Introduction: There are
several diseases of apple that are considered "minor diseases" because they are
sporadic in occurrence or are managed well in programs developed for our more important
diseases. Nevertheless, we see them almost every year somewhere.
Brooks Spot, Mycosphaerella pomi
I. Introduction: Brooks spot is a minor disease that occurs throughout the northeastern and mid-Atlantic
apple growing regions of the U.S. and occasionally as far westward as Iowa. Some of the
more susceptible common commercial cultivars include 'Jonathan', 'Golden Delicious',
'Stayman Winesap', 'Grimes Golden', and 'Rome Beauty'.
II. Symptoms: Brooks spot first appears as
irregular, slightly sunken dark green lesions typically on the calyx end of immature apple
fruit (photo 2-37). As the fruit matures, the lesion turns dark red or purple on red areas
of the fruit and remains dark green on green or yellow areas (photo 2-38). The disease is
sometimes confused with the physiological disorders Jonathan spot, cork spot, and bitter
pit. However, Brooks spot usually appears earlier in the season and shows little browning
of the flesh underneath the lesion. Jonathan spot lesions are usually more round and the
lesion edge is more abruptly sunken, with a shallow browning of the flesh underneath.
Disease Cycle: Primary infection is initiated by ascospores which are discharged from
overwintering leaves in late spring and early summer. Ascospores germinate in six hours at
61 to 75 F (16-24 C). Leaf infection through stomata can occur after 96 hours of
continuous wetting at 68 F (20 C), but infection may be enhanced by alternating wetting
and drying conditions. Fruit lesions appear in July and August. Secondary infection is not
known to occur. Leaf infections remain inactive until late summer when small purple
lesions begin to appear. Following leaf fall, the fungus colonizes the leaf extensively.
IV. Monitoring: Ascospores are discharged from
overwintering leaves in late spring and early summer. Wetting periods of six hours or more
at 61 to 75 F (16-24 C) can be recorded as infection periods. Fruit lesions may begin to
appear in early July. Monitoring of fruit symptom development (photo 2-37) is useful to
improve recognition of this minor disease and its control in subsequent years; however, by
the time symptoms are recognized it is usually too late to control in the year of
infection and there is no secondary infection.
V. Management: Most of the fungicides used
in the early cover sprays for summer diseases are effective against the Brooks spot
fungus. The sterol demethylation inhibitor (DMI) fungicides used for early season
scab, mildew, and rust control are ineffective. In Virginia when there is
significant infection in scattered orchards, typically it results from over-extended spray
intervals a month to six weeks after petal fall.
Black pox, Helminthosporium papulosum
I. Introduction. Black pox is a minor
fungal disease which can affect apple bark, fruit, and foliage. The disease is more common
from southern Virginia southward than in the northern mid-Atlantic region.
II. Symptoms. Infection first appears on current
season twigs as well-defined, conical, shiny black lesions which enlarge by the end of the
first season. Extensive secondary lesion enlargement may girdle small, heavily infected
twigs. Smoothbarked twigs remain susceptible for several years and twig infection is
cumulative throughout this period. Twig symptoms may be confused with the nutritional
disorders measles and internal bark necrosis. Fruit lesions are small, black, circular and
slightly sunken (photo 2-44). Leaf lesions begin as red halos with light green centers,
and become tan to brown with purple borders. Severe leaf infection may cause defoliation.
Differences in cultivar susceptibility have been noted, with 'Rome Beauty' and 'Grimes
Golden' being more susceptible than 'Yellow Transparent' and 'York Imperial'.
III. Disease Cycle. This wet weather fungus
overwinters and produces conidia in old bark lesions. The optimum temperature for growth
is 82F (28C). Leaf infections may become evident in midsummer, and fruit and stem lesions
slightly later. The incubation period is three to six months on fruit and three to ten
months on bark. If fruit is harvested relatively early in the season, infection may occur
after the last preharvest fungicide application, allowing spread of the disease to
unprotected new growth and new trees.
IV. Monitoring. Beginning in the dormant season,
look for bark lesions on ten shoots per tree in blocks where this minor disease has been a
problem. Record the number of shoots with lesions. At midseason, black pox Infection may
occur during warm wetting periods. Monitor for leaf and fruit (photo 2-44) infections
which may appear in problem areas by midsummer. The incubation period is three to six
months on fruit and three to ten months on bark.
V. Management. Disease-free planting stock
is important to help prevent spread of the disease. Most fungicides used in the
cover sprays at 14-day intervals for summer diseases will control black pox. On
early-maturing apple cultivars, postharvest sprays may be needed to reduce the buildup of
the disease on leaves and twigs after harvest. The disease is often more severe on
'Golden Delicious' than other cultivars.
I. Introduction: Although now uncommon,
incidence of X-spot as high as 86 percent was reported on unprotected 'Golden Delicious'
and 'Rome Beauty' fruit in 1950. Other susceptible cultivars include 'Jonathan', 'Stayman
Winesap', and 'York Imperial'.
II. Symptoms: The names X-spot, X-rot, and
Nigrospora spot have been applied to a small, circular, depressed, necrotic spot typically
on the calyx end of apple fruit in the mid-Atlantic region (photo 2-43). A fungus has been
associated with X-spot lesions, but the typical lesion symptom has not been duplicated by
III. Disease Cycle: The identity of the causal
organism has not been confirmed and the disease cycle has not been resolved.
IV. Monitoring: Symptoms of this minor disease
appear late in the season on susceptible cultivars such as 'Golden Delicious', 'Rome',
'Jonathan', 'Stayman Winesap' (photo 2-43), and 'York'. Infection may occur from July to
September. Observe 25 apples on each sample tree for the presence of this disease where
there has previously been a problem.
V. Management: Fungicides applied
for summer diseases provide effective control of X-spot.
Chemical control -
Chemical control - home
orchardists (pdf file - Acrobat Reader required)
Text prepared by
K. S Yoder, Virginia Tech,
A. R. Biggs, West Virginia University.