Friday, September 6, 2013

Battling early blight (tomatoes)

Sad tomato plant. All its bottom leaves succumbed to
the early blight disease (leaf spot).
I was so excited with the progress of my tomatoes. Really, they were green, bushy, and producing lots of new tomatoes. After my vacation, some of the bottom leaves looked a little worse for wear... they had gotten a little too wet. I trimmed off all the bottom leaves and laughed at my top heavy plant.

But then, bam, the problems started spreading up the plant. A little (a lot) internet research led me to a diagnosis as well as treatment options. My tomatoes have early blight. I probably did a lot of things wrong that led to disease susceptibility: mainly, I encouraged my tomatoes to get bushy and grow upward in the confines of their cage, allowing all the leaves to touch. I also spent way too much time touching the leaves. It rained for about 5 days straight, and it was during this time that I trimmed the bottom leaves and inspected the top ones without washing my hands.

Below, I'll describe what I've learned about early tomato blight, its prevention, its treatment, and my own experimental prevention/treatment ideas.

What is early blight?

  1. Early blight is caused by a fungal infection. The Alternaria solani fungus primarily infects leaves and stems, but it can also infect the fruit.
  2. A. solani can also infect potatoes, peppers, and other members of the solanum genus, which includes the nightshade family. It is best known for infecting tomatoes and potatoes.
  3. The common name early blight is to distinguish it from the much more severe late tomato blight. Late tomato blight is caused by a completely different strain of fungi called Phytophthora infestans. It moves much faster and is famous for causing the Irish potato famine. Early blight typically hits in mid summer, while late blight hits toward the end of the growing season.
  4. Early blight (A. solani) reproduces through spores that can survive through freezing and drying. The spores land on the leaf of a tomato plant and then germinate (become active and start to divide). The single spore leads to a visible colony (a black spot on the tomato leaf). This sucks nutrients from the leaf and leads to yellowing in the surrounding area. Germination requires water and relatively warm temperatures. Actively growing A. solani can also produce spores when temperatures decrease and moisture increases (like overnight!). This more technical reference contains a nice figure outlining the life cycle of A. solani in potatoes, which is essentially the same as the life cycle in tomatoes.
  5. Early blight is spread through spores or direct contact between an infected leaf and uninfected tissue (leaves, fruit, or stems). 
  6. Once early blight hits, you can't really get rid of it. You can contain the infection, though, and you can protect new growth on the plant.
  7. The soil around an infected plant will likely contain spores. When it rains, spores can splash onto lower leaves, spreading infection. Spores can also be wind borne.

Early blight prevention
There are tons of references outlining how to prevent early blight. I found several gardening message boards incredibly helpful, as well as these two resources: Sweet Domesticity: The Battle of Blight and
The Rusted Garden: Ten Tips For Preventing Early Tomato Blight: A Disease.

I'm not going to repeat everything that's already been written out in so many places, but here are the take-away messages:
  1. If you see any sign of early blight on your tomato plant, you have to dispose of it and the surrounding soil properly. Clean all your tomato stakes and cages, etc. for reuse. The spores won't survive on those.
  2. Don't let the leaves get too wet. Water from the bottom of the plant. Once your plant is thriving, trim off all the bottom leaves so they don't touch the soil. Leave yourself a "splash zone" of several inches (the recommended amount varies), so that spores can't splash up from the soil onto the leaves of your plant.
  3. Don't touch the leaves of your plant excessively, especially when they're wet. This can spread the disease before you even know it's there.
  4. Give the leaves lots of room to breathe. Steak stems so they're off the ground, but encourage them to spread out to avoid touching leaves.
  5. Keep the plants healthy and fertilized. They're less likely to become infected.
  6. Spray plants proactively with a fungicide. Start spraying a few weeks before your plants "typically" become infected or at the first sign of blight.*
  7. Spray the plants with a newly marketed "bio-fungicide," like Serenade.**
*I'm not sure I agree with this last one. I'll go into more detail in a bit.
**I haven't tried this product. It's not a real fungicide though. It is a living soup of bacteria designed to keep the infectious fungi in check.

Early blight treatment
Take home message: Once your plants have early blight, they have it. Unless you're willing to resort to really serious fungicides, you're not going to get rid of it. Here's what I've learned about containing the spread, though:

  1. Review the early blight prevention list. Make sure your plants are healthy, avoid wet leaves, and give the leaves lots of room to breathe. Trim off all the bottom leaves, even if they're not infected.
  2. Remove all infected leaves. Make sure you clean your scissors/hands/shears after this. Don't let the infected leaves drop into the soil! Discard of them far away from your tomato plants (and not somewhere you're likely to plant tomatoes in the future).
  3. Spray healthy leaves with a fungicide at the first sign of infection. For example, use a copper fungicide, which prevents germination. If a new spore lands on a leaf, the copper fungicide should prevent it from becoming an active, growing colony.
  4. Spray leaves with a "bio-fungicide," like Serenade.
  5. Try one of many "homemade" methods that have not been verified to work on early blight, including but not limited to baking soda, cornmeal, vinegar, hydrogen peroxide.*
  6. After doing the above, mulch over the soil so any spores that are living on the soil surface or that dropped onto the surface while trimming/spraying are covered up and can't splash onto the plant.
*I'll mention some of these homemade methods below.

What I've done
As soon as I determined that my plants were infected with early blight, I started removing infected material. Once it was all removed (the best I could), I tried some old wives tales (baking soda spray and cornmeal). I figured they couldn't hurt, even if they might not help. I then purchased a copper-based fungicide to prevent germination of spores on healthy leaves. So far, I've sprayed both plants twice, and they're doing much better. I also have been fertilizing the plants with fish emulsion to keep them healthy and to encourage new leaf growth. The plants look sad, but the rate of disease spread to new leaves has slowed drastically. I'm continuing to inspect the leaves daily, and I adjusted my staking to allow for more air flow and less leaf touching. Finally, I separated the pots. The plants had gotten big enough that they were touching on my patio. I moved the Better Boy plant over near the air conditioner unit. An added benefit seems to be increased air flow every time the A/C kicks on.

Today, I decided to try another homemade remedy, which may or may not work. Read on to learn a bit more about treatment options.


Initial trimming of the cherry tomatoes,
8/18/13 
Cherry tomato plant, 8/18/13
Better Boy tomato plant before showing
any symptoms, 8/18/13
Cherry tomatoes 9/6/13:
They seem to be bouncing back quite a bit.
This plant was most severely infected ~8/20.
Stem sores on Better Boy tomato, 9/6/13
Still some infected leaves on Better Boy
tomato, 9/6/13, despite extensive trimming.
New growth looks healthy on Better Boy, 9/6/13
(knock on wood).
Do you see the few colonies hiding away?
Better Boy, 9/6/13
Now the leaves are more spread out (and the plant looks
sadder). Hopefully new growth will fill in some of the gaps.
Compare this picture to the one from 8/18/13-this plant has
lost ~80% of its leaves :(
My thoughts on preventative fungicide application and "natural remedies"
As I commented above, I'm not sure that I agree with spraying plants with fungicide in anticipation of an early blight infection. 

I would think that a healthy flora of naturally occurring fungi and bacteria would help prevent infection by the bad guys, and using a general fungicide before early blight hits would just make the plant more susceptible to other infections. To me, the idea sounds similar to taking antibiotics before going to the dentist to prevent infections if you have even very minor heart conditions. This was in favor for a long time, but new guidelines suggest only those of high risk use this approach, as using antibiotics only applies a selective pressure that allows only those bacteria resistant to the antibiotic to survive. The ultimate public health in the case of preventative antibiotics before dental work is antibiotic resistance, but current research also emphasizes how important your natural bacteria are in keeping you healthy. Antibiotics kill the good guys in addition to the bad guys!

Once the plant has early blight, however, I'm not one of those people who totally avoids products that are tried and true to control my problems. I have sprayed with a copper fungicide (supposed to be an "organic" option, safe to eat tomatoes from the sprayed plants) and will continue to do so.

Trends in plant health are catching on to trends in human health, though, specifically with regard to encouraging healthy bacterial and fungal growth to maintain health. The new product I mentioned above, Serenade, is a bio-fungicide. It contains a patented strain of Bacillus subtilis, a bacterium that battles with the disease causing fungi. This species of bacteria is included in some probiotics for human consumption, too, though certain rare strains of it can cause food poisoning. The idea is that by encouraging the growth of this beneficial bacteria, you can prevent the growth of detrimental fungi (and detrimental bacteria, too). I haven't tried this product yet. From the reviews I've read, it sounds like once you have early blight, Serenade can't really get rid of it. However, I think the idea has huge potential, and I might buy a bottle of it next year to spray proactively.

One of the old wives tales I mentioned also relies on this same concept. There is a lot of online information suggesting that cornmeal encourages the growth of a naturally occurring soil fungus, Trichoderma, that can compete with the bad fungi. The reason I call this an "old wives tale" is that it has never been formally proven. It is true that under lab conditions, corn meal supports the growth of this fungus, but that is far from proving that cornmeal supports Trichoderma growth in soil containing growing plants or that it successfully competes with A. solani or other fungi. Despite the lack of real evidence (like publication of a controlled study in a peer-reviewed journal), there is a lot of anecdotal evidence to suggest that it works. I don't see how trying it could hurt in addition to tried and true techniques.

Other "homemade remedies" involve making the conditions unfavorable to the growth of the fungus. For example, baking soda increases pH, and the bicarbonate somehow damages the cell walls of the spores. This technique has been well-vetted scientifically. Alternatively, spraying the plants with apple cider vinegar decreases the pH, making the environment less favorable. Any vinegar will do this, but most sources I've read tout the trace elements and other benefits of apple cider vinegar. As far as I know, an apple cider vinegar treatment for early blight has not been confirmed scientifically.

So here's my new idea: I sprayed my tomato plants with kombucha (fermented tea containing a mixture of bacteria and fungi (yeast) that tastes like vinegar). You might think I'm crazy, but hey, I had it in the fridge, and it contains a whole range of microbial life that might help fight off the early blight. I don't plan to stop using the copper fungicide, but I don't think this attempt would hurt. I'm hopeful that the decreased pH will damage the early blight fungi and that inoculation with all the bacteria and fungi from the kombucha will encourage the growth of healthy microflora that can outcompete the bad guys. I'll post an update to let you know how it goes. If I can get my hands on some Serenade to test, I'll spray with that as well, since it contains bacteria that has been patented for this specific purpose.

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