Viruses are complex single molecules that act like living organisms. There are more than 200,000 of these marauding molecules to an inch. To identify viruses, plant pathologists usually group them together by what they do to the plant. Viral diseases often show up as a distortion (puckering or curling) of leaves, flowers, or fruits; stunted plants; yellow streaking; or mottled leaves.
A few of the viruses that affect vegetables are aster yellows (plants are stunted and yellow), curly top (dwarfed plants with bunched, curled leaves), mosaic (leaves have mottled yellow or light green areas), ring spot (yellow or brown concentric rings), and yellows (plants are uniformly yellow, may wilt and die).
Curly Top Virus. Courtesy T.A. Zitter, Cornell University
Mosaic Virus. Courtesy T.A. Zitter, Cornell University
Ring Spot Virus. Courtesy Missouri Botanical Garden
There is no chemical control for a virus. Good cultural practices and use of virus-resistant vegetables provide the best protection. Since many viruses are spread by sucking insects such as aphids and leafhoppers, you can also limit the viral disease in your garden by controlling insects that spread the diseases.
Parasitic nematodes that attack plants are vigorous, slender, tiny roundworms. They are generally included under diseases because the symptoms they cause are similar.
Most nematodes are harmless because they feed on decomposing organic material and other soil organisms. Nematodes that attack living vegetables suck the green color and cause stunting of vegetables, wilting, dieback, and similar signs.
The root-knot nematode causes galls to form on the roots of many vegetables. The first indication of nematode injury in a garden or field is often the appearance of small circular or irregular areas of stunted plants with yellow or bronzed foliage. This area gradually enlarges. There are a number of chemical and nonchemical ways to protect your vegetables from nematodes.
Root-Knot Nematode. Courtesy Missouri Botanical Garden
(Chemical control was discussed under each of these diseases in previous blogs.)
What to look for: Dark green spots or streaks that later turn gray, brown, reddish; can ooze gelatinous fluid.
Prevention and/or natural controls: Rotate crops; keep plants vigorous by fertilizing.
Soft bacterial rot
What to look for: Infected areas on leaves, branches, or fruit bordered by yellow or tan area; advanced infection causes large sunkened dark areas, frequently oozing gelatinous fluid.
Prevention and/or natural controls: Avoid planting in undrained soil; rotate crops on long rotation.
Bacterial wilt (spread by insects)
What to look for: Plants wilt and die; symptoms identical to fusarium and verticllium wilt.
Prevention and/or natural controls: Destroy infected plants; grow resistant varieties.
What to look for: Grayish patches on upper surface of leaves (powdery mildew); pale green or yellow areas on upper surface, light gray or purple below.
Prevention and/or natural controls: Rotate crops; avoid overhead sprinkling; plant resistant varieties.
What to look for: Yellow, orange, red, or brown pustules on underside of leaves and stems.
Prevention and/or natural controls: Destroy nearby weeds that show rust; collect and destroy infected plants when first seen.
What to look for: Stems, leaves, roots, and/or fruit become mushy and spongy.
Prevention and/or natural controls: Plant in well-drained soil; collect and destroy infected material and plant debris; keep fruit off soil.
What to look for: Sunken or swollen discolored dead areas on stem that sometimes girdle stem.
Prevention and/or natural controls: Destroy infected plants; use four-year rotation; purchase healthy-looking plants.
What to look for: Roughened crustlike raised or sunken area on surface of leaves, stem, fruit, roots, and tubers.
Prevention and/or natural controls: Practice long crop rotation; plant resistant varieties; remove weeds.
Fungal Leaf Spots/Blight
What to look for: Spots on leaves; centers may fall out; spots may enlarge to form blotches.
Prevention and/or natural controls: When severe, collect and burn infected material.
What to look for: Leaves turn pale green to yellow; plants wilt and die.
Prevention and/or natural controls: Use resistant varieties; practice long rotation; collect and destroy infected plants.
What to look for: Dark brown to black sooty-looking masses inside swollen white blisters.
Prevention and/or natural controls: Pick off and burn infected parts before blisters open; grow resistant varieties.
What to look for: Distortion of leaves, flowers, fruit; stunted plants; yellow streaking or mottling.
Prevention and/or natural controls: Destroy diseased material; keep down weeds.
What to look for: Yellowing, stunting, wilting, dieback; knots on roots.
Prevention and/or natural controls: Destroy infected plants; plant resistant varieties; rotate plantings.
© Copyright by Karen Newcomb
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