Critical Uses and Needs

A broad spectrum of fungicides with differing modes of action are required for effective apple disease management and to prevent the emergence of fungicide-resistant disease organisms. With the diversity of disease-causing organisms that must be effectively managed to produce a profitable crop, and the varying effectiveness of the available fungicides against individual organisms, loss of any class of fungicide materials listed below would seriously threaten the stability of the apple industry.

Benomyl (Benlate 50W) is registered for use in controlling several diseases of apples. It is registered for control of powdery mildew, sooty blotch, and fly speck. It is registered in combination with captan for control of other diseases. Benomyl is compatible with oil and may be used at the 2.0 to 3.0 oz rate per 100 gal in combination with superior spray oil at the rate of 2.0 gal per 100 gal, or with oil at the 1.0 qt rate plus mancozeb 75DF 1 lb per 100 gal in a seasonal spray program. Because captan is not compatible with oil, it must not be added to the Benomyl + oil tank mix. Where resistance is not present, the benomyl-oil combination is effective against apple scab and powdery mildew when applied before bloom at intervals not longer than 7 days and after bloom at intervals between sprays not exceeding 12-14 days.

Strains of benomyl-resistant apple scab have been found in the Virginia counties of Botetourt, Clarke, Floyd, Frederick, Madison, Nelson, Roanoke, Rockingham, Shenandoah, Warren, Wise, and Smyth, and in Berkeley, Hampshire and Jefferson Counties, West Virginia. Apple scab strains are also cross-resistant to thiophanate-methyl (Topsin-M). If resistance is suspected, use of Benlate or Topsin-M should be discontinued, and they should be replaced by full rates of other fungicides. Do not apply to apples within 14 days of harvest.

Thiabendazole (Mertect 340-F) is benzimidazole-type fungicide registered for postharvest treatment of apples and pears for control of Penicillium blue mold and several other rots. Use a suspension of 16 fl oz per 100 gal water for application as a dip, flood or spray to harvested fruit. Apples may be treated before and after storage; pears may be treated only once. Do not treat longer than 3 minutes. Because thiabendazole is a benzimidazole-type fungicide, it will not be effective on strains of fungi resistant to Benlate or Topsin-M. To prevent infection of fruit in the dip tank by strains of resistant rot fungi, it is suggested that captan be included in the postharvest treatment.

Thiophanate-methyl (Topsin-M) is formulated as a 70WP and is available in water-soluble bags. Thiophanate-methyl (Topsin-M) belongs to the same family of chemical compounds as Benlate. Generally, those disease-causing fungi that are resistant to benomyl (Benlate) also are resistant to Topsin-M.

Thiophanate-methyl is registered for apple scab, powdery mildew, apple black rot, sooty blotch, and fly speck. Strains of apple scab resistant to thiophanate-methyl and benomyl have been detected in many counties in Virginia and West Virginia. See additional comments on resistance under benomyl. Thiophanate-methyl appears to have no compatibility problems with those pesticides that commonly are used in apple orchards. However, do not tank-mix with copper-containing chemicals or with highly alkaline pesticides such as Bordeaux mixture or lime sulfur.

As with most sterol-inhibiting fungicides, Rubigan is not effective against all moldy core fungi and summer diseases and is less effective for scab control on fruit than on foliage. Also, there is evidence that the apple scab fungus can develop cross resistance to all sterol-inhibiting fungicides (Rubigan, Nova, Procure) when used alone for scab control. To avoid these potential weaknesses and to lengthen the effective life of this class of fungicides, it is recommended that the sterol-inhibiting fungicides be used in conjunction with protectant fungicides (captan, mancozeb, metiram, ziram, dodine, etc.), preferably as a tank-mix combination as permitted by the label.

Do not apply Rubigan closer than 30 days to harvest or use more than 84 fl oz per acre per season on apples and pears. Rubigan may be used on cherries up to the day of harvest at a maximum of 6 fl oz/A per application and 26 fl oz/A per season prior to harvest.

Myclobutanil (Nova 40W) is a sterol-inhibiting fungicide which is highly effective for control of apple scab, powdery mildew, cedar rust, and quince rust. It can be used as a preventive or 96-hour post-infection treatment on scab. The dilute rate selection of 1.25 to 2.5 oz per 100 gal, depending upon the target disease and time of treatment, is concentrated per acre according to tree size as indicated on the label. When used on a post-infection schedule, reddish or yellowish, partially inhibited scab lesions may appear and a follow-up application should be made 7 days after the first post-infection application (totaling two complete applications within 11 days after the infection period) to inactivate such lesions which would appear 10 to 20 days after the infection period. When spraying on an alternate middle schedule, two half-sprays should be applied as soon as possible after the infection period, followed by two more half-sprays starting 7 days after the first half-spray following the infection period.

As with most sterol-inhibiting fungicides, Nova is not effective against all moldy core fungi and summer diseases and is less effective for scab control on fruit than on foliage. Also, there is evidence that the apple scab fungus can develop cross resistance to all sterol-inhibiting fungicides (Rubigan, Nova, Procure) when used alone for scab control. To avoid these potential weaknesses and to lengthen the effective life of this class of fungicides, it is recommended that the sterol-inhibiting fungicides be used in conjunction with protectant fungicides (captan, mancozeb, metiram, ziram, thiram, dodine, etc.), preferably as a tank-mix combination as permitted by the label.

Nova is sold in water-soluble PVA bags. Some precautions should be taken to assure that the material is properly suspended in the spray tank. The bag should be dissolved and the fungicide fully-suspended before adding other spray materials to the tank. This is particularly true with spray oils and Solubor (and other materials releasing boron) because these materials cause a reaction which prevents the bag from dissolving. Once in suspension, Nova is compatible with most common spray materials except basic copper-containing fungicides. Nova is not fully compatible with Ambush 2E and requires strong spray tank agitation to keep it in suspension.

Do not apply Nova to apples within 14 days of harvest and do not apply more than 5 lb per acre per season. Do not graze livestock in treated areas or feed cover crops grown in treated areas to livestock.

Triadimefon (Bayleton) is a sterol-inhibiting fungicide registered for the control of cedar-apple rust and powdery mildew on apples. Test data in northern Virginia indicate that Bayleton is also effective for quince rust control. It is formulated as a 50DF to be used at the rate of 0.5 - 2.0 oz per 100 gal dilute or 2-8 oz per acre concentrate. Bayleton may be applied up to the day of harvest but no more than 24 oz Bayleton 50DF may be applied per acre per season. The following tank mix is not compatible: Bayleton + Cyprex 65W + Guthion 2S. Bayleton is not registered on stone fruits.

Triflumizole (Procure 50WS) is recently registered sterol-inhibiting fungicide. Procure is registered for control of powdery mildew and scab on pears. Formulated use rates on both apples and pears are 2-4 oz per 100 gal dilute and 8-16 oz per acre. Restrictions on both apples and pears are a limit of 4 lb per acre per year, a 14 day pre-harvest interval and a 24 hour restricted entry interval.

Ferbam (Carbamate) is formulated as a 76WDG powder and used at rates of 0.6 to 2.0 lb per 100 gal. Ferbam may be used on processing fruit to control most apple diseases because finish is not as critical. It has tended to roughen the finish of Golden Delicious, and has been an important factor in the development of enlarged lenticels on Stayman and Delicious varieties. York and Rome usually finish well with ferbam. Fruit coloring may be slightly delayed when ferbam is used.

Mancozeb (Dithane C45, 75DF, Manzate 75DF, Penncozeb 80W, and/or various wettable and flowable formulations) is a coordinated product of zinc ion and manganese ethylene bisdithiocarbamate. It differs from maneb, or maneb plus zinc ion products. It is formulated as an 80W powder, flowable or 75DF dry flowable and when used at the rate of 1 lb per 100 gal (or equivalent rate of another formulation), it provides supplemental control against a broad range of apple diseases including: apple scab, apple rust, black rot, bitter rot, sooty blotch, and fly speck. Note that EBDC labels now permit the use of mancozeb and metiram (Polyram) interchangeably on the same crop, but the total amount applied per acre per year is now governed by the most restrictive label (usually a 75DF formulation).

Metiram (Polyram), an EBDC fungicide, is formulated as an 80DF powder and is registered for the control of apple scab, cedar apply rust, fly speck, sooty blotch, and as an aid in reduction of European red mite. It has been effective in reducing the severity of leaf blotch defoliation on Golden Delicious. Note that EBDC labels now permit the use of mancozeb and metiram (Polyram) interchangeably on the same crop, but the total amount applied per acre per year is now governed by the most restrictive label (usually a 75DF formulation).

Thiram is formulated as a 65W powder and used at the rate of 0.6 to 2.0 lb per 100 gal of spray. Thiram is a dithiocarbamate chemical, as are ferbam and ziram. It is frequently used in place of ferbam because it is light colored and generally less injurious to the finish of sensitive varieties, including Golden Delicious. Thiram combines readily with dormant oil sprays, while ferbam may mix poorly. Thiram will control rust infections, but is less effective than ferbam. It is less effective against apple scab than captan and other dithiocarbamate fungicides.

Ziram: Ziram is a dithiocarbamate fungicide which was recently re-registered for use on the following tree fruit crops in the Eastern U. S.: apples, pears, peaches, nectarines an cherries. Diseases for which ziram is labeled include scab, quince and cedar-apple rust, sooty blotch, fly speck, bitter rot and necrotic leaf blotch on apples; scab and Fabrea leaf spot on pears, leaf curl on peach, brown rot on cherries, peaches and nectarines, scab on peach and nectarine, and cherry leaf spot. On apples and pears, the registered rates for the 76DF formulation are 6-8 lb per acre per application and a limit of 56 lb per acre per year. Rates of 4.5-8 lb per acre per application and 72 lb per acre per year are permitted on peaches and nectarines. On cherries, the use rate 5-8 lb per acre with a maximum of 40 lb per acre per year. On all Eastern U. S. tree fruit crops, the pre-harvest interval is now 14 days. Ziram dust may cause irreversible eye damage and irritation of nasal passages, throat and skin. To avoid exposure, loader/applicators should be properly protected and use low-dust DF, WDG or flowable formulations.

Captan may produce frogeye-like spotting of the foliage of Delicious, Stayman, and Winesap early in the season. The small spots do not enlarge and are no cause for alarm. The inclusion of sulfur in the spray mixture may increase this type of injury.

Captan should not be used with lime or other alkaline materials. Do not use it with oil or within four days of an oil spray. Do not use in combination with EC formulations of parathion. Captan should be used with caution in bloom sprays, especially on varieties which are hard to pollinate (e.g. Red Delicious). Captan has been shown to severely reduce pollen viability for 24 - 48 hours after application.

Although new Captan labels permit application to apples up to the day of harvest, Captan has a 4-day re-entry interval which makes pre-harvest application more restrictive. A label exception is made for the last 48 hours of the re-entry interval during which workers may enter the treated area to perform hand labor or other tasks involving contact with anything that has been treated, without time limit, if they wear all of the following: coveralls, waterproof gloves, shoes and socks, and protective eye wear.

Coppers: Fixed copper is a term that refers to several relatively insoluble forms of copper which are safer and more conveniently prepared than Bordeaux mixture. The addition of spray lime is usually necessary for applications on fruit crops, depending upon timing. Copper fungicides are effective against many diseases, however they must be limited to only certain sprays on specific fruit crops because of the potential for injury to fruit and foliage. Fixed coppers are especially useful on apples and pears as an early season spray (dormant to 2 inch green) to reduce overwintering fire blight inoculum. Fixed copper compounds are available under many trade names although they can be grouped into several general categories: (Kocide 101, 50% copper, Kocide DF, 40% copper), and tribasic copper sulfate (53% or 26% copper). Dust preparations contain from 5% to 7% metallic copper.

Some pesticide labels warn about incompatibility with copper materials due to their alkalinity. Copper materials also have potential for phytotoxicity to leaves and fruit. Phytotoxic potential is generally increased when copper-containing spray mixtures are acidified.

Bordeaux mixture is a mixture in water of copper sulfate and hydrated spray lime and is usually used as a dormant application on apples and pears to reduce overwintering fire blight inoculum, on peaches for leaf curl, and on cherries in postharvest sprays for leaf spot. The recommended amount of each ingredient varies according to use and is designated by a three number formula, e.g. Bordeauxx 8-8-100. The numbers represent the amounts of copper sulfate in pounds, spray lime in pounds, and water in gallons, respectively. Bordeaux mixture is generally unsafe for use on fruit crops after the 1/2-inch green stage. Pears seem to tolerate copper better than apples, and it can be used during bloom for fire blight if disease pressure is not severe. It also has some activity against collar rot. Bordeaux mixture has some compatibility problems, therefore, when used in combination with other materials, the labels of the pesticides involved should be examined thoroughly.

Dodine (Syllit) is formulated as a 65W powder and is used from 0.4 to 0.75 lb per 100 gal dilute. It has been outstanding in its ability to control apple scab; however, it is not effective for control of rust infections, fruit rots, or powdery mildew. In some areas under heavy usage for scab control over a period of years, tolerance to dodine has developed in the apple scab fungus.

Dodine may russet yellow varieties, particularly Golden Delicious. It has sometimes caused fruit spotting of Stayman and some other red varieties, so excessive rates of usage should be avoided. Resistance of the apple scab fungus to dodine has been shown in some areas of the eastern U.S.A. including some orchards in Clarke and Warren Counties, Virginia; therefore, dodine should be used with caution under heavy scab conditions.

Fosetyl-Al (Aliette 80WDG) is registered for control of Phytophthora collar rot on bearing and non-bearing apple trees. Dilute rate per 100 gal is 2.5-5.0 lb. Begin applications at tight cluster. Use 3-4 foliar sprays during the season at 60 day intervals at the 5.0 lb per 100 gal rate or 6-8 applications at 2.5 lb per 100 gal on a 30 day interval. Do not apply more than 5.0 lb of Aliette per acre per application and no more than 20.0 lb per acre per year. Alliette can not be applied within 14 days of harvest. Can be applied to the tree after harvest but do not apply within 2-3 weeks of leaf senescence.

Aliette is registered in West Virginia as a pre-plant tree root dip for control of Phytophthora root and collar rot at the rate of 3.0 lb per 100 gal. Mix the appropriate amount in the desired volume of water and dip the entire root system for 30-60 minutes prior to planting in the field. The Aliette label indicates that it can be used for fire blight control, but experience in the mid-Atlantic region and elsewhere indicates that it is not as effective as streptomycin and may give less than adequate control.

Metalaxyl (Ridomil 2E) is registered as an aid in control of Phytophthora crown rot on bearing and non-bearing apple trees. Applications should be made before symptoms appear, especially in orchards favorable for disease development. Metalaxyl should not be expected to revitalize trees showing moderate to severe crown rot symptoms. To apply, Ridomil 2E is diluted at the rate of 1 qt per 100 gal and poured around the trunk of each tree. The amount of diluted mix applied per tree is based on the trunk diameter as follows: diameter less 1 inch, 1 qt; 1-3 inch diameter, 2 qt; 3-5 inch diameter, 3 qts; greater than 5 inch diameter, 4 qt. Make one application at the time of planting or in the spring before growth starts. Make another application in the fall after harvest. Ridomil is highly specific and will not control other agents causing similar tree decline symptoms such as other root rots, graft union necrosis (Tomato Ring Spot Virus) and vole damage.

Ridomil Gold EC is a more highly refined, more biologically active, 46.6% formulation of metalaxyl isomer. Because of its increased biological activity and higher percent formulation the diluted use rate is lower: 0.5 pt/100 gal. The amount of diluted fungicide to apply as a drench is the same as listed above for Ridomil 2E. The restricted entry interval for the recently registered Ridomil Gold EC is 48 hours.

Oxythioquinox (Morestan 25W) is registered as a prebloom miticide and mildewcide on apples. Morestan provides protective powdery mildew control when applied at the registered rate of 0.5-1.0 lbs per 100 gal dilute during the tight cluster to pink stage. Do not apply after bloom appears. Injury of foliage may occur when applied under slow drying conditions. Do not apply in combination with oil or spreader-stickers or with EC pesticide formulations, or within 10 days of an oil spray.

Streptomycin is an antibiotic widely used against fire blight of apples and pears, is formulated as streptomycin sulfate 17W. It is commonly used at 60 to 100 ppm in blossom sprays and at 100 ppm in post-blossom sprays. The effectiveness of streptomycin can be increased by including the adjuvant Regulaid at the rate of 1 pt per 100 gal of tank mix; however, the increased uptake of streptomycin with Regulaid is more likely to result in streptomycin injury.

Sulfur: Wettable sulfurs are finely divided, elemental sulfur particles with a wetting agent added so that the sulfur can be mixed with water and remain in suspension while being applied. The most readily available forms of sulfur are dry, wettable powder (95% sulfur) and fused bentonite sulfur (30% to 81% sulfur depending upon the brand). Sulfur dusts are available and generally are more finely divided and therefore more adherent and effective than the coarser wettable powders. Flowable sulfur products are available and their advantage over wettable sulfurs include being effective at lower rates and having better retention properties. Generally, sulfur is used in apple programs for the control of apple scab and powdery mildew. For scab, dry wettable sulfur (95WP) is used at a rate of 5 lbs per 100 gallons in early-season sprays in a protective program. It can be used during bloom without reducing fruit set significantly, however fruit russetting and yield reduction may result if it is used under high temperature conditions. However, many orchardists growing fruit for the processing market routinely use sulfur in post-bloom sprays without adverse economic effects. Sulfur is also the cornerstone for early-season and summer disease control in organic orchards. Sulfur is very effective against powdery mildew and can be combined at reduced rates (3 lbs/100 gallons) with most pesticides.

Do not use any sulfur products within two weeks before or after an oil spray. Copper, sulfur, and liquid lime-sulfur should be used by growers who intend to produce fruit for the organic market. Growers are cautioned to be aware of the disadvantages and limitations of these materials, compared to synthetic fungicides: sulfur is incompatible with oil, it has poor residual activity, it acidifies soil when used in seasonal programs, and it is phytotoxic to fruit and foliage when used in hot weather; liquid lime-sulfur is extremely caustic and may be dangerous to apply, it may also be phytotoxic to foliage and it may reduce leaf size and yield, several consecutive applications may need to be made to effectively eradicate scab lesions; copper sprays have better residual activity than sulfur sprays and some coppers can be used to tight cluster if surface russetting of the fruit is not a problem. Only a few copper formulations are registered for application after petal fall. Because of these problems, organic growers are best served by planting many of the excellent scab-immune cultivars that are available commercially.