It's July. The monsoon rains have finally lifted and the ground is starting to bake. You head out to the garden one morning and find that tomatoes that looked perfectly healthy yesterday are hanging their leaves limp. You assume they're thirsty and give them a deep soak, but by the next day they're drooping even more, and within a few days the whole plant has collapsed. Cut a stem and set it in a glass of clear water, and a cloudy thread of bacteria will stream out of the cut end. This is bacterial wilt.
A Disease That Can Topple a Tomato in a Day
Bacterial wilt is caused by a soil-dwelling bacterium, Ralstonia solanacearum, which slips into the roots through tiny wounds and clogs the plant's water-conducting vessels. With those vessels blocked, the leaves can't draw up water, so the plant suddenly wilts in the midsummer heat. Once this bacterium settles into the soil, it can linger for years and move on to tomatoes, eggplants, and chili peppers in turn. There's no reliable treatment, either. Until now, the only real option for home gardeners has been to pull up infected plants and burn them.
What Everyday Yogurt Bacteria Can Do
A research team recently tested the lactic acid bacteria found in yogurt. When these bacteria are grown in culture and the cells are filtered out, what's left is a clear broth that holds only the substances the bacteria released as they grew. That broth contains organic acids such as lactic acid and acetic acid. Broth from Lactobacillus plantarum yielded up to 4.4 grams of lactic acid per liter, and broth from L. rhamnosus produced up to 3.9 grams of acetic acid per liter. These acids slowed the growth of the wilt bacterium and kept its cells from clumping together into a sticky film called a biofilm. Because that film is what plugs the water vessels, preventing it makes it much harder for the disease to spread.
What Happened in the Seedlings
When the researchers applied the broth to tomato seedlings that had been inoculated with the pathogen, noticeably fewer plants wilted. Infected seedlings that got no treatment all fell ill, and severely so. But in seedlings given broth from Lactobacillus salivarius, the infection rate dropped to 19 percent and disease severity fell to around 5 percent. The broth did more than hold back the pathogen. Infected tomatoes build up reactive oxygen species in their tissues, which damage cells. Seedlings treated with the broth showed less of this oxidative damage, along with more chlorophyll and higher levels of antioxidant enzymes. They also produced more phenols and flavonoids, compounds plants use to defend themselves. In short, the treatment both kept the disease at bay and grew greener, sturdier leaves.
What You Can Try in Your Garden
These results come from the lab and from seedlings, so it's too early to say that pouring drinkable yogurt straight onto your soil will do the same thing. Still, the findings point in a useful direction. If bacterial wilt has already swept through your garden, here's what to keep in mind this year.
- Build your rows up high so water drains well, and take care not to nick the roots when you weed.
- Don't plant tomatoes in the same spot year after year; move them to a new bed each season.
- If you use a store-bought lactic acid bacteria or microbial product, apply it to the soil before planting rather than after disease shows up, so beneficial microbes can settle in around the roots first.
- If you spot a wilting plant, don't hesitate. Pull it out right away so the disease doesn't spread to its neighbors.
Reference — BMC Plant Biology, a study on suppressing tomato bacterial wilt with lactic acid bacteria culture broth
