Sooty Blotch and Flyspeck

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

Sooty blotch and flyspeck on Golden Delicious (Clemson University - USDA Cooperative Extension Slide Series, 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. Let’s look at 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 skin. These patches are usually ¼” in diameter or larger. They can merge to cover large areas of the fruit. These smudges are made up of hundreds of tiny fungi that are connected by hyphae. Sooty blotch is caused by several different fungi: Peltaster fructicola, Geastrumia polystigmatis, and Leptodontium elatius.


Flyspeck

Flyspeck is caused by Zygophiala jamaicensis. It gets the name because of the clusters of black flecks that look like fly poop on the fruit skin. Those tiny flecks can coalesce into large scabby or blotched areas. If you rub or scratch off these scabs you will find normal healthy fruit skin underneath.


Together, sooty blotch and flyspeck (SBFS) create dark blemishes on 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 mostly cosmetic. This is because the fungi live on and in the waxy cuticle and do not actually enter the fruit.

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

It is believed that spores hibernate on ash and willow trees, hedgerows, and wild brambles, along with 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 fruit 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 disease, as does removing mummies and keeping the area under trees clear of weeds. Commercial growers spray fungicide 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 with baking soda. They are NOT the same thing.


Keep your fruit dry and sooty blotch and flyspeck will never get to your harvest.