Most gardeners have heard of black rot. It’s mentioned in gardening books and troubleshooting articles. It’s whispered about by old growers and complained about by newbies. It strikes fear into the hearts of veggie gardeners across the world.
For a while, all I knew was that black rot meant death to any plants infected by it. It came to my little apartment garden one warm, rainy summer and wiped out my kale, radishes, and turnips. That’s when I went searching for answers.
Unfortunately, I was too late to save my little garden, but these days I know what to do. The apartment and its postage-stamp patch of earth are long gone, and so are my troubles with black rot. Knowing this sneaky disease, that could change at any moment. Here’s what I learned so you, too, can dodge the dreaded black rot.
What is Black Rot?
Many diseases go by the common name of black rot, so let’s clear up a little confusion. I’m focusing here on Xanthomonas campestris pv. campestris, a bacterial disease that primarily, but not exclusively, affects vegetables in the crucifer family. Think broccoli, cabbage, kale, arugula, and radishes.
It’s considered one of the most important and destructive diseases of plants in the Brassica family.
Xanthomonas campestris pv. campestris tends to affect the aerial parts of these plants first, doing only minor damage to the root – until the whole plant dies, that is. The leaves and stems show dark brown or black lesions that are dry and dead. Then, the disease spreads throughout the infected plant.
When the infected leaf is cut open, it exudes a sticky, rotting residue – and this residue is spreading throughout the plant – clogging its veins and depriving it of nutrients.
Black rot can appear anytime throughout the life of the plant. It can wither away young seedlings or riddle mature cabbages with rotting black patches. Few garden diseases are more overwhelming and frustrating to confront.
What Black Rot Isn’t:
This article is not about fungal diseases known as black rot. There are a lot of “black rot” fungi. If you’re growing grapes, fruit trees, citrus, or sweet potatoes, there’s a different black rot for you.
Botryosphaeria obtusa or Physalospora cydoniae are fungal diseases that affect apple, pear, quince, and cherry trees. Guignardia bidwellii is a fungi affecting grapevines. It’s known as black rot or grape rot interchangeably. In sweet potatoes, a different fungal black rot causes the disease Ceratostomella fimbriata.
I’m also not writing about potato black rot (Erwinia atroseptica), which is caused by a different bacterium than Xanthomonas campestris pv. campestris. Don’t let the same common name confuse you – make sure you’re hunting for the right Latin name.
Who knew there could be so many different meanings behind one, simple name!
How to Recognize Xanthomonas campestris pv. campestris
So how do you recognize black rot among your crucifers? First, know the conditions that contribute to the spread of black rot. Heat, humidity, and close-quarters work together to give this disease an ideal environment in which to spread.
Continual humidity, drizzly, overcast days, and lack of airflow between plants also create prime conditions for black rot bacteria to bloom.
When it first starts to affect your plants, you’ll see yellow, V-shaped lesions on the leaves. These yellow lesions are dry. When you touch them, they’ll feel fragile and wilted. As the disease progresses, the yellow lesions turn dark brown or black.
Black rot then spreads down the leaf and throughout the plant. Inside, infected plants are full of think, slimy bacterial excretions. Eventually, this disease turns your once-lovely kale plants into a pile of black mush.
Worst of all, it’s almost impossible to stop black rot from devouring your plants once they’re showing symptoms. Instead, the best way to fight Xanthomonas campestris pv. campestris is by preventing it altogether.
Stop the Spread
When you notice the early signs of black rot on your plants, take immediate action. Pull up any plants with the V-shaped lesion and burn them. Then, clean up the soil to remove any traces of infected plant material. This includes fallen leaves, weeds, and other debris. If your plants are growing close together, thin them out to give the air a chance to circulate.
Unfortunately, there’s no guarantee that Xanthomonas campestris pv. campestris isn’t in the soil already. This bacteria spreads easily and can settle into your garden for up to four years. Practice rotational gardening to keep black rot from destroying your harvest again and again.
Solarizing the Soil
After an infestation of black rot, try solarization for a year to kill off the bacteria hiding in the soil. To solarize, clear the garden bed of all plants and debris. Then, cover it with clear plastic – weigh down the edges and leave the plot covered for at least a month. You should solarize during the hottest, sunniest part of the summer.
Solarization heats up the soil and kills off soil-based pathogens – including black rot. Once solarized, your soil should have a lot less bacteria living in it. That applies to both good and bad bacteria, however. After the solarization is done, you should work hard to restore balance to your soil by working in lots of compost.
Of course, you can’t grow anything in a bed while solarizing it. But sometimes you can plant late season crops in a solarized bed right after cleaning up the soil. It’s frustrating to leave a spot in your garden unplanted, especially if you have a small garden. But remember, solarization will help next season’s plants grow better and healthier. It’s worth the wasted time.
For more information on solarizing, you might be interested in our guide.
Once you notice signs of black rot in one plant, treating with a copper fungicide like the one made by Bonide can help stop the spread. Copper won’t heal your plants, and it can’t drive out Xanthomonas campestris pv. campestris completely, but it can help slow the spread.
Pull the infected plants. Remember to either burn them or throw them in the trash. Don’t compost diseased plants or you’ll just keep spreading the problem around. Then treat the rest of the garden with an anti-fungal spray containing copper.
Of course, crucifer black rot isn’t a fungal disease, but thankfully it’s still susceptible to copper. This should slow the spread and save some of your harvest.
Prevention is the Best Cure
The best way to beat black rot is to keep it far from your garden in the first place. That’s easier said than done, but there are a few options.
Buy seeds of black rot-resistant varieties, or seeds heat-treated to prevent black rot. Unfortunately, many cases of Xanthomonas campestris pv. campestris start with infested seeds. Many seeds carry this bacterium and are just waiting for the right conditions for an outbreak.
Keep your garden clear of decomposing leaves, weeds (many of which can be carriers of black rot), and other debris. Wild crucifers like shepherd’s purse and wild mustard can be carriers of black rot.
Make sure the air can move freely around your garden and in between your plants. Airing out the garden makes it harder to maintain those warm, humid conditions that black rot needs to spread.
Sometimes, we get into habits that are hard to break. Remember that in hot, humid weather, spraying down the garden with a hose isn’t always necessary. It can even be damaging. Too much water during an already wet season creates a perfect atmosphere for black rot.
If you have to water in humid weather, avoid getting the leaves of the plant wet. Water at the base of each plant to give the roots moisture, while the rest of the plant stays dry. Since plants have a hard time drying off in humid weather, wet leaves can create excessive dampness and the perfect environment for bacteria growth.
Space Plants Appropriately
Don’t crowd out the plants in your garden. Tightly packed plants create an easy opportunity for Xanthomonas campestris pv. campestris to hop from plant to plant. Give your plants the full, recommended planting space between each one. Then, maintain that spacing with regular weeding and thinning.
Change up your garden plan each year. Try to avoid growing crucifers in the same spot year after year. If you can, give your crucifer plot a year or two of tomatoes and peppers before bringing the cabbages and broccoli back. Rotational gardening will help keep your garden free of black rot and other plant-specific diseases.
It may seem counterintuitive to soak seeds to fight black rot, but this really does work. If you’re buying seeds that aren’t heat-treated or naturally resistant to black rot – treat them yourself!
All you have to do is soak the seeds in water that is about 120°F. Leave the seeds in the hot water for 25 minutes before scooping them out and letting them air dry. If there is any bacteria in the seed, this hot water bath will kill it.
The hot water soak can also be used to treat other bacterial diseases like Alternaria leaf spot and black leg.
Choose Resistant Varieties
Look at seed packets before you purchase them to see if the particular cultivar you’re considering is resistant.
Cabbage cultivars that are resistant include: