Introduction: Sooty blotch and flyspeck are surface blemish diseases that commonly
appear together on apple or pear in late summer and fall. Although these diseases may
shorten the storage life of fruit due to increased water loss, they do not cause decay,
and losses are attributed to unacceptable appearance. During wet growing seasons, losses
of 25 percent or more are commonly found even in orchards treated with fungicides.
II. Symptoms: Sooty blotch appears as sooty smudges
or olive-green spots on mature fruit (photo 2-28). Individual spots or smudges vary from
discreet circular colonies to large lesions with diffused margins. Different colony
appearances are attributable to several different pathogens which comprise the disease
complex. Flyspeck is characterized by clusters of 10 to 50 sharply defined black shiny
specks on the fruit surface (photo 2-29). These superficial colonies are round to
irregular and usually measure 1/16 to 1 inch (8-25 mm) in diameter. The individual dots or
specks are fruiting structures in which spores are formed that cause secondary spread.
Although these diseases may appear separately, they are commonly found together on the
same fruit. Typically fruit symptoms are observed by the first of July and become more
easily found as the season progresses. There are no significant differences among apple
cultivars in susceptibility to these diseases, but symptoms are more apparent on yellow,
green, or light colored fruit. Fruits of apple and pear having the thickest cuticle appear
to be more severely affected.
III. Disease Cycle: These fungi are commonly found
on the stem surfaces of many woody plants, including apple shoots. Infections may occur on
fruit as early as two to three weeks after petal fall, and are highly favored by frequent
rain periods and poor drying conditions. Mycelial growth that forms the sooty blotches can
occur in the absence of free water at relative humidity greater than 90 percent. Symptom
development of both diseases is relatively slow, typically requiring 20 to 25 days in the
orchard, but may occur in 8 to 12 days under optimum conditions. Optimum conditions for
conidial production for the flyspeck pathogen are 60 to 70 F (16-21 C) and relative
humidity greater than 96 percent.
Monitoring: At midseason, observe 25 fruit in the interior canopy of sample trees.
Symptoms (photos 2-28, 2-29) are more likely to be found in poorly pruned trees in the
wetter, foggy, slow-drying areas of the orchard. Expect first symptom expression by early
to mid-July. Fungicides should be applied to fresh fruit showing any infections. Presence
of these diseases is a good indicator that fungicide surface residues are lacking or very
low, and signals potential need for treatment to control these diseases or the
decay-producing fungal pathogens.
V. Management: The diseases are managed by
orchard sanitation and the use of fungicides (Table of
effectiveness of apple fungicides). Removing reservoir hosts, especially brambles,
from the orchard and surrounding hedgerows helps reduce the amount of inoculum from
external sources, but in wet years this practice alone may not be adequate for disease
control. Some cultural practices may help prevent the diseases and/or reduce the severity
of sooty blotch and flyspeck. These include dormant and summer pruning to open up the tree
canopy and thinning to separate fruit clusters. In addition to facilitating the drying of
fruit after rain or dew, these practices favor better spray coverage and improve fruit
quality. Both diseases are difficult to control in orchards with restricted air movement.
A predictive model for sooty blotch was developed in North
Carolina by Turner Sutton. The model is driven by the accumulation of wetting hours
beginning 10 days after petal fall. The goal of the model is to help time the first spray
for sooty blotch based on the appearance of sooty blotch symptoms.
Credits: Text prepared by A. R. Biggs, from the original
text in the Mid-Atlantic Orchard Monitoring Guide (original text by K. D. Hickey and K. S.
Yoder). Table of fungicide effectiveness from the 1997 Va./W.Va./Md. Spray Bulletin for
Commercial Fruit Growers, table compiled by K. S. Yoder and A. R. Biggs.