Lesion on top of leaf
Sporulation on bottom of leaf
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?
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:
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.
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
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)
Extension.org - 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
Maine.gov - Late Blight Indentification (PDF)
MetaPathogen.com - phytophthora/
Smithsonian.com - 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