ONE GARDENER TO ANOTHER: Diagnosing plant disease | Columns

ONE GARDENER TO ANOTHER: Diagnosing plant disease | Columns

If you’re reading a gardening article, chances are you are a plant enthusiast, as I am. There is something very satisfying about creating a garden full of healthy, thriving plants, shrubs and trees. But, what do we do when one of our babies shows signs of disease?

Of course, after having a mini panic attack, it would seem the best course of action would be to determine what is harming your plant so that you can fix it. While that may be true, effective disease control starts before the plant shows symptoms. In order to control the development of disease, a combination of four basic methods are implemented. These include cultural controls, resistant varieties, biological controls and the use of chemicals.

Cultural controls are an important part of disease control. Proper sanitation methods, such as removing dead or diseased leaves or entire plants, as well as disinfecting garden tools, lessens the chances of spreading disease, bacteria and viruses from infected to healthy plants. Crop rotation is another cultural control aimed at reducing the disease agents that live in the soil, called pathogens, by removing their preferred host plant.

Plant varieties that have been developed to be resistant to a particular disease agent are the most effective method of disease control. Resistant varieties have been developed to stunt rusts, root rots, wilts and viruses; unfortunately, there are more diseases than there are resistant varieties.

Biological control is the use of one organism, whether it be a fungus, nematode or bacteria, to impact the activity of pathogens in the soil. For instance, marigold roots release terthienyls, which reduces parasitic nematodes in the soil, yet the introduction of another nematode, steinernema feltiae, is used to control several fly species, including leaf miners.

Chemical controls are applied as protectants, preventing the disease from becoming established. Fungicide and bacterial sprays are popular for foliage protection. Very few are developed to eradicate the disease agent after the plant has become infected. Fungicides are aimed at killing the fungus causing the disease, but not all fungi can be controlled with a fungicide. For example, it has no effect on verticillium wilt.

Diseases are categorized into three groups according to the part of the plant affected. Foliar diseases affect the leaves, fruit or stems of the plant. Crown and root diseases can result in the death of the plant, since they are unable to function normally by moving water and minerals through the plant. Wilt disease is easily detected by the onset of drooping plant parts caused by bacteria, fungi and pathogens that create obstructions, blocking water from reaching the foliage.

Diagnosis and treatment of common plant diseases

• Powdery mildew – white, dusty coating on leaves stems and flowers;

Common on: apples, cucumbers, grapes, peas, daisies, lilacs, phlox and roses

Cause: prolonged wetness; usually does not occur on plants in full sun

Treatment: Remove infected plants; prevent with fungicide, proper watering

• Downy Mildew – upper leaves discolor, lower leaves have a white/grey mold

Common hosts: broccoli, cauliflower, lettuces, grapevines, columbine, impatiens, pansies

Cause: prolonged wetness; watering in the evening

Treatment: Remove affected plant parts, or entire plant if necessary; use preventive controls

• Verticillium Wilt – leaves wilt, turn yellow and drop prematurely

Common hosts: trees, shrubs, edibles and ornamentals

Cause: pathogens enter through the roots

Treatment: Fungicides are not effective; use preventive controls; remove annuals, perennials or edibles; prune affected branches from trees and shrubs

• Rust – spots on foliage are a rusty reddish-orange that turns black

Common hosts: tomatoes, beans, daylilies, hollyhocks, snapdragons, roses

Cause: Pathogens spread by air, water, insects; overhead watering; insufficient room between plants

Treatment: plant resistant varieties; remove affected plant parts; use drip irrigation water methods

• Root Rot – yellowing and wilting leaves; black roots

Common hosts: Yews, rhododendron, heather, English lavender, japonica

Cause: overwatering; poor drainage; fungus

Treatment: Visually inspect roots and trim black roots. Wash remaining root ball with fresh running water, dispose of old soil and replace.

• Fusarium Wilt – wilted leaves, root rot, plant growth is stunted

Common hosts: asparagus, beans, peas, tomatoes, dianthus

Cause: fungus found in soil

Treatment: no chemical treatment; plant resistant varieties, remove entire plant and do not plant the same plant in the area for several years.

• Black Spot – black spots on foliage which will turn yellow and drop

Common host: Roses

Cause: fungus

Treatment: use fungicide for prevention; remove affected leaves and canes

• Tomato Mosaic virus – blotchy yellow and green leaves, curling, may form bumps on foliage

Common hosts: tomatoes, beets, cucumbers, lettuce, peppers, squash, petunias

Cause: virus; commonly caused by rotting debris; easily transmitted

Treatment: no chemical treatment; plant resistant varieties; crop rotation

When using fungicides or other chemical treatments, follow the label directions and warnings. Diseased plant matter should be removed from the garden and disposed of in the trash. Use preventative measures, and keep an eye on your plants. Early detection means less stress to the plant.

Until next week, happy gardening.

— Irland, a member of the Limestone County Master Gardeners, can be reached at Visit for more information on the Limestone County Master Gardeners.

2021-05-10 12:00:00

Leave a Reply

Your email address will not be published. Required fields are marked *