Sooty Blotch and Flyspeck

Sooty blotch and flyspeck are two different conditions. They occur together so often that the combined name is now an all-in-one fungal disease. It is also known as apple summer disease.

Sooty blotch and flyspeck on Golden Delicious (Clemson University - USDA, Bugwood.org) CC BY 3.0 US

Apple summer disease can affect bananas, blackberries, citrus, crabapple, papaya, pears, persimmons, raspberries, and several other tree and vine crops, along with apples. We will explore the two conditions individually and then learn what we can do about them.

Sooty blotch 

Sooty blotch looks like small gray patches just under the fruit skin. These patches are usually ¼” in diameter or larger. They can merge to cover large areas of the fruit. These smudged areas are hundreds of tiny fungi connected by hyphae. Peltaster fructicola, Geastrumia polystigmatis, and Leptodontium elatius fungi are all responsible for sooty blotch.

Flyspeck

Zygophiala jamaicensis fungi infect fruit, generating black flecks that look like fly poop on the fruit skin. Those tiny flecks can coalesce into large scabs or blotchy areas. Rub or scratch off these scabs and see normal healthy fruit skin underneath. 

Sooty blotch and flyspeck (SBFS) work together to create dark blemishes on the fruit. These mycelia can also grow on leaves, stems, and twigs.

Summer apple disease lifecycle

Scientists are still learning about this disease. We do know that the damage is primarily cosmetic. The fungi live on and in the waxy cuticle and do not enter the fruit.

Sooty blotch and flyspeck on Haralson apples (Wuerzele) CC BY-SA 4.0

Spores hibernate on ash and willow trees. They also occur in hedgerows, wild brambles, and our beloved edibles. Humidity favors the growth of these fungi. In particular, temperatures between 60°F and 70°F with a relative humidity greater than 96% set the stage for massive fungal growth.

As apples and other fruits mature, they exude a tiny bit of fruit juice through microscopic tears in the skin. Fungal spores land on this juice and use it as food, entering the skin through the tears. Surprisingly, infection with summer apple disease does not lead to fruit decay. But it does reduce storage time.

Sooty blotch and flyspeck management

Sooty blotch and flyspeck is most likely to occur on lighter-colored, thicker-skinned, slower-to-mature fruit from older trees growing in regions with fog or frequent rain. Hot, dry weather halts the growth of these fungal pathogens.

Pruning for good airflow is the best way to prevent sooty blotch and flyspeck disease. Proper fruit thinning will also reduce the chance of this disease, as does removing mummies and keeping the area under trees clear of weeds. Commercial growers spray fungicides to treat summer apple disease along with apple scab. Organic growers spray with cocoa soap or lime sulfur, neither of which I’ve ever heard of. Home growers are finding that sprays of potassium bicarbonate (KHCO3) work well. Do not mistake potassium bicarbonate for baking soda. They are NOT the same thing.

Keep your fruit dry to prevent sooty blotch and flyspeck from harming your harvest.