By: Bonnie L. Grant, Certified Urban Agriculturist
Scab can affect a wide variety of fruits, tubers, and vegetables. Scab on vegetables and fruits causes malformed and damaged crops. The crop may become infected by bacteria or other organisms. Learn how to treat scab disease to prevent further scarring and damage. Management of your garden site can prevent future crops from becoming affected by the disease.
What is Scab Disease?
Scab is usually caused by Cladosporium cucumerinum. These fungal spores overwinter in soil and plant debris and become most active and reproductive in spring when temperatures begin to warm and there is plenty of moisture.
Scab on vegetables may also be introduced to your crops from infected starts, contaminated machinery, or even from wind-blown spores. Cucurbits, which include cucumbers, gourds, squash, and melons are particularly susceptible. It is also common on potatoes and some other tubers.
Scab of Cucurbits
Scab of cucurbits is the most commonly seen and affects melons, summer squash, cucumbers, pumpkins, and gourds. Only most strains of watermelon, however, are resistant.
Symptoms first appear on the leaves and present as water spots and lesions. They start out light green and then turn white and finally gray surrounded by a yellow halo. The center eventually tears away, leaving holes in affected foliage.
Unchecked, the disease moves to the fruit and produces small oozing pits in the skin which enlarge to deep sunken cavities.
Potato Scab Disease
Tubers such as potatoes are also often infected. Potato scab disease produces corky spots on the skin, which can go quite deep and affect the upper layer of flesh.
Potato scab is caused by a different organism, a bacterium. It lives in soil and can also remain in the earth over winter.
How to Treat Scab Disease
Are vegetables affected by scab disease safe to eat? They aren’t dangerous, but the texture and appearance is greatly affected. You can cut out the lesions and use the clean flesh of the edible.
When it comes to treating scab on vegetables, some scab disease responds to fungicide when applied early, just as the plant begins to bloom. However, prevention is easier.
Don’t overhead water and avoid working among the plants when they are wet. Remove all old plant material and rotate crops every three years if possible.
Use disease resistant plants and seeds and don’t start tubers from affected roots. If your soil is alkaline, acidify the soil with the appropriate amount of sulfur as spores dislike acidic soils.
Always use clean tilling and pruning tools to prevent the spread of the disease.
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Read more about General Vegetable Garden Care
Potato Pests and Diseases
Potatoes can be bothered by a variety of pests and diseases. Here are the most common and what to do about them.
Damaging pests can work quickly in a potato patch. Stroll through the plot regularly looking for insects and the damage they cause. It's a lot easier to deal with a pest before it becomes a disaster. If you choose to use sprays or dusts to prevent or control a pest problem, read the directions and follow them carefully.
Colorado Potato Beetle
This pest is present and working in just about every state. Destroy any potato bugs you see and check the underside of leaves for their orange egg masses. Both the adults, which are yellowish with black stripes, and the larvae, which are dark red or orange with black spots, feed on potato foliage. Pick them off or spray Bacillus thuringiensis San Diego on the young larvae. Bt 'San Diego' attacks only the potato beetle larvae and is harmless to beneficial insects, animals and humans.
Flea beetles are tiny, black or brown, and pesky. They chew small holes in plant leaves and can do serious damage fast if they attack young plants. To foil these pests, cover young plants with fabric row covers as soon as you set them out. Keep flea beetle populations low through crop rotation and by maintaining high soil organic matter.
These tiny insects can transmit virus diseases. They suck juices from the leaves and stems of potato plants, injuring them badly. Insecticidal soap sprays are an effective control.
Wireworms are the larvae of the click beetle. They're a problem when potatoes are planted in a section of garden that was recently sodded. Fully grown wireworms are 1/2- to 1 1/2-inches long. They're slender, brownish or yellowish white and tunnel into plant roots and tubers, spoiling them. If your soil is heavily infested, contact your extension service agent for advice on solving the problem, or try growing your potatoes in the "tower" fashion mentioned in our article "Planting Potatoes."
You may have a disease problem in the potato patch one year and none at all the next. The weather plays a big part in the health of a potato crop. Moisture and temperature conditions may trigger certain diseases, which will spread rapidly through the potato rows. But there's no need to simply sit back and let the environment determine the fate of your crop.
To protect your crop, rotate the potato plot each year. Plant healthy, certified seeds. If you have severe disease problems, consider using a standard potato dust or spray regularly throughout the season. These are chemical mixtures that prevent some diseases such as late blight. They thwart some pests, too, such as the Colorado potato beetle. If you use a potato dust or spray, read and follow the directions carefully. To be effective, most standard dusts must be applied to the potato foliage every 7 to 10 days, beginning when the plants emerge from the ground.
If you have questions about diseases or pests, your local cooperative extension service agent can help you. Here are brief descriptions of a few of the common potato diseases.
The fungus that causes common scab lives in the soil for many years. It's not active, though, when the soil pH is below 5.4, so if you have a serious scab problem, take a soil pH test. You may want to lower the pH by not liming or adding wood ashes to the potato section of the garden.
Early blight injures foliage and reduces overall yields. Affected leaves develop small, dark brown spots that often grow in size, and which eventually kill the leaves. Gardens in central, southern and eastern states are most susceptible. Planting certified seed and mulching with hay can prevent this disease.
Late blight is caused by the downy mildew fungus -- Phytophthora infestans, which triggered the Irish crop failures of 1845 and 1846. You'll notice the disease first by water-soaked areas on the leaves that turn brown and black as the leaf dies. The disease strikes often during cool, wet weather and may spread rapidly if the weather warms up. Plants can die in a severe case, and potatoes can be seriously affected, especially in storage. Plant certified seed and use a potato dust to guard against late blight.
Aphids can spread mosaic viruses, which cause potato leaves to curl and appear almost two-toned (light and dark green). Mosaic occurs throughout the United States and cuts down on the harvest, but it won't kill the plants. 'Kennebec' and 'Katahdin' varieties have some resistance to certain kinds of mosaic.
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What Is Scab Disease: Information About Potato Scab Disease And Scab Of Cucurbits - garden
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Scab, in botany, any of several bacterial or fungal plant diseases characterized by crustaceous lesions on fruits, tubers, leaves, or stems. The term is also used for the symptom of the disease.
Scab often affects apples, crabapples, cereals, cucumbers, peaches, pecans, and potatoes. Leaves of affected plants may wither and drop early. Potatoes are especially susceptible to common scab, caused by a bacteria ( Streptomyces scabies and related species) that spreads rapidly in dry alkaline soils. It can be prevented by avoiding the use of materials such as wood ash, fresh manure, and lime that will add alkalinity to the soil. Other disease-prevention methods include planting resistant varieties or disease-free seeds, tubers, and corms destroying diseased parts removing weeds rotating vegetables and flowers and regularly spraying plants with fungicides, if appropriate.
This article was most recently revised and updated by Melissa Petruzzello, Assistant Editor.