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Alternaria Leaf Spot

Alternaria Leaf Spot

Alternaria species

Fungalalso: Alternaria leaf blight, Black spot of brassicas

A common fungal leaf spot of the cabbage family that rings the leaves with brown target-like spots and can rot into the heads of broccoli and cauliflower. It shows up on cabbage, kale, broccoli, and their kin, especially in warm, wet weather, downgrading the harvest and sometimes spoiling the part you eat.

🔎 How to spot it

Look for round brown spots with dark concentric rings, like a target, often surrounded by a yellow halo, on the older, lower leaves first. The spot centers may dry and crack or drop out, and heavy spotting merges into large dead patches that yellow and kill the leaf. On broccoli and cauliflower the fungus can spot and rot the head itself.

🥀 Damage it causes

Spotting kills leaf tissue and defoliates plants, weakening them and cutting yield, and on the cole crops grown for their heads the disease can spot and rot the head directly, ruining the harvest. Spotted, browning leaves of leafy brassicas are also directly unmarketable.

🔬 What causes it

Alternaria leaf spot is caused by Alternaria fungi, which overwinter on crop debris and related weeds and ride in on seed. Spores spread by wind, splashing rain, and overhead watering and infect through the leaves. The disease is favored by warm, humid weather and extended leaf wetness, building through the warm part of the season.

🛡️ Prevent it

Start with clean or hot-water-treated seed and rotate brassicas to a new spot for two to three years. Space plants for good airflow, water at the base rather than overhead, and clean and sanitize seed-starting trays each year. Straw mulch lowers disease by blocking soil splash, and removing whole infected plants early limits spread.

🧯 If it is already here

Remove and destroy whole infected plants and the lower spotted leaves as they appear, and keep the foliage dry. On a valued crop, an approved organic fungicide such as copper, applied early and repeated per the label, can slow spread. Clean up all crop debris at season end so the fungus does not carry over.

💡 Good to know

The target-like concentric rings make Alternaria easy to spot and to tell from the bacterial black rot that also hits brassicas, which makes V-shaped edge lesions instead. Because the fungus rides on seed and debris, clean seed, rotation, and cleanup matter as much as any spray. It is one of the most common diseases of broccoli and kale.

🌱 Plants it affects

336 plants in the library can be affected by this problem

Adirondack Blue PotatoAfrican MarigoldAgapanthusAgeratumAji Amarillo PepperAlice du Pont Mandevilla🥔All Blue PotatoAmethyst Falls WisteriaAmish Paste TomatoAnaheim PepperAnemoneAngelique TulipAngeloniaAnnabelle Smooth HydrangeaAnnual VincaApeldoorn TulipApril Tryst CamelliaArizona Sun Blanket FlowerArmenian CucumberArugula🥕Atomic Red CarrotAugust Beauty GardeniaAutumn Joy SedumBachelor's ButtonBanana PepperBarbara Karst BougainvilleaBeauregard Sweet PotatoBecky Shasta DaisyBee Balm🍅Beefmaster TomatoBenarys Giant ZinniaBengal Tiger CannaBetter Boy Tomato🍅Big Beef TomatoBig Boy TomatoBitter MelonBlack Beauty EggplantBlack Cherry TomatoBlack Krim Tomato🥕Black Spanish RadishBlack-Eyed PeaBlack-eyed Susan VineBlood OrangeBlue Bird DelphiniumBlue Bird Rose of SharonBlue FescueBok Choy🥕Bolero Carrot🥒Boston Pickling Cucumber🍅Box Car Willie TomatoBrandywine TomatoBroccoli RabeBrunneraBrussels SproutsBurning BushButterfly Blue Pincushion FlowerButterfly Marguerite DaisyButterfly WeedCafe au Lait DahliaCalabrese BroccoliCaladiumCalendulaCalibrachoaCalifornia Giant ZinniaCalifornia PoppyCampanulaCardinal FlowerCarolina GeraniumCarolina JessamineCarolina Reaper PepperCasaba MelonCascara SagradaCayenne PepperCelebrity Tomato🥬Champion CollardsChantenay CarrotCheddar CauliflowerCherokee Purple TomatoCherry Belle RadishChinese BroccoliCleyeraClimbing HydrangeaClimbing Prairie RoseCocktail Vodka BegoniaColeusCoral Drift Groundcover RoseCornelian CherryCosmic Purple CarrotCosmosCreeping PhloxCrenshaw MelonCrimson Cherry RhubarbCubanelle PepperCupani Sweet PeaDaikon RadishDanvers CarrotDavid Garden PhloxDe Cicco BroccoliDelft Blue HyacinthDenim n Lace Russian Sage

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.