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Late Blight

Late Blight

Phytophthora infestans

Water moldalso: Tomato and potato blight

The most destructive disease of tomato and potato, late blight is the water mold that caused the Irish potato famine. It does not appear every year, but when cool, wet weather brings it in it can rot an entire planting in a week or two. Fast scouting and fast removal are the keys to keeping it from spreading.

🔎 How to spot it

Look for large, dark brown to gray-green greasy blotches on leaves and stems, often with a pale green or water-soaked edge, that enlarge quickly. In humid weather a fine white mold appears at the edge of the spots on the leaf underside. Tomato fruit develops firm, dark, greasy-looking patches; potato tubers show brown to reddish discoloration. Whole plants can collapse as if hit by frost.

🥀 Damage it causes

Late blight can cause total crop failure if it is not stopped. Under cool, wet conditions entire plants turn brown and wilted within days, and fruit and tubers rot. Because each lesion makes thousands of windborne spores in under a week, it spreads explosively from plant to plant and garden to garden.

🔬 What causes it

The disease is caused by the water mold Phytophthora infestans, not a true fungus. It favors cool, damp weather around 60 to 70 F and prolonged leaf wetness, and stalls in hot, dry spells. It arrives on infected potato seed tubers or tomato transplants, in cull piles, or on spores blown in from the south, and it can overwinter in living potato tubers.

🛡️ Prevent it

Plant certified disease-free seed potatoes and inspect transplants before buying. Destroy cull piles and volunteer potato and tomato plants before the season. Water at the base, space and stake plants for fast drying, and choose resistant varieties where available. Scout in the morning before the dew dries, and stay current with regional late blight alerts.

🧯 If it is already here

Act fast: if only a few plants are affected, pull and bag them or bury them deeply right away to protect the rest. Once it is established, water mold specific fungicides applied preventively can slow it, but ordinary fungicides do not work; follow the label and rotate products. Never compost infected plants, and dig and destroy any infected potato tubers so they do not carry the pathogen into next year.

💡 Good to know

Late blight is a community problem, because spores travel on the wind for miles, so reporting and removing it protects neighboring gardens too. It is mainly limited to tomato and potato, though related weeds like hairy nightshade can carry it. Track regional outbreaks at resources such as USAblight.org.

🌱 Plants it affects

73 plants in the library can be affected by this problem

Adirondack Blue PotatoAji Amarillo Pepper🥔All Blue PotatoAmish Paste TomatoAnaheim PepperBanana PepperBeauregard Sweet Potato🍅Beefmaster TomatoBetter Boy Tomato🍅Big Beef TomatoBig Boy TomatoBlack Beauty EggplantBlack Cherry TomatoBlack Krim Tomato🍅Box Car Willie TomatoBrandywine TomatoCarolina Reaper PepperCayenne PepperCelebrity TomatoCherokee Purple TomatoCubanelle PepperEarly Girl TomatoFairy Tale EggplantFingerling PotatoFresno PepperGerman Butterball Potato🍅German Queen TomatoGhost PepperGreen Bell PepperGreen Zebra TomatoHabanero PepperHungarian Wax PepperIndigo Rose TomatoItalian EggplantJalapeño PepperJapanese EggplantJuliet Grape TomatoKellogg's Breakfast TomatoKennebec Potato🍅La Roma IV TomatoLemon Boy Tomato🍅Mortgage Lifter TomatoMr. Stripey TomatoNorland PotatoOrange Bell PepperPadrón PepperPimento PepperPineapple TomatoPoblano PepperPurple Bell Pepper🥔Purple Majesty PotatoPurple TomatilloRed Bell PepperRed Pontiac PotatoRoma Tomato🍆Rosa Bianca EggplantRusset Potato🍅Rutgers TomatoSan Marzano TomatoScotch Bonnet PepperSerrano PepperShishito Pepper🍅Striped German Tomato🍅Sungold Cherry TomatoSweet 100 Cherry TomatoSweet Banana PepperThai Chili Pepper🍅Tiny Tim Cherry TomatoToma Verde TomatilloTrinidad Scorpion PepperYellow Bell PepperYellow Pear TomatoYukon Gold Potato

For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.