Late blight

The drought is over! Fungi are ascendant, mold is rampant. Mushroom hunters are delighted — and your tomatoes are probably blighted! Late blight (Phytopthora infestans), the pathogen of Irish Potato Famine infamy, is the likely culprit. It caused a potato-crop failure all across the Emerald Isle (and, by the autumn of 1845, throughout the European continent). All of Europe suffered, but actual famine struck Ireland. According to historian Henry Hobhouse, the seeds of this tragedy lay in the nature of English domination and the Irish peasantry’s response to their subjugation (see his fascinating book Seeds of Change for details).

The fact that late blight arrived from the New World at the Isle of Wight in June of 1845 and, within a single growing season, had devastated the entire European potato crop will surprise few local growers hereabouts.

After several years of drought, this year’s incessant rains are giving us a graphic demonstration of Phytopthora infestans‘ virulent rate of infection. I heard reports of the pathogen on farms in Madison and Buncombe counties in mid-July. By early August, my tomatoes were infected. It rained every day, and I sprayed every day. Sometimes it would start raining again just as I’d finished spraying. A week later, virtually all my fruit and some of the stems of my tomato plants were turning black. Worse, I found late blight at the Mountain Air Community Garden near Burnsville, N.C.

I’m responsible for disease- and pest-control there, and our literature assures members that I have a solution for most any eventuality. But I’d forgotten about our so-called “normal” summer weather pattern. My partner in life, Diane, now reminds me that after our last round of “normal weather,” I vowed to grow tomatoes only under tunnels. True — but since then we’ve had all those arid summers. Also, I have to confess that I’d grown cocky about my success last year in dodging modest disease pressure. I accomplished this with a one-two combination of the no-longer-organically-certifiable fungicide OxiDate (essentially, stabilized hydrogen peroxide) immediately followed by an application of compost tea. I brewed the tea with a compost screened and tested for its disease-suppressive qualities. Around midsummer, however, I confirmed that the Organic Materials Review Institute had dropped OxiDate as an allowed input for organically certifiable crops. I therefore finished out the year using only compost tea. This regime stopped an infection of early blight and kept my solanaceous crops disease-free while other local growers were reporting both early and late blight.

I began this tomato season already behind. I was late getting my tomatoes in and feeling too overwhelmed to even remember, let alone execute, my tunnel strategy.

Instead I switched to Serenade, which I hoped would require less time and focus, as my mainline defense. I’d heard very positive reports on Serenade’s efficacy from Dr. Frank Louws at this year’s Organic Growers School. I was also motivated by an impressive description in The Peaceful Valley Farm Supply Catalog. And then there was my nagging uneasiness about being unable to use OxiDate to wipe the tomato-leaf surfaces clean of pathogens before replacing them with the beneficials in my compost brew. Although I’d had success in the second half of last year’s tomato season using compost tea alone, I worried that the OxiDate might have been a key factor in ensuring the tea-borne microbes’ ability to dominate the ecology of tomato-leaf surfaces early on. Without the clean slate provided by OxiDate, would my microbes prove too wimpy to compete?

Serenade’s active ingredient is a bacterium, Bacillus subtilis, that’s similar to compost tea in the way it works. The difference is that a private company has isolated this particular biological-control organism and developed it to the point that they could register it with the U.S. Environmental Protection Agency and label it as being effective against an impressive array of diseases, including late blight. Compost tea, on the other hand, is brewed from compost that has the potential to suppress disease. The specific recipe for the tea is designed to allow the suppressive organisms to multiply. Based on research, anecdotal evidence and personal experience, I’m quite hopeful about compost tea’s potential effectiveness. But this year, what with the farm and the job, I needed what passes for certainty with as little trouble as possible.

I began applying Serenade to my tomato-family seedlings about two weeks before setting them out. Despite exhaustive and exhausting efforts on my part, however, I had failed to ensure that these transplants were always protected from the many 40-degree nights and cold rains that Celo endured late this spring. As a result, the seedlings were all showing signs of alternaria infection. I seriously considered pitching five flats of eggplant that past experience told me were doomed. Impressively, however, Serenade actually stopped — and then eliminated — the alternaria on everything, including the eggplant.

As June gave way to July, I was feeling pretty confident. I sprayed Serenade at least every seven days, and more often if warranted. The ideal conditions for Phytopthora infestans spores to germinate are rain/mist/fog, cool nights, humidity above 90 percent, and temperatures around 70 (or at least below 80). According to Rodale’s Garden Insect & Weed Identification Guide, late blight produces two types of infecting agents: sporangia (at temperatures above 65 degrees) and swimming spores (at temperatures from 56 to 65 degrees). Swimming spores can travel up to 30 miles. They can live in the soil for several weeks until the conditions favor germination. And once a spore infects a tomato or potato, it takes a mere seven days until a new crop of spores starts emanating from the victim. But although swimming spores can live in the soil for several weeks, the disease needs a living host plant to make it through the winter. Those of us who plant uncertified potatoes or leave potatoes in the ground to overwinter may contribute significantly to the next year’s disease pressure.

By the second week of August, my 200 or so tomatoes showed infection on virtually all the fruit and quite a few stems, and I was beginning to give up hope. At that point, however, I should have refreshed my memory of how Serenade works. According to the description, Serenade not only halts spore germination, it also disrupts mycelial growth, which will stop the infection even if it has progressed to the vascular and fruiting parts of the host plants. And that’s exactly what happened. I actually despaired of being able to keep the Serenade on the tomatoes and quit spraying for 10 days or so, yet my plants have survived and the foliage looks great (though it remains to be seen whether they will actually produce good-tasting tomatoes). In the meantime, I’ll be brewing some compost tea and including a few other enhancements — such as seaweed, soluble phosphorous and humates — before I apply it.

I’ve also checked with others who used Serenade to fight late-blight infection, and the reports are mixed. Some started too late, and though Serenade slowed the blight, they still lost their crop. Others gave up spraying about the same time I did but also pulled out their plants. Still others did pretty much the same thing I did and also experienced tomato survival and new growth. Considering the weather conditions and the presence of serious infection, everyone who managed to save their crop is impressed that they still have tomato plants at all. One such person is a conventional grower who operates a greenhouse; I gave him some Serenade because, despite using Daconil and copper sulfate, he couldn’t seem to stop the infection in a plot of heirlooms he was growing out for seed. The Serenade worked. This man is now a believer, and Troy’s Greenhouse of Burnsville will be stocking Serenade as soon as he can find a source for 1-pound bags.

I wish I could say that Serenade is the certain solution to incredible disease pressure from late blight, but I’m afraid the jury is still out. I do know that it’s a powerful tool, however, and combined with maximized air circulation, it looks real good. The members’ tomatoes at Mountain Air’s Community Garden are still virtually blight-free. These plants are growing on a gentle slope, and the lay of the land ensures excellent air drainage. Each plot holds no more than five tomatoes, all well-staked. But for my part, I’m still planning on using tunnels next year. I’ll probably need Serenade, too — but at least it won’t get washed off right after I apply it!