Thursday, July 16, 2009

Tomato Late Blight: A Lesson Learned the Hard Way

     If you are from back East, or have been following this story via blog posts from here in the dry West, then you know the big tomato story of 2009. Tomato plants are being cleared from the shelves of the big box stores east of the Mississippi. 

     The culprit? A fungus, late blight (Phytophthora infestans). Infected tomato plants are dying by the thousands along the eastern seaboard as this fungus spreads from tomato plant to tomato plant, a condition that is brought on by the mild temperatures and high humidity that are part of this year's wet East Coast summer.
     According to news reports, many of the infected plants came from the same source, a wholesale nursery that grew tomato seedlings for vendors throughout the East.

     Late blight in tomatoes is summed up in this University of California webpage:

"Leaf symptoms of late blight first appear as small, water-soaked areas that rapidly enlarge to form purple-brown, oily-appearing blotches. On the lower side of leaves, rings of grayish white mycelium and spore-forming structures may appear around the blotches. Entire leaves die and infections quickly spread to petioles and young stems. Infected fruit turn brown but remain firm unless infected by secondary decay organisms; symptoms usually begin on the shoulders of the fruit because spores land on fruit from above.

Late blight is found when humid conditions coincide with mild temperatures for prolonged periods. When humidity is above 90% and the average temperature is in the range of 60° to 78°F, infection occurs in about 10 hours. If conditions are ideal for disease development, disease development is rapid and losses can be severe. The fungus overwinters in potatoes, tomatoes, hairy nightshade, and possibly in the soil. Spores of the fungus are easily spread by wind to other plants.

Tomato varieties resistant to certain races of the late blight fungus are grown where the disease occurs regularly. Remove any nearby volunteer tomato and potato plants and nightshades. Check transplants to ensure they are free of late blight before planting. Avoid sprinkler irrigation, if possible, because it favors the development of late blight."

     So, why is it happening here, in the dry Central Valley, in a garden bed with no overhead irrigation? To one of MY tomatoes, an heirloom Giant Belgium (rest in peace), started by me from seed? Being raised Catholic, I have to ask the question: How did I screw this up?

     First of all, the Central Valley ain't no hotbed of late blight. High humidity? Mild temperatures? Not here. Maybe along coastal California, but not here.

The plant was started, along with another 20 varieties, from fresh seed from commercial sources. In truth, the Giant Belgium plant, even when young, never looked great. It was scrawny and slow to develop. Even in July, it lagged behind the others, producing only three green tomatoes while most of the others had already been producing harvestable tomatoes for a few weeks.

     Like most gardeners, I let it ride. It wasn't in a serious decline, just slow with some browning branches. Anyone who grows lots of tomatoes can identify with that: give it a chance, maybe it will bounce back.

     As news of the late blight outbreak in the East spread, I decided to take a closer look:

Hmm...browning branches scattered throughout the plant.

 Uh, oh: purple spotted leaves!

      I charged back inside to find out more about late blight, and checked one of my favorite reference books, "IPM For Tomatoes".
I found out more than I really wanted to know about late blight:

"Late blight occurs in all tomato growing areas of California, in sprinkler irrigated fields... The fungus overwinters in commercial potatoes and in volunteer or abandoned potato and tomato plants in fields...and gardens. From these sources, spores are carried by wind to spring tomato plantings and to greenhouses where transplants are grown for commercial and home use. In each infected plant, the fungus produces new sprores, which again spread by wind to other plants. The disease develops most rapidly when humidity is near 100% and when the temperature is about 68-76 degrees (F). Development slows or stops in hot, dry weather, but may resume when favorable conditions return. All California varieties appear to be susceptible. Avoid sprinkler not use sprinklers on bush-type tomatoes that have developed a dense canopy."

      Another tomato disease that could be mistaken for late blight is an infection of powdery mildew. The "IPM for Tomatoes" book says how to spot the difference. In the case of powdery mildew: "...fold a yellowed leaf in half, keeping the lower surface toward the outside. With a hand lens, check along the edge of the fold for bunches of straight, hairlike structures slightly longer than normal leaf hairs; these are the spore bearing structures of the (powdery mildew) fungus."

According to the late blight experts at Cornell, late blight disease can spread fast, even in warmer weather: 
"It can develop in very warm daytime temperatures (95°F) if conditions are extremely wet, and night temperatures are moderate (60°-75°F). Epidemics can be rapid and devastating because of the high reproductive potential of this pathogen. Individual lesions can produce 100,000 to 300,000 sporangia per day." That Cornell site also has a link to great pictures of late blight of tomatoes.

 According to the University of Hawaii: "On tomato petioles and stems, (late blight) lesions begin as indefinite, water-soaked spots that enlarge rapidly into brown to black lesions that cover large areas of the petioles and stems."
Hey, I got that!

And since tomato late blight is a strain of Phytophthora, there may be some root discoloration, as well. 

After digging up the plant, well...what do you know!

So, where did I screw up? Was it the seeds? Probably not. Multiple university sources say  that late blight fungus is not known to infect tomato seeds. 

Did I plant this year's tomatoes in the same bed where tomatoes were planted last year? Ummm, yeah. I forgot to rotate my crops. The spores for late blight can survive in the soil.

But the plant didn't look too healthy since the time it started growing in late winter in the greenhouse. Every other variety looked fine. Why did this one, the Giant Belgium, get off to such a slow start?

Did I remember to thoroughly clean out the four inch pots that I reuse each year for those tomato seedlings? Ummm, I kinda skipped that step this year, instead just giving them a quick wipe with a rag instead of a thorough rinse in a  water and bleach solution. The spores could have overwintered in those pots.

And finally, Cornell University offered the best advice: "Remove diseased plants by digging them up." And, "Only plant healthy looking tomato transplants."

A lesson learned, the hard way.


  1. I am constantly learning lessons the hard way. This is especially difficult because like you, we are suppose to be experts.

    People new to gardening could easily just throw up their hands and give up. Maybe at least it will instill a sense of appreciation for those who, despite all odds bring food to the table. Gardening is loads of fun, but when you are talking about food the stakes get higher. It's a serious business.

  2. Noooooo! I was feeling smug about having started my tomatoes from seed (therefore being immune to this plague) until I read that you had done the same.

    So far, everything is looking fine on my side of town...but I'm making a note about preventative measures in my garden book now.

  3. See, that's what you get for starting your own tomatoes from seed. Clearly you need to buy them at a nursery....

  4. And to add insult to injury, after I dug out the plant, I placed the empty tomato cage firmly back on the garden bed. I discovered this morning, after turning on the drip irrigation, I had managed to puncture three holes in two drip lines with the tomato cage! (The drip lines were beneath red plastic mulch, so cut me some slack! still...)

  5. Good to know even seasoned veterans as yourself sometimes fail also. Somehow, it makes me feel loads better.

  6. I had avoided the blight until last week. It's really destroyed everything here in Massachusetts. My question now is what do I do next year? I have a very small 6 by 14 foot bed. Do I just do tomatoes in pots from now on? I'm organic so I can't treat the soil with anything, can I?

    I did a rant post about this too. I was so disappointed, but these things happen!

    Thanks for the info, glad I stumbled upon your blog! -kate

  7. I had 13 plants of many varieties, including some early girls, in Davis, CA, near Sacramento. Had cover crop of inoculated veatch, beautiful black soil. Buried drip hose under red mulch plastic. Watered once/week for 20 min. Beautiful growth until mid July. Then one plant started getting brownish limbs, then a few days later, all were stricken. Post says to rotate the crops. Does that mean no tomatoes there next year? Raised bed is 8' x 25'.:(