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Department of Plant Pathology

Southeastern Apple Production

PLANT PATHOLOGY

apple with flyspeck disease

Sooty Blotch, Flyspeck

Sooty blotch, a disease complex caused by Peltaster fructicola, Leptodontium elatius, Geastrumia polystigmatis and other fungi, and flyspeck, caused by Schizothyrium pomi, are the most common diseases of apple in the southeastern United States. Without the use of fungicides, they would occur on nearly 100% of fruit year in and year out, and probably cause more losses to growers than any other disease (Fig 1). The fungi causing these diseases affect all cultivars of apple but grow only on the surface of the fruit, causing cosmetic damage that results in fruit being downgraded from fresh to processing or juice grades. Both diseases often occur on the same fruit.

Symptoms

Sooty blotch colonies appear dusty to olive green or black on mature fruit (Fig 2), depending on the particular fungus species involved. Colonies vary in size from a few tenths of an inch in diameter to large colonies that cover much of the fruit. Colonies of P. fruiticola are characterized by the presence of numerous fruiting structures known as pycnothyria (Fig 3). Colonies of G. polystigmatis are dark and have scattered pycnothyria, and colonies of L. elatius do not have any pycnothyria. Flyspeck colonies are characterized by the presence of small, shiny black fungal fruiting structures, known as thyrothecia, which are grouped in an irregular to circular pattern (Fig 4). Colonies may be small with only a few fruiting structures, or large and contain hundreds of fruiting structures.

Fig 1 - Extensive infection

Fig 1. Extensive infection

Fig 2 - Sooty blotch

Fig 2. Sooty blotch

Fig 3 - P. fruiticola pycnothyria

Fig 3. P. fruiticola pycnothyria

Fig 4 - Flyspeck colony

Fig 4. Flyspeck colony

Disease Cycle

The fungi that cause sooty blotch survive from one season to the next on apple twigs and any other perennial vegetation with a waxy cuticle that can serve as a reservoir host. These fungi are dispersed by wind and in windblown rainwater to developing fruit in the spring and early summer. Secondary spread from these primary colonies occurs throughout the summer. Fruit infections can begin 2 to 3 weeks after petal fall. The optimum temperature for growth of P. fruiticola is between 65° and 80°F. Growth does not occur at temperatures above 86°F. L. elatius is more tolerant of high temperatures and will grow up to 90°F. Neither fungi grow at relative humidities less than 95%. It usually takes 20 to 25 days for the colonies to appear on fruit after infection occurs. Little information is available on the other fungi in the sooty blotch complex.

Schizothyrium pomi, the fungus that causes flyspeck, also overwinters on apple twigs and other perennial hosts. The initial infection occurs from airborne ascospores that are produced for a period of about 2 months beginning around bloom. Symptoms appear in the orchard about 3 to 6 weeks after infection. Secondary spread is by airborne conidia produced on infected apple fruit and twigs as well as on numerous reservoir hosts that surround the orchard. The optimum temperature for fruit infection is about 65°F, but infection can occur at temperatures up to 82°F.

Control

Control of sooty blotch and flyspeck is achieved though dormant and summer pruning, tree training and fungicide sprays. Sooty blotch and flyspeck are usually most severe in the middles and tops of poorly pruned trees where the fruit stay wetter longer and fungicide penetration is poorest. Both dormant and summer pruning facilitates drying and fungicide deposition within the tree canopy. The diseases are also common on tightly clustered fruit. Removal of reservoir hosts, especially brambles (blackberries), will also aid in controlling these two diseases.

Fungicides applied on a preventive basis should begin about second cover and continue at 10- to 14-day intervals until harvest. The benzimidazole fungicide thiophanate methyl (Topsin-M 70W) has some eradicant activity on the two diseases and its inclusion in the cover spray program can be timed according to the hours of leaf wetness. A minimum of 100 gallons of water per acre as a carrier is needed to assure good fungicide distribution throughout the canopy and within clusters of fruit.


Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

 

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Department of Plant Pathology
North Carolina State University
Raleigh, NC 27695 USA


Web Crafters: Anne S. Napier and Steve Schoof
Email: steve_schoof@ncsu.edu

 

Format updated March 29, 2011