or tank mix with a compatible fungicide with a different group number to help prevent the development of fungicide-resistant pathogens.

Contact the Penn State Extension for recommendations of fungicide products currently labeled for use on your crop in Pennsylvania.

Prepared by Gary W. Moorman, Professor Emeritus of Plant Pathology