Scaffolds 01 index

(Bill Turechek wwt3@nysaes.cornell.edu & Dave Rosenberger dar22@cornell.edu, Plant Pathology)

Sooty blotch and flyspeck are two of the most important summer diseases of apple in New York. The diseases do not result in direct losses in yield, but rather cause a reduction in fruit quality, which can lead to economic loss due to downgrading in fresh market fruit. Losses can exceed 25%, especially in much warmer climates such as those encountered in southeastern NY.

Until recently, sooty blotch was thought to be caused by the fungus Gloeodes pomigena. However, recent studies have shown that sooty blotch is a disease complex caused by at least 3 different fungi: Peltaster fruticola, Leptodontium elatius, and Geastrumia polystigmatis. All three fungi are not necessarily present in all sooty blotch lesions. Flyspeck is caused by the fungus Schizothyrium pomi (= Zygophiala jamaicensis).

Symptoms

Sooty blotch appears as various shades of olive-green on the surface of the fruit. Colonies range in shape from nearly circular with distinct margins to rather large, amorphous blotches with diffuse margins. The variation in shapes and color can be attributed to the interaction between the different fungi causing the disease and environmental conditions, specifically temperature and relative humidity.

Flyspeck appears as distinct groupings of shiny, black fungal bodies (called thyriothecia) on the surface of the fruit.

APPLE FLYSPECK

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The number of thyriothecia associated with a single infection ranges from a few to over fifty. Although flyspeck thyriothecia appear to exist individually, close examination reveals mycelium connecting the individual structures.

For both flyspeck and sooty blotch, the causal fungi grow only within the wax cuticle of the fruit and are quite superficial. Rubbing the fruit with a cloth will often be enough to "clean-up" an apple that is only lightly affected.

Disease Cycle

The specific details of the life cycle of the sooty blotch fungi P. fruticola, L. elatius, and G. polystigmatis are not specifically known because they have only recently been identified as the cause of sooty blotch. However, the disease cycle is assumed to be similar to that described for Gloeodes pomigena. The fungi overwinter on infected twigs on apple and on its numerous wild hosts. Conidia are formed in late spring and early summer and are dispersed to developing fruit by wind and splashing rain. Fruit infection typically occurs from late-April to mid-May in the southeastern United States and in June in the northern and northeastern United States. The first symptoms are generally apparent 20-25 days after infection, but can be visible in 8-12 days under optimal conditions.

In Pennsylvania, it was found that the development of sooty blotch was highly correlated with the amount of rainfall received in July, and to a lesser degree in August and September. In laboratory studies, conidia of P. fruticola germinated between 12-24°C and between 12-32°C for L. elatius at a relative humidity of 95%. The optimum temperatures for fungal development were between 12-24°C and 16-28°C for P. fruticola and L. elatius, respectively. The production of conidia of both fungi was greatest when the relative humidity exceeded 97%.

Flyspeck overwinters as thyriothecia on apple twigs, culled apple fruit, and on numerous wild hosts. Ascospores mature and are discharged shortly after bloom and initiate infection (Williamson & Sutton, 2000). The time of discharge varies from region to region and in relation to environmental factors. Symptoms are visible 10-12 days after infection under optimal conditions, but may not occur for 1 month under less than ideal conditions. Initial infections will give rise to conidia, which initiate secondary infection throughout the remainder of the season.

Numerous observations in the field have shown that warm and wet or humid conditions are needed for the development of disease. Laboratory studies have shown that conidia can germinate within the range of 8-24°C, colony development occurs over the range 5-28°C, and spore production readily occurs between 12-24°C (Ocamb-Basu et al., 1988a; Williamson & Sutton, 2000). All three processes require that the relative humidity exceed 96%. The development of asci was initiated at temperatures between 4-6°C and ascospore maturation occurred at various temperatures between 9 and 21°C. Again, both processes require a high relative humidity.

Disease Management

Control of sooty blotch and flyspeck truly requires an integrated approach. Perhaps the single most important practice for reducing the damage caused by these diseases (outside of the use of fungicides) is to ensure that pruning is adequate to promote rapid drying of fruit surfaces. It was shown that the incidence of sooty blotch and flyspeck could be reduced by an average of 30% by "severe pruning" (Latham & Hollingsworth, 1973). In a later study, dormant pruning in a non-sprayed orchard reduced the incidence and severity of sooty blotch in 2 out of the 3 years, but the results were inconsistent with respect to flyspeck (Ocamb-Basu et al., 1988b).

In a 2-year study conducted in Massachusetts, Cooley et al. (1997) showed that summer pruning could reduce the incidence of flyspeck by nearly 50% in an unsprayed orchard. In the same study, they indicated that the number of fruit downgraded from USDA Extra Fancy was reduced when summer pruning was done in commercial orchards. They concluded that summer pruning helped to decrease the incidence of flyspeck by reducing the number of hours of relative humidity >95% and allowing increased penetration of pesticides to the upper two-thirds of the canopy when applications were made with an airblast sprayer.

However, tHhe primary means of managing sooty blotch and flyspeck is by regularly scheduled applications of fungicides. In northeastern United States, fungicides are applied to apples from mid-June through August primarily to control sooty blotch and flyspeck. Four or five summer fungicide applications may be needed to control these diseases in wet years, whereas only two or three well-timed applications are needed in dry years. Omitting summer fungicide sprays is risky because gaps in fungicide protection during critical periods in summer can result in the sudden appearance of numerous flyspeck infections just before harvest.

Field research conducted in the Hudson Valley over the past 10 years has been used to develop a model for timing apple fungicide sprays during the summer. The model targets flyspeck because fungicide programs that control flyspeck will nearly always control sooty blotch under N.Y. conditions. The concepts used to develop the N.Y. Flyspeck Model are outlined below. Omitting fungicides is always risky because potential losses from disease on fruit can quickly obliterate any savings that accrue from withholding sprays. Nevertheless, the information and concepts used to develop the N.Y. Flyspeck Model may be useful in deciding how to time summer fungicides, even if the ultimate decisions on fungicide timing end up being considerably more conservative than those suggested by the model.

The first step in constructing the N.Y. Flyspeck Model was the development of a table of estimated residual activities for various summer fungicides (Table 1). This table was developed using data from small-plot field trials conducted in the Hudson Valley from 1987-1996. Residual activities shown in the table are shorter during summer than for the last spray before harvest because cooler conditions in the fall slow development of sooty blotch and flyspeck, and also because late infections will fail to develop symptoms before harvest, and therefore are of no concern.

In addition to the residual activity of fungicides shown in Table 1, research has shown that benzimidazole fungicides (Benlate and Topsin M) and strobilurin fungicides (Sovran and Flint) provide post-infection activity against sooty blotch and flyspeck. Their post-infection activity decreases as the time between infection and fungicide application increases. Although there are still some data gaps with Sovran and Flint, tests completed to date suggest that all four of these fungicides have reasonable activity against flyspeck infections that have accumulated fewer than 100 hours of wetting after infections occurred. Working in North Carolina, Brown & Sutton (1995) showed that sooty blotch and flyspeck appear on fruit only after fruit are exposed to 275-300 hours of accumulated wetting following infection. Thus, it appears that Benlate, Topsin M, Sovran, or Flint will provide post-infection control of flyspeck and sooty blotch so long as the infections are less than one-third of the way through the incubation period, with "incubation period" defined as 275-300 hours of accumulated wetting after infection.

By taking advantage of both the residual activity of fungicides and the post-infection activity of Benlate, it may be possible to eliminate one or two summer fungicide sprays after the last scab spray is applied in early to mid-June. The logic is as follows:

• Assume that the last spray for apple scab (usually first or second cover spray) will provide the residual activity noted in Table 1. If mancozeb is used for the last scab spray, then fruit will be protected for the shorter of either 21 days or through 3.5 inches of accumulated rain following the mancozeb application.

• After the residual activity from the last scab fungicide spray is exhausted, a "protection gap" of up to 100 hours of leaf wetting (including dew periods) can be tolerated if Benlate, Topsin M, Sovran, or Flint is used later in the season. A leaf wetness recorder will be required to monitor hours of leaf wetting, but data from a regional recording station may suffice for orchards within 10-15 miles of the recording station. During the protection gap, fruit will not be protected by fungicides, so sooty blotch and flyspeck infections may occur on fruit if inoculum is present in the vicinity of the orchard.

• At the end of the protection gap, one of the four fungicides with post-infection activity must be applied to eradicate infections. To be conservative and allow for unexpected rains that might intervene before sprays are completed, the post-infection program should be initiated after the accumulated wetting during the protection gap reaches 80 hours. A minimum of two fungicide applications should be used following the protection gap and prior to harvest to ensure complete suppression of incubating flyspeck infections. The two post-infection sprays should be 14-21 days apart and, in dry years, will most likely coincide with insecticide applications timed to control apple maggot. Including Benlate, Topsin M, or Flint in August applications should also control black rot infections that may develop in fruit lenticels as the fruit begin to ripen. Sovran is also effective against black rot, but it has a 30-day to harvest restriction.

In simple terms, following this model will usually result in eliminating fungicide sprays for about 30 days sometime between June 7 and July 25, with timing of the spray gap dependent on when the last June spray is needed for apple scab or cedar apple rust. In dry years, the spray gap might extend for 45 days or more, whereas in wet years, it might only be three weeks.

CAUTION: Omitting fungicide sprays during July is not recommended if tree canopies are dense (as in trees left unpruned last winter) or if fruit are clustered. In orchards with dense canopies or clustered fruit, complete fungicide coverage will almost certainly be impossible during late summer when the canopy reaches maximum density and the clustered fruit prevent fungicide from reaching the center of clusters. In such orchards, Benlate, Topsin M, Sovran, or Flint should be applied during July, when the likelihood of decent coverage is better than it would be in August. Even a very tight fungicide program may fail to control flyspeck during wet seasons in orchards with dense canopies.

Table 1. Suggested fungicides, rates, and spray intervals for controlling sooty blotch and flyspeck in orchards considered at moderate risk for these diseases.

Adapted from Agnello, A., Kovach, J., Nyrop, J., Reissig, H., Rosenberger, D., and Wilcox, W. 1999. Timing sprays to control flyspeck. Pages 22-23 in: Apple IPM: A guide for sampling and managing major apple pests in New York State. NY State IPM Program, Geneva, Publ. 207.

Literature cited:

Agnello, A., Kovach, J., Nyrop, J., Reissig, H., Rosenberger, D., and Wilcox, W. 1999. Timing sprays to control flyspeck. Pages 22-23 in: Apple IPM: A guide for sampling and managing major apple pests in New York State. NY State IPM Program, Geneva, Publ. 207.

Brown, E.M., and Sutton, T.B. 1995. An empirical model for predicting the first symptoms of sooty blotch and flyspeck of apples. Plant Disease 79, 1165-1168.

Cooley, D.R., Gamble, J.W., and Autio, W.R. 1997. Summer pruning as a method for reducing flyspeck disease on apple fruit. Plant Disease 81, 1123-1126.

Latham, A.J., and Hollingsworth, M.H. 1973. Incidence and control of sooty blotch and flyspeck on apples in Alabama. Auburn Univ. Agric. Exp. Stn. Circ. 208.

Nasu, H., Fujii, S., and Yokoyama, T. 1985. Zygophiala jamaicensis Mason, a causal fungus of flyspeck of grape, Japanese persimmon, and apple. Ann. Phytopathol. Soc. Jpn. 51, 536-545.

Ocamb-Basu, C.M., Sutton, T.B., and Nelson, L.A. 1988a. Effect of temperature and relative humidity on germination, growth, and sporulation of Zygophiala jamaicensis. Phytopathology 78, 100-103.

Ocamb-Basu, C.M., Sutton, T.B., and Nelson, L.A. 1988b. The effects of pruning on incidence and severity of Zygophiala jamaicensis and Gloeodes pomigena infections of apple fruit. Phytopathology 78, 1004-1008.

Williamson, S.M., and Sutton, T.B. 2000. Sooty blotch and flyspeck of apple: Etiology, Biology, and Control. Plant Disease 84, 714-724.

 

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