Understanding Black Rot in Turnips: Causes, Symptoms, and Prevention
Black Rot, caused by the bacterium Xanthomonas campestris pv. campestris (Xcc), is a destructive disease affecting crucifers such as turnips. First identified in 1893 on turnips, this disease continues to pose a significant threat to growers today. Xcc spreads easily between plants and can survive in infested plant material in the soil for years, especially if buried deeply. It also impacts other cole crops, including close relatives in the Brassica rapa species like bok choy, as well as cousins within Brassica oleracea. Given its delayed symptom onset and rapid spread, prevention is critical, particularly for organic growers.
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In this comprehensive guide, we’ll explore how Black Rot spreads, its symptoms, and effective prevention and treatment strategies to protect your crops.
How Black Rot Spreads
The Xcc bacterium does not require wounds to infect plants. It often enters through water pores (hydathodes) on leaf margins that release sap in the morning. However, wounds from hail, insect feeding, or mechanical damage can also serve as entry points. The bacteria spreads easily through rain splashes, wind, machinery, animals, insects, and even irrigation or drainage water.
Once inside, Xcc can become systemic, infecting the veins of turnip plants. It thrives in warm, humid conditions, with optimal temperatures ranging from 75-95°F. The pathogen can survive in infected plant material in the soil for up to two months—or even years if deeply buried. It also persists on cruciferous weeds like wild mustard and shepherd’s purse. In greenhouses, Xcc can linger on organic matter, tools, benches, and other surfaces.
Experts advise against working in wet fields to minimize the risk of spreading the bacteria.
Symptoms of Black Rot in Turnips and Crucifers
Symptoms of Black Rot can take up to 14 days to appear, often making it too late to save an infected plant. The manifestation of symptoms can vary depending on the plant species, cultivar, age, and environmental conditions.
- Seedlings: Infected seedlings exhibit systemic infection, with yellowing of lower leaves as the first sign. The disease progresses, causing premature leaf drop and plant death.
- Mature Plants: A hallmark symptom is the appearance of V-shaped lesions along leaf edges, with the base of the “V” following a vein. Initially, veins may discolor, turning brown or black over time—giving the disease its name, Black Rot. Eventually, the entire plant may die, becoming a source of inoculum for nearby plants.
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Effective Strategies to Prevent Black Rot
Since Black Rot spreads rapidly and is challenging to control once established, prevention is the most effective management strategy. Key approaches include planting clean seeds or seedlings, practicing crop rotation, and minimizing soil and leaf wetness. This is particularly important in regions with warm winters, where the pathogen is more likely to persist in the soil, compared to colder areas like upstate New York.
1. Plant Clean Seeds
Xcc can survive under the seed coat, making bleach treatments ineffective. Planting infected seeds is a common cause of severe outbreaks. Opt for heat-treated turnip seeds whenever possible. If unavailable, you can treat seeds yourself by soaking them in water heated to 122°F for 15 minutes. Resources from institutions like The Ohio State University offer detailed guidelines for this process.
2. Rotate Crops with Non-Crucifers
The pathogen can survive on wild or cultivated crucifers and in the soil. To break the disease cycle, rotate with non-cruciferous crops for at least four years. This practice helps cleanse the soil of Xcc and reduces the risk of reinfection.
3. Use Clean Transplants
If you grow your own transplants, maintain strict hygiene in your greenhouse. Sterilize benches, tools, and reusable flats. Ensure compost is free from diseased plant material. Keep seedling leaves as dry as possible and avoid trimming or brushing plants when they are wet.
4. Reduce Wetness During the Growing Season
Minimize leaf wetness to prevent the spread of Black Rot. Water plants in the morning to allow leaves to dry during daylight. Increase spacing between plants and align rows with prevailing winds to enhance air circulation. Avoid sprinkler irrigation, as it can splash water and spread the bacteria.
Treating Infected Crops
If Black Rot is detected in your crop, immediate action can help limit its spread. Apply a bactericidal compound like copper hydroxide (e.g., Kocide 3000) at a rate of 0.5 to 7.5 pounds per acre. Consider combining it with Actigard™ (acibenzolar-S-methyl), which stimulates the plant’s immune system and may protect uninfected plants. However, treatment is often less effective once symptoms are visible, underscoring the importance of prevention.
Why Prevention Is the Best Control for Black Rot
Black Rot is a devastating disease that can remain undetected for up to two weeks, allowing it to spread unnoticed. The best defense is to prevent its establishment through proactive measures. Planting clean seeds, maintaining dry growing conditions, and controlling weeds are essential steps. If infection occurs, apply bactericidal treatments promptly to reduce further spread. Additionally, rotating with non-cruciferous crops for four years can safeguard future turnip plantings.
Have you encountered Black Rot in your turnips or other crucifers? Share your experiences and management strategies in the comments below—we’d love to hear how you’ve tackled this challenging disease!
