Late Blight Rears Its Ugly Head Again

Late blight, the disease that wrecked many gardeners’ dreams of a fresh tomato sandwich last year, has been found, wide-spread, in western Pennsylvania. Many horticulturists, me included, were optimistic that we would be spared in 2010 but that hope now appears to be dashed. At this time, late blight has only been confirmed in three western PA counties but all Pennsylvania tomato and potato growers need to be vigilant. For a good discussion of the disease including images, see this site; or this site
To recap…. Late blight is a devastating fungus-like disease of both tomato and potato. Symptoms include rapid blighting of foliage and fruit. Tomato stems exhibit chocolate brown lesions. Leaves have blotchy brown spots and may produce fuzzy, whitish masses on the underside of leaves, especially in the morning when dew is present. Note that this is very different from early blight which results in yellow-brown lesions on lower leaves, containing concentric rings of dead tissue.

What’s a gardener to do? First, be vigilant. Inspect tomatoes and potatoes daily for late blight symptoms. Penn State will be tracking the disease in an attempt to alert commercial growers about the threat of late blight in their vicinity. It’s one thing to miss out on a tomato sandwich…quite another thing to lose a big chunk of your income. Farmers have a lot at stake with a crop of tomatoes or potatoes. Contact your extension office if you think you see late blight. Feel free to bring samples to the office for confirmation of the disease.

How about protecting those plants? Conventional gardeners and farmers can use products containing chlorothalonil, for starters. New foliage needs to be protected as it grows. Organic gardeners have copper fungicides, which will not provide the same level of protection as chlorothalonil but are better than nothing. If you plan to let Mother Nature take its course, and do nothing, at least be prepared to destroy infected plants at the first sign of disease… your neighbors will thank you. Best bet in small gardens is to bag the plants in plastic and cook them in the sun.

Any chance we’ll escape late blight in eastern PA? I was an optimist in the winter because it is known that the organism does not survive without living tissue in the North and Old Man Winter took care of that (except for potato cull piles) . But now that the cat is out of the bag…. I think we’re in for another year of greasy, black fruit in the tomato patch.

Dang… I was feeling good; tying up plants in the rain yesterday. Now I wonder if those raindrops were accompanied by late blight spores. Current conditions are ideal for late blight infection.

Late Blight - 2010

Ok. First, the take home message…. Tomatoes growers should not expect the 2009 epidemic of Late Blight to return in 2010. And… the disease organism that causes late blight has not survived in soil, pots, stakes or other non-living tissue in Pennsylvania. So, chill out on all of those elaborate plans to sanitize the garden. If Late Blight interests you, read on. If you grew potatoes last year, and have any left over, be sure to read to the end of this blog because you could cause a problem.

Last week I spent an entire day, in one room….learning about tomato production at the Mid-Atlantic Fruit and Vegetable Growers Conference in Hershey, PA. I was in charge of making sure the projector worked and the lighting was conducive to learning, otherwise I would have slipped out to catch that session on horseradish, among others, down the hall.

A lot of time in the tomato marathon was devoted to late blight, the disease that caused widespread destruction of tomatoes in the Northeast US last summer. Two things were responsible for that epidemic. One… infected transplants were sold throughout the Northeast thru the “Big Box" stores. The stores were supplied by a large greenhouse business that had late blight (a disease caused by a fungus-like organism) in their production system. I am sure that neither the stores nor the greenhouse business intended to create such an epidemic. But the end result was distribution of infected plants over the Northeast... a devilishly effective Step One: spread a very contagious organism over a wide geographic area. Late Blight spreads by spores which can blow 30-40 miles in moist air. It can infect tomato and potato as well as some weeds in the tomato family (and petunia, I learned).

Thanks to an eagle-eyed plant pathologist from Cornell, the problem was diagnosed every early. He blew a whistle and everyone paid attention, otherwise, it could have been worse. He said it may have been the most constructive thing he has done in his long career.

Step Two was Mother Nature. She picked 2009 to provide excess moisture and cooler than normal temperatures from June until September. Perfect for the Late Blight organism. Hey, whose side are you on Ma, the tomatoes or some pathogen? I guess we know now. Seems she loves all of Her creations, including Late Blight. Hmmm.

The good news is that the organism that causes Late Blight has no history of overwintering in the Northeast U.S. Once infected plants die, so does the disease. Plant pathologists are concerned about a situation in which late blight does develop the capacity to overwinter here, but so far that has not occurred (as far as they know).

So again, the really good news is that both gardeners and farmers have no reason to expect late blight to be any more of a threat in 2010 than it was in any other year. And 2009 can be considered an unusual year for the disease. Since the organism requiures a living host, there is no need to plan elaborate crop rotations or go to extremes in trying to kill what is already dead. Dead tomatoes equal dead late blight. Unless the spuds get us….

Now about those potatoes…. Recall that late blight can survive in living tissue and since the same organism infects both tomatoes and potatoes … do you know where your spuds are? The concern is that infected potatoes are laying cull piles or worse, stockpiled for planting. Don’t do this! Buy certified, clean, potato planting stock!. It ain’t worth starting the epidemic of 2010 to save a few bucks on seed potatoes. And that means your precious heirloom varieties, too. If you grew potatoes in 2009 and experienced late blight, be alert for volunteer spuds next spring and destroy them.

No problem with saved tomato seed.

I located an excellent summary of late blight and its management at UMass extension. It provides and excellent overview and advice for gardeners, including those using the organic approach.

For a utube segment by PSU's own plant pathologist, Beth Gugino, see this.

Can I Compost those Diseased Tomato Plants?

Yes. That’s the short and simple answer to a question that is on many gardeners minds. The disease that concerns most gardeners this year is late blight, Phytophthora infestans, which plagued so many gardens and farms this year. I have spoken to too many gardeners who are wasting time solarizing tomato vines, planning elaborate crop rotations or sending tomato debris out with the trash. Compost them. It’s also good to know that most diseased plants can be composted without the fear of aggravating the disease situation next year.

Let’s take late blight first because it is easiest. The late blight organism requires a living host to survive. Since tomatoes cannot survive our winters, any late blight fungus will die along with the plant. May as well compost the diseased plants. Or you could simply turn them into the soil. For that matter you could let the dead tomato skeletons hang out all winter on their stakes. Dead tomatoes = dead late blight. Late blight does not form overwintering spores in Pennsylvania that could cause new infections next year.

Potatoes are another story. Late blight infected potato foliage can be treated like tomato foliage. But infected tubers should not be put into the compost pile. Tubers may survive the winter and start up new infections next spring.

Most other tomato diseases do have the ability to survive and infect tomato again next year. Early blight, Septoria leaf spot and anthracnose can survive either on plant debris, in special fungal survival structures, on other plants and even on pots and containers. Crop rotation provides some small measure of control but you can expect these diseases to return each year regardless of crop rotation. And let’s face it, we often have limited ability to rotate crops in our small vegetable gardens. Do it, as a good general pest management strategy, but realize it will not eliminate re-occurrence of disease.

Back to that compost pile… since the environment in the compost pile is much more competitive for fungal pathogens than soil, and these disease organisms will survive thru other means, why not compost those diseased plants?
There are a few garden diseases that surely should not go into the compost pile. Fusarium and Verticillium wilt come to mind, but they are relatively rare these days because of good plant breeding. If you suspect that these are involved then trash them. Otherwise… everything into the compost pile.

Late Blight of Tomato Continues to Spread

You’ve been warned. Late blight is here and continues to affect commercial and home plantings of tomato and potato. A farmer friend called me minutes ago to report a farm right across the river where tomato was confirmed with late blight.

Now it seems a matter of time before we see wholesale injury to this popular crop. Although relatively dry weather has prevailed recently, morning dew in enough to support late blight infection.

I could say more but my co-horts in Lehigh Co have done a great job. See this link.

The best pictures I've seen are from Cornell.

Late blight of tomato and potato arrives early this year

I had hoped that I would not write about late blight this year. Usually it is August or September before late blight rears its head in our area. So writing about it in early July is a bad sign for gardeners and farmers. But it is here. A perverse combination of the widespread sale of infected tomato plants and abnormally cool, wet, weather in June was the perfect storm. The disease has been confirmed in almost all Northeastern and mid-Atlantic states, including in Lancaster and Lehigh counties in southeastern Pennsylvania.

This devastating fungal disease is a threat to all tomato and potato growers. It is the same fungus that was involved in the Irish potato famine of the 1840’s. While no one will starve in 2009 due to late blight, it appears that we are in for an unusually early and wide spread epidemic. Plant pathologists are reporting that this is the earliest the organism has been found in such a wide spread area of the Northeast U.S. The problem is, this fungus can travel many miles in the air, settling out on host plants and spreading infections. Apparently, widespread sale of infected transplants has inoculated the northeast. Many of these locations will be home gardens where it will go unnoticed until it is too late. Nearby commercial plantings are at risk.

The fungal pathogen involved in late blight disease is Phytophthora infestans. It thrives under cool (60-70 degree) moist (dew rain, humidity) conditions. Host plants include tomato, potato, other tomato family plants and, believe it or not, petunia. Foliage, stems and fruit blackens. Unlike the slow progression of early blight described in the previous blog, late blight moves like lightning. That’s why plant pathologists watch like hawks for its emergence and alert commercial growers to its presence.

Fungicides can protect plants and limit the spread of the disease. Famers have good options but must be vigilant and proactive. Gardeners have the fungicides chlorothalonil and copper. Copper is a poor second choice but some forms are organically approved. More details, instructions and pictures from Cornell University can be found here. And at this Penn State site. Plant pathologists recommend treating now, rather than waiting for signs of infection. This fungus is amongus.