Minor Diseases of Apples - Fruit Disease Focus - August, 1998
Mycosphaerella pomi, Helminthosporium papulosum  
Brooks spot
Brooks spot
Black pox lesions on fruit.
X-spot lesions on apple fruit.

 

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.

III. 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.

X-Spot

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 controlled inoculation.

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.

General Recommendations:

Chemical control - commercial growers

Chemical control - home orchardists (pdf file - Acrobat Reader required)

 

Text prepared by K. S Yoder, Virginia Tech, and A. R. Biggs, West Virginia University.