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Fungicide Use Strategies

Tree fruit growers are well aware of the complex of pathogens that produce disease on leaves, branches, and fruit spanning the season from bud break to postharvest. There are 10 or more major diseases of apple trees alone. Managing these pathogens at levels below economic injury requires the integration of cultural and sanitation measures with prudent use of agricultural chemicals.

In this section we focus on different fungicide-use options for efficient and effective disease management. It is assumed that acceptable pruning, fertilization, and sanitation measures known to reduce disease inoculum levels are being used and that minimal fungicide rates to maintain low disease levels are desirable.

Types of Fungicides
Pesticides used for managing fungi-caused fruit diseases are either fungicidal (they kill fungi) or fungistatic (they inhibit fungal growth). Fungicides can be separated into two categories: protectants and systemics.

Protectant fungicides protect the plant against infection at the site of application. Their characteristics are as follows:

They provide protection against infection.
They do not penetrate into the plant.
They require uniform distribution over the plant surface.
They require repeated application to renew deposit.
They have a multisite mode of action against fungi.
Fungi are not likely to become resistant to protectant fungicides.

Some common protectant fungicides are Bravo, captan, copper, Dithane, Manzate, Polyram, sulfur, and Ziram.

Systemic fungicides prevent disease from developing on parts of the plant away from the site of application. Their characteristics are as follows:

They penetrate into the plant.
They move within the plant.
They control disease by protectant and/or curative action.
They often have a very specific mode of action against fungi.

Some systemic fungicides are Elite, Flint, Indar, Nova, Orbit, Pristine, Procure, Rubigan, and Sovran.

Modes of Action
Fungicides can act on fungi by (1) inhibiting energy production (e.g., sulfur, copper, Manzate, Dithane, captan, Flint, Pristine, and Sovran), (2) interfering with cell structure (e.g., Syllit), and (3) interfering with growth (e.g., Elite, Procure, Nova, and Rubigan).

Fungicide Options for Apple Trees
Growers used to have few fungicide options for apple scab control other than protective fungicides that had to be applied before infection occurred. With the introduction of dodine, benzimidazole (Benlate, Topsin-M), and sterol-inhibitor fungicides (Funginex, Rubigan, Nova) came greater versatility in timing of application and often more complex scenarios of usage patterns. These fungicides were either more effective when used in combination with chemically unrelated compounds (captan, Manzate, Dithane, Polyram, thiram) or were perceived as effective in the selection of less sensitive pathogen strains in the orchard.

It is now recognized that strains of the apple scab pathogen tolerant of the benzimidazole fungicides can be found rather commonly in mid-Atlantic orchards. This factor, along with the reduced usage of the EBDC fungicides (Dithane, Manzate, Polyram), have made decisions about fungicide selection and application timing more difficult. Dodine (Syllit) usage in the past has been limited essentially to early season because of its narrow spectrum of disease activity and is currently not recommended due to the high risk of existing fungicide resistance. There is also concern that resistance is developing in Pennsylvania orchards to the sterol-inhibitor fungicides (e.g., Elite, Procure, Nova, and Rubigan) and strobiluron fungicides (Flint, Pristine, and Sovran). Another complicating situation has been the desirability of spray oil usage for European red mite management and the incompatibility of captan or sulfur with oil.

Moreover, captan is apt to cause leaf spotting on Delicious, and sulfur is more injurious to most apple cultivars when the temperature is above 90°F. In sorting out the various options, dividing the various diseases and control options during the growing season into early, mid-, and late-season periods will help to optimize disease-management decision making.

Early season (green tip through second cover)
Fungal diseases of primary concern during this period are scab, powdery mildew, and rust, all of which occur on both leaves and fruit. Although scab may infect leaves as early as green tip under favorable environmental conditions, the number of infections increases with ascospore maturation rate from 1/2-inch green through second cover with peak periods between pink and first cover. Mildew infections blanket the period between the open cluster and second cover growth stages and become significant in orchards having overwintering infections when the temperature is above 50°F.

Cedar-apple and quince rust must be controlled in areas where susceptible red cedars are common. Infections occur from pink through second cover but are more likely to cause economic injury between bloom and first cover. The frog-eye leafspot phase of the black rot disease may be of concern in some orchards during petal fall through second cover.

The choice of which fungicides to use during the early season may vary, depending on which diseases are present in the orchard and whether oil is used for mite control at the 1/2-inch green stage. A wide range of fungicides with different modes of action may be used during this time. Protective fungicides effective at this time (listed in descending order of effectiveness) include Vangard, Manzate, Dithane, Polyram, captan, thiram, ferbam, and Ziram.

The sterol-inhibitor (SI) fungicides (Rubigan 1E, Nova 40W, Procure) are highly effective for control of scab, rust, and mildew. They are recommended during open cluster through the second cover. They are more effective against scab when used in combination with a protectant fungicide. To avoid or delay the development of tolerant strains of pathogens, we strongly recommend that the use of sterol-inhibitor fungicides be limited and that they be used preventively, not curatively, and that they be tank-mixed, alternated, or used in alternate groups of sprays with a chemically nonrelated fungicide.

The SI fungicides are very effective for scab control when used in postinfection applications timed 72 to 96 hours from the beginning of the infection period. A single post-infection application generally is not as effective as two or more used in sequence. A good method is to follow the post-infection spray with another application 5 to 7 days later during times when infection periods occur at intervals of 10 or more days. However, the threat of SI fungicide resistance limits the potential for post-infection use of these materials.

The strobilurin or "stroby" fungicides, Flint, Sovran, and Pristine, represent a different class of fungicides registered for use on apple and pear. These fungicides have both protective and curative activity against a number of early season diseases of apple and pear including scab, powdery mildew, frog-eye leaf spot, apple rusts, and fruit rots. As with other curative fungicides, the stroby fungicides are likely to select out resistant strains of one or more pathogens if used excessively. There is concern that fungal tolerance to the mode of action of this group of fungicides has already begun in Pennsylvania. To avoid this selection and increase the effective life of these fungicides, limit their use to the restrictions on the label. Unlike the sterol inhibitors, it is not necessary to mix these fungicides with protectants; however, using them in alternating applications with unrelated fungicide chemistry is strongly encouraged. Spray programs using the stroby fungicides in a limited number of applications alternating with other fungicides should provide control of the fungal disease complex common in many orchards.

Midseason (third, fourth, and fifth cover sprays)
The major fungal pathogens of apple are generally less active during this period, so fungicide usage may be safely limited by reducing fungicide rates, lengthening application intervals, or eliminating one or more sprays. Where scab has been controlled there is little risk of fruit infection during this period because the primary inoculum has been dispensed. Powdery mildew and rust are no longer a threat, and fruit is less susceptible to the decay- producing pathogens. Sooty blotch and flyspeck may cause infections as early as third cover when rainfall is frequent, but they are generally not a problem until later in the season.

Fungicides of choice include captan, thiram, and Ziram. The rates of each can be reduced to 50 to 60 percent of full rates without appreciable risk unless rainfall is frequent.

Late season (sixth, seventh, and eighth cover sprays)
Rainfall is frequently heavier during late July, August, and September. This heavier rainfall is favorable for the pathogens causing fruit decay (black rot, bitter rot, bot rot, and Alternaria rot) and blotches (sooty blotch and flyspeck). Bitter rot, particularly, is more prevalent when rainfall is frequent and temperatures are above 85°F. Sooty blotch and flyspeck may not be readily apparent until early September, although infections may occur as early as mid-June. Under conditions of frequent rainfall, control of these diseases requires using protective fungicides at a minimum of three applications and at rates near or at full labeled rates.

Fungicides such as Pristine, Topsin-M, Ziram, captan, and thiram are effective for late-season control when used in complete sprays at 14-day intervals or in alternate-side sprays at 7- to 10-day intervals. Because captan, thiram, and Ziram are not as persistent on the fruit during frequent rains, using them along with Topsin-M reduced label rates will provide better disease control than if either is used alone. Topsin-M is ineffective against bitter rot and provides only fair control of black rot. In orchards with a history of these diseases, captan used in combination at 70 to 80 percent of full rate in the fifth through eighth cover sprays is more likely to provide satisfactory control. The interval between the last spray and harvest should not exceed 45 days. A recently registered fungicide, Pristine is highly effective against the late-season diseases, fruit rots, sooty blotch, and flyspeck.

Sooty blotch and flyspeck often become visible within 40 days after the last application on fruit sprayed with captan, thiram, or Ziram. Since the incubation period for these pathogens is about 30 days in the field, the effective residual protection is about 10 to 14 days for these protectants. The residual protection from a combination with Topsin-M is approximately 20 days. Thus symptoms should not be evident for 50 days after the last application. Combinations are preferred for late-maturing cultivars grown for fresh markets.

The "stroby" fungicides, Pristine, Flint, and Sovran, are registered for control of several apple diseases that occur during the late summer months. Research data from the Mid-Atlantic region on application timing and efficacy against bitter rot, Brooks fruit spot, black rot, bot rot, sooty blotch, and flyspeck indicate that the stroby fungicides are very effective. The choice of using these fungicides during the summer should be balanced against the need for control of early season diseases and the limited number of applications recommended on the label.

Fungicide options for peach and nectarine trees
Diseases infecting peach and nectarine trees span the entire growing season, from dormancy in the spring through harvest and postharvest. As on apple, sanitation measures that eliminate or reduce inoculum levels in the orchard or packing house used in combination with effective fungicides are necessary for acceptable control. A minimum of five fungal and bacterial diseases must be managed in most commercial orchards. Their intensity varies with cultivar susceptibility, inoculum levels, and favorable environmental conditions.

Peach leaf curl is a threat unless inoculum is maintained at very low levels. A single application of ferbam, Bravo, copper fungicides, or Ziram is effective when applied in spring before bud swell or in the autumn when most leaves have fallen. In orchards where leaf curl has not been present for two or more years, this application can be safely omitted until the first symptoms become evident. Careful monitoring for low disease incidence is important if spray omission is planned.

Brown rot is the most destructive fungal disease of stone fruit. Infections occur first in blossoms, then grow into shoots, causing them to blight, and later produce heavy inoculum (spores) that cause fruit decay. Nectarines are particularly susceptible and require closer attention to sanitation measures and thorough applications of fungicides. Effective fungicides include Pristine, Elite, Indar, Orbit, Funginex, Rovral, Bravo, Vangard, Scala, captan, thiram, and sulfur.

In orchards where blossom blight has been a problem, two applications from pink through bloom are required for control when rain periods occur with temperatures 55°F or above. The removal of mummified fruit from trees, light cultivation to disturbed mummies on the ground beneath trees, and careful timing of applications during bloom before infection periods can eliminate the need for one or possibly both of these applications. The necessity of applications on green peach fruit for brown rot control is in question, even though growers routinely spray during this period.

Losses from brown rot are more apparent during and after harvest. Fruit becomes more susceptible as it matures, and fungicide protection from one or two weeks before harvest and during harvest is required for control.

Peach scab causes minor losses in Mid-Atlantic orchards but can build up if no fungicides are used. It is common in orchards where complete crop losses from freezing occurred the previous year and fungicides were not used. Three fungicide applications at 2-week intervals starting at late shuck split are effective for controlling scab. Captan and sulfur are highly effective and either is a good choice during this period.

Bacterial leaf and fruit spot is a very destructive disease along the eastern seaboard on susceptible cultivars (Sunhigh, Loring). The disease generally is not difficult to control in most Mid-Atlantic orchards unless it builds up during rain periods with temperatures of 70 to 85°F. Cultivars show varying levels of resistance from highly susceptible to moderate to highly tolerant (Redhaven, Encore). Infection begins on leaves, twigs, and fruit at shucksplit and continues through the growing season.

Maintaining good tree vigor is helpful in control. Myco-Shield 17SP is effective if applied weekly as a dilute spray, but can be expensive because as many as eight applications may be required. Susceptibility of the cultivar and frequency of favorable environmental conditions are important factors in choosing the cultivar to be grown.

Rhizopus rot is a fungal disease that infects ripe fruit at harvest and may cause extensive losses in harvested fruit in transit or on grocery shelves. The fungus is widespread in nature and gains entry through wounds or may spread from rotting fruit to adjacent fruit in storage containers. Sanitation and cultural practices that maintain low inoculum levels are essential for control. Fungicides applied for brown rot at harvest generally provide good protection.