Black Root Of Radish : How To Treat A Radish With Black Root
By: Bonnie L. Grant, Certified Urban Agriculturist
Radishes are quick to produce from seed to harvest. If your roots have dark cracks and lesions, they may have black root disease. Radish black root disease is very contagious and causes severe economic losses in crop situations. Good cultural practices can help minimize the occurrence of the disease.
Symptoms of Black Root of Radish
Black root in radish is a fairly common disease in cool, wet soils. It can occur at any point in plant development, resulting in seedling death or rotten roots. Black root of radish has no cure, but there are several cultural methods that can help protect your crop from this fungal disease.
The signs of radish black root disease are unmistakable once the roots are harvested, but initial symptoms can be a little trickier to recognize. In the earliest infections, seedlings will die quickly. More established plants will develop yellowing at leaf margins in a wedge shape. The veins will begin to turn black.
A radish with black root that exhibits leaf signs is already developing dark patches on the root. These spread and become cracks and fissures that turn necrotic. The entire root soon becomes black, hence the name of the disease. All plants with signs of the disease should be destroyed, as it is very contagious.
What Causes a Radish with Black Root?
The culprit is a fungus-like organism named Aphanomyces raphani. The organism attacks not only radish but other crucifer vegetables. Cool, wet soil encourages the growth of the disease. Rounded root types seem to be less susceptible to black root than the elongated root forms. Some, such as French Breakfast, can even be planted in areas where previously contaminated crucifers were housed and will remain relatively blemish free.
The disease spreads by wind, water splash, insects and animals. It can also be harbored on host plants in the crucifer family or in plant waste. The organism can survive in soil for 40 to 60 days, giving it the potential to re-infect a new crop.
Preventing Black Root in Radish
Crop rotation every 3 years seems to be the most effective way of preventing disease. Clean up old plant debris and remove crucifer type plants in a 5-foot (1.5 m.) radius.
Sow seeds in raised beds with excellent drainage. Keep air circulation free around the plants. Practice good cultivation methods and sanitize tools.
Solarization of soil may be beneficial. There are currently no registered fungicides for treatment of the disease. Use plant varieties that are resistant such as:
- French Breakfast
- White Spike
- Red Prince
- Belle Glade
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Learn About Radish
How to Sow
- Sow radish seeds in well-worked soil after danger of frost in early spring, again in late summer for fall crop. In frost free areas, sow in the fall.
- Sow thinly in rows about 6" apart. Cover with 1/2" of fine soil firm lightly and keep evenly moist.
- Seedlings emerge in 4-6 days.
- Thin to stand about 2" apart.
- Sow winter radishes in midsummer for late fall and winter use. They can grow large with several inches of root showing above ground.