Thursday, September 29, 2011

Sooty Blotch and Flyspeck of Apples

Sooty Blotch and Flyspeck (Photo: Bugwood)
With fall coming fast, it’s time to gear up for apple season. Before you make your trip to the local farmer’s market, we decided to introduce you to the two most common apple diseases in North Carolina, sooty blotch and flyspeck. Without the use of fungicides, these two diseases would affect virtually all apples grown in the southeastern United States. These diseases are found on all cultivars of apples and both diseases can be found on the same apple at the same time.

The most common symptom associated with sooty blotch is the growth of feathery, olive green fungal colonies on the surface of mature fruit. Flyspeck can be recognized by the presence of very small, shiny, black, dots arranged in an irregular or circular pattern on the fruit surface. This gives fruit the unappetizing appearance of being covered with fly droppings.

Sooty Blotch and Flyspeck  (Photo: Bugwood)  
Both of these diseases cause only superficial damage on the apple surface. The fungi that cause them, grow on the surface of the cuticle and do not damage the apple itself or affect its flavor or quality. Sooty blotch and flyspeck cause losses in commercial apple production because affected fruit are downgraded from valuable fresh market grades to much cheaper processing or juice grades.
Sooty Blotch and Flyspeck (Photo: PDIC Database)

Over 60 putative species of fungi have been associated with sooty blotch and flyspeck worldwide. In the Southeast, Peltaster fructicola, Leptodontidum elaitus, Stomiopeltis spp. and Geastrumia polystigmatis are the most common species associated with sooty blotch and Schizothyrium pomi is the most common species associated with flyspeck. Fungi that cause sooty blotch survive the winter on apple twigs and reservoir hosts surrounding the orchard. Spores are dispersed by wind and windblown rainwater to developing fruit in the spring and early summer. Secondary spread occurs throughout the summer. Usually, symptoms can be seen 20 to 25 days after infection.

S. pomi also overwinters on apple twigs and other perennial reservoir hosts. Airborne ascospores are the primary means of infection and are usually produced for about 2 months beginning around bloom. Specks, which are actually fruiting bodies of the fungus, appear about 3 to 6 weeks after infection. Secondary spread occurs through windblown conidia produced on infected fruit and twigs and on reservoir hosts surrounding the orchard.

Sooty Blotch (Photo: PDIC Database)
Commercial growers control these diseases with cultural practices and fungicide applications. Cultural control practices include pruning during dormant and summer seasons and fruit thinning. Pruning and fruit thinning reduce the drying time within the canopy and allow better fungicide penetration through the canopy. Sooty blotch and flyspeck fungi require wetness to infect. Pruning and fruit thinning discourages these fungi by increasing airflow through the canopy. Mowing weeds and grasses under apple trees also helps trees stay dry. Another important cultural practice is removal of surrounding reservoir hosts, especially brambles. Surrounding blackberry plants act as harbors where these fungi are able to multiply during the season and then spread to nearby apple fruit.

In addition to cultural practices, these diseases are controlled with preventative fungicide applications. Preventative sprays need to be applied beginning about second cover (about a month after bloom) and continuing at 10 – 14- day intervals until harvest.

Since sooty blotch and flyspeck are harmless to people and apples, there is no reason to apply fungicides to control them at home – nor do you need to avoid buying apples with sooty blotch and flyspeck, especially for cooking or baking. Most of the fungal colonies can be removed by washing the apples in a mild solution of bleach.

For more information:
Apple Disease Factsheet

Special thanks to Dr. Turner Sutton for helping with this post!