Infected Fruit

Infected Stem

Infected Leaves

Lesion on top of leaf

Sporulation on bottom of leaf

Tomato Blight

Out, Out Damn Spot: Late Blight at OVF

By Dean Cleverdon

Lady Macbeth: Yet here's a spot. Out, damned spot! out, I say!
One; two: why, then, 'tis time to do 't.

Macbeth Act V, scene 1

Lady M may have been dreaming of murdered Scottish kings, but more likely she was seeing something nasty on her tomato plants. Late Blight gives me bad dreams, too.

Phytophthora infestans (phyto — plant; phthora — destroyer; infestans — devastating), Late Blight, is a water mold that is especially virulent for potatoes and tomatoes, but can infect other host plants as well. It caused the Irish Potato Famine in 1845 and wiped out tomato crops on the American East Coast in 2009. Now it is attacking plants at Ocean View Farms.

Late Blight can infect tomato fruit, stems and leaves (see top 3 pictures at left). Fruit will have a greasy brown discoloration and become mushy. Stems become dark brown, brittle and will snap off with very little pressure. The top surface of the leaves will form dark brown and/or black lesions; the underneath surface of the leaf will be fuzzy gray-white with new sporangia (see bottom 2 pictures at left).

Sporangia — one stage in the Late Blight Life Cycle: Oospore, Sporangium (singular), Sporangia (plural), Zoospores, Mycelium. Late Blight reproduces both sexually and asexually. Sporangia Mating Types A1 & A2 hook up and beget Oospores. Oospores germinate and create Sporangium (yep, plural Sporangia). Sporangia are little sacs containing Zoospores which are delivered to the plant via wind or rain (or splashed up from the soil when overhead watering). When the Sporangia hit the plant they burst, releasing the Zoospores which penetrate the cell wall of the leaf with a germ tube (an intact Sporangia can do the same). This can take as little as 2 hours when sufficient moisture is available. Once established Mycelium are created which break down the plant tissue. After the Mycelium works its way down it through the leaf it creates new Sporangium (sporulation). The gray-white fuzzy stuff on the bottom of the leaf is new Sporangium coming through the stomata and the cycle begins again. If Sporangia Mating Types A1 & A2 are not romantically inclined (or available) the Sporangium can reproduce asexually. Germination to sporulation takes one to three days. Each lesion on the leaf can create up to 300,000 new Sporangia per day. Each Sporangia contains 6 to 12 Zoospores. Count the lesions on any infected plant and do the math. Billions of new Sporangia are released into the wind daily.

P. infestans must have a moist, temperate climate to survive. Remember, it is a water mold. It must have moisture to stay alive long enough to penetrate and infect the plant. If it gets too much sun or heat it dries out and dies. A study by the University of Wisconsin points out, "...sporangia survived for at least 2 hours in the shade at 98°F, but did not cause infection if exposed to sunlight for 4 hours or longer at 95°F or above." (Q: When was the last time it was 95°F in Santa Monica? A: 9/28/2010)

According to the University of Hawaii the best weather conditions for Late Blight are daytime temperatures between 60 and 70°F, night temps between 50 and 60°F, and relative humidity near 100%. Weather data from the Santa Monica station of the California Irrigation Management Information System (CIMIS) for the period 4/18/2012 to 6/23/2012 shows that all but 15 days met the criteria for perfect Late Blight conditions.

Or, closer to home, the University of California Davis says, "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." All but 7 days from 4/28/2012 to 8/8/2012 were perfect for Late Blight.

Ocean View Farms has the perfect micro-environment for Late Blight no matter what source you use. P. Infestans loves that coastal fog because the moisture and shade give it sufficient time for the Sporangium to get established. In reviewing the CIMIS data for the last ten years, the only years we did not meet the criteria for Late Blight were during the droughts. Although the odds of getting Late Blight are high at OVF there are preventative measures to reduce the risk and/or severity of the disease.

Federal, State and University agricultural sources provide suggestions for keeping Late Blight at bay. A few are listed below but the best defense is using your common sense. P. Infestans likes a moist, temperate environment. It reproduces by the billions. It is mostly a wind-borne disease. It needs about 8 to 10 hours of moisture to germinate and infect a plant. It goes dormant in high heat. It dies in solar radiation. So, what can you do to create a hostile environment for it?

  1. Keep your garden clean and weed free. All it needs is a living host like an infected potato from last year or weeds to survive.
  2. Consider planting later in the season when the weather warms up. However, keep in mind we do not get sufficient heat or sunlight to kill off the spores until the middle of summer. (Q: How many days this year through 8/14/2012 had an average air temperature above 80°F? A: Zero).
  3. Select varieties that are disease resistant.
  4. Select fast-growing varieties to reduce the time from planting to harvesting.
  5. Never EVER have water on the foliage when the sun goes down.
  6. Keep moisture off foliage by spacing the plants apart to allow for good air circulation.
  7. Keep moisture off by pruning away excess foliage to allow for good air circulation. Trim all bottom foliage 6"-9" off the ground.
  8. Keep moisture off foliage by watering in the early morning. Sunlight will dry off the foliage and kill the spores.
  9. Keep moisture off foliage by watering the soil underneath the plant, not the foliage. (This does not apply to foliar feeding).
  10. Do not splash soil onto your plant as it may contain spores.
  11. Keep your plants healthy with proper feeding and watering. Overwatering will weaken a plant's resistance as much as under-watering.
  12. Cover your soil with a plastic mulch to keep the spores out. This will also raise the soil temperature and reduce evaporation.
  13. Erect a barrier and/or put a canopy over the plants to keep the Sporangia from landing on them. (Not sure how practical this one is as it also reduces air circulation).
  14. Inoculate the plants with an Organic Materials Review Institute (OMRI) approved prophylactic spray (i.e. Serenade, Rhapsody, Sonata, etc.) which contain beneficial bacteria which eat and/or destroy the P. Infestans Sporangia. These non-GMO organic sprays must be on the plant before the Sporangia land. They do not cure a plant after it has been infected. Do not use copper based fungicides. Although many copper based fungicides are OMRI approved for commercial farmers, the copper may build up in the soil over time to toxic levels. Probably don't want to do that, eh?

If your plants do get Late Blight (and they probably will) do not panic, all is not lost. Infected plants can still produce harvestable fruit. Since we are not commercial growers we do not have to pull out our few precious tomato plants. Before yanking out your plants (and using common sense) you might consider the recommendations below:

  1. First off, keep in mind that absolutely nothing will get rid of the disease once it is inside the plant. It is there for the duration. However, from personal observation of plants at OVF and anecdotal information from other gardeners, plants infected with Late Blight can still produce tomatoes. If you are the only one in the garden with Late Blight (unlikely) then by all means be a good neighbor and get rid of the plant (it takes a community to kill a Blight). Otherwise invest some time and effort to salvage what you have ('salvage' not 'save').
  2. Barbara Spencer (Windrose Farms) verbally passed on a tip to start watering an infected plant as much as possible (without overwatering) to keep its vascular system functioning. This and keeping it well fed will help the plant stay strong enough to better defend against the disease.
  3. Spray the plants with an OMRI approved biofungicide to keep the Sporangia from spreading to healthy tissue. We do a foliar feed of worm tea combined with Serenade once a week. Any remaining liquid gets poured on the root base to help the plant resist the disease from the inside as well.
  4. Prune off, bag and throw out any diseased leaves. Late Blight is a contact "Spore-t" and these leaves will infect other healthy tissues. Do not waste your time with individual leaves. Take the entire leaf stem back to the stalk. And keep after it. Once is not enough. We prune our plants at least once a week; more if time allows.
  5. If a stalk or producing stem thicker than a pencil is infected give it a close examination. If the foliage and flowers on the leading end remain Blight-free, then leave it alone and keep an eye on it. If the foliage becomes infected and/or the stem becomes infected all the way through, then cut the whole thing off and dispose of it.
  6. Every so often wipe off your hands (or disposable gloves) and cutters with rubbing alcohol to prevent cross-contamination. Wipe down your tools with alcohol before storing them. Wash your clothing. (Never enter the OVF Greenhouse if you have been around diseased plants).
  7. At the end of the plant's life, bag and put it in the dumpster. Even if the stalk looks like it may have healed itself the Sporangia are still there and are particularly virulent. Trash them.

One last point: most agricultural research sources indicate that Late Blight will not live in soil or organic debris because it needs a living host. Not quite true. The living host caveat only applies to asexually reproduced Sporangia which will die without a living host (i.e. potato=living host). The Oospores (sexually reproduced) have a much tougher cell wall and are able to survive in the soil. Per a report published in December 2000 "Production, survival and infectivity of oospores of Phytophthora infestans" Oospores in infected potato fields in the Netherlands survived for 48 months. Both Oospores and Sporangia can overwinter in potatoes both in and out of the ground and sporulation will occur again when the temperature and moisture conditions are right. A 2007 Brazilian study "Management of Late Blight with Alternative Products" states, "...the pathogen survives from season to season in infected tubers and to a lesser extent on crop debris." When you yank diseased plants out, bag them and put them in a dumpster, not the compost. Unless you bring a microscope to distinguish the Oospores from the Sporangia better to not risk spreading the disease.

With any luck you might salvage a few harvestable plants over the season. No promises; it requires a lot of hard work and there is the risk of keeping a virulent disease alive in the garden. Use common sense and your powers of observation. See anyone overhead watering their plot? Are they overhead watering at dusk? Yep, they are creating the perfect environment for a Late Blight epidemic. Where is Lady Macbeth when we need her.

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Additional Reading:

University of Delaware Fungicides and Bactericides (PDF)

University of Wisconsin BioWeb

University of Massachushetts Extension Late Blight Management (PDF)

University of Massachushetts Tomato Late Blight

New York State Integrated Pest Management

Ohio State University Extension

Wiley Online Library - Plant Pathology (Full)

Wiley Online Library - Plant Pathology (Abstract)

Toxipedia Connecting Science and People

Food and Agriculture Organization of the U.N. - Late_Blight_Cycle (PDF)

Michigan State University - Tomato Late Blight (PDF)

Cornell University - Crop Management Practices

Cornell University - Material Fact Sheets

Windrose Farm

British Columbia Ministry of Agriculture

American Phytopathological Society Late Blight of Potato and Tomato

American Phytopathological Society 2009 Late Blight Pandemic in Eastern USA

American Phytopathological Society Mini Review (PDF)

Bangor University (U.K.) Phytophthora (PDF)

Colorado State University Late Blight Management Plan

University of Hawaii Cooperative Extension Late Blight of Tomato (PDF) - Organic Management of Late Blight

Fresh from Florida Tomato Late Blight (PDF)

Global Science Books - Organic Management of Late Blight (PDF)

UC Davis - Pest Management Guidelines - Late Blight Indentification (PDF) - phytophthora/

OMRI - How the Potato Changed the World

U.S. Department of Agriculture - Soil-Borne Oospores of Phytophthora

Inoversity of Vermont - Late Blight of Tomato and Potato (PDF)

California Irrigation Management Information System

Swedish University of Agricultural Sciences (PDF)

Library 4 Farming - Plant Diseases

Subrealism Blog - Late Blight in New England