What are Sooty Blotch and Flyspeck?
Sooty blotch and flyspeck are two distinct fungal diseases that typically occur on apple fruit at the same time in the Summer. The specific environmental conditions that favor these diseases cause them to appear together. Both diseases have negative economic impacts to growers since they damage the aesthetic appearance of the apple fruit, often making them unfit for fresh market sale. Sooty blotch is caused by Gloeodes pomigena and flyspeck is caused by Schizothyrium pomi.
What do Sooty Blotch and Flyspeck look like?
Sooty blotch can be identified as dark, brown, or olive-green fungal colonies that can resemble patches of soot on the surface of the apple. These patches have irregular, undefined edges and can eventually envelop the whole fruit. The infection is restricted to the fruit surface, so the patches can be wiped off, but this does not guarantee full removal. The infection can start as early as 3-4 weeks after petal fall, but more severe infections are observed in late summer. The beginning and end of the growing season are well suited for the infection because the cool humid environments promote disease progress.
Flyspeck symptoms are mainly small black shiny dots on the fruit surface that can resemble minor insect damage. The spots will group in clusters of 6 to about 50 and can range from small contained groups or span the whole fruit in severe cases. These blemishes are strictly superficial and can reduce the value of the fruit. Symptoms and spore production are optimal once the temperature approaches 65F(18ºC). Disease progression nearly ceases in activity at 85F(30ºC). Both sooty blotch and flyspeck will appear around the same time, but they are easily distinguishable.
Where do Sooty Blotch and Flyspeck come from?
Sooty blotch can overwinter in the orchard or be spread from a wide range of alternative wild hosts. This fungus will overwinter inside any twigs and branches left in the orchard at the end of the season. During the early spring, rain and high humidity signals the fungus to begin active growth and development. The spores infect branches on the trees first, then after an incubation of about 2-3 weeks, the infection spreads to the fruit. Over the course of the season, large amounts of spores collect into a gelatinous mass on the fruit. This will then spread by water splash or strong wind carrying a mist of the spores.
Flyspeck has a disease cycle that is very similar to sooty blotch. Flyspeck can overwinter in any infected wood material and begin spreading the following spring. Each individual speck is a fungal fruiting body that can produce spores. These spores are carried by the wind to infect new hosts, whether in the orchard or on a wild host. This activity is typically only on days with a relative humidity of 95% or higher. Within 15 days, new symptoms will begin to emerge, and the cycle will repeat. There are not many known apple cultivars with genetic resistance to infection by these diseases.
How do I prevent and control Sooty Blotch and Flyspeck?
– Prune tree canopies to reduce density and allow proper air circulation
– Mow middle grass rows to reduce orchard humidity in the summer
– Mitigate the number of alternative hosts around the orchard
– Choose a planting site that has a lower average humidity
For more information on Sooty Blotch and Flyspeck:
A New View of Sooty Blotch and Flyspeck (Gleason et al. 2011)