While there are at least three vegetable diseases called “black rot,” including a rot of pumpkins and a rot of sweet potatoes, I am the only bacterial black rot. The importance of this crucial difference will become apparent later.
My given name is Xanthomonas campestris pathovar campestris or Xcc for short. From the “xanth-“ part of my Greek surname, Xanthomonas, you can tell that I’m yellow, a nice cheery butterscotch yellow, that is, when I’m growing in a laboratory culture dish and have more than enough to eat.
Out in nature, where I have to fend for myself, I’m often not visible, at least not until I get together with, oh, 99,999,999 of my closest friends. That’s per quarter square inch of leaf, or a spot smaller than a dime. Yeah, it’s pretty crowded, and a crowd can get a bit rowdy and cause some mischief. That’s when the party really gets going, because that’s when we’re strong enough to force our way into a leaf.
Even though you still can’t see us, a few days later you will see how the leaf reacts. A yellow spot will develop, usually starting at the edge of the leaf, where we slipped in through a pore called a hydathode. A few days later, the leaf veins inside the yellow spot turn dark purplish black. That’s where the “black” part of the name “black rot” comes from.
Don’t ask me what the “rot” refers to. I am blamed for the rot, but that’s not my responsibility, honestly, even if I leave the door open and let other rotters slip in. By the way, I don’t smell. The odor comes from the cabbage leaf, not me.
Don’t you love all the rain we’ve had this summer? I sure do. A nice wet leaf makes life so much easier for us bacteria. It’s easier to grow and easier to move around, that is, find a new home (plant) in this crazy housing market.
My favorite foods are any vegetable in the cabbage family, which gives me lots of choices. If I want something peppery, there’s horseradish, radish and turnip. For salad, there’s baby kale, arugula and cabbage. For variety, I might have some Chinese cabbage, broccoli raab (Chinese broccoli) or mizuna, a Japanese turnip green. Of course, I love the southern greens: collards, turnip and mustard. Don’t offer me anything else, though, because I won’t bite.
Over the years, I’ve found the perfect place to hide. After the damage to the plant is done, I hide in the seed. It’s totally awesome! The hard seed coat protects not only the embryo plant but me, too. Even better, I, or more precisely, my offspring, don’t have to spend time or energy looking for a new, fresh plant to colonize. Our new home is right there in the seed.
When the seed germinates, we just chill and give the young plant its space to get going without interfering too quickly. By the time the first or second true leaf appears, though, we are ready to go. Soon the typical, triangular yellow leaf spot with black veins shows up.
You would think that by looking for the symptoms of black rot, it would be easy-peasy to eliminate contaminated seeds by getting rid of diseased plants before the seed is harvested. Sometimes I slip into my “Cloak of Invisibility” and sneak up the flower stalk and into the young seed without being noticed. That works best when the weather is cool.
Scientists have devised tests to check cabbage seed to see if I’m hiding in it. Lucky for me, they can test only a small percentage of the seed from one field, so some of my progeny might slip through anyway.
As a bacterium, I’m naturally immune to most fungicides and biopesticides that plant pathologists recommend. That’s one reason black rot has been called “the most important disease worldwide of vegetable brassicas.” Pretty impressive, right?
Even the one product that usually can hurt bacteria, fixed copper fungicides, doesn’t bother me that much. Once I slip inside the plant, copper can’t touch me, thank goodness.
If you grow brassica vegetables, we’ll probably meet each other sooner or later.