While the organically accepted fungicides and insecticides individually do not offer the same degree of efficacy or longevity as their conventional counterparts, when used in concert with each other along with conservation of biological control agents and cultural practices to reduce inocula, it is possible to produce a high percentage of fruit free of insect damage and disease symptoms within organic certification restriction on allowable materials (Berkett et. al., 2013).
Ideally, organic fruit production involves a whole systems approach not just a substitution of organically-acceptable pesticides for non organically-acceptable ones. Research is continuing in New England to examine the challenges and opportunities of organic apple production. Information from this and other research will be incorporated into future extension publications. The following information is based on observations by researchers and Extension specialists in Vermont and New York.
Sulfur materials include elemental sulfur and liquid lime sulfur (calcium polysufide). Elemental sulfur (e.g., Microthiol Disperss) is effective for controlling some fruit diseases, but it must be applied prior to infection. Sulfur is easily removed by rain. Thus, coverage must be renewed much more frequently than is required with conventional fungicides with better rain resistance. Sulfur is not effective for controlling rust diseases on apples. [Note: Rust diseases in organic apple orchards can be minimized if eastern red cedars and ornamental junipers within 500 ft can be removed or if new orchards are established in areas isolated from existing or potential cedar habitat.] In more southern areas of the region, sulfur is also relatively ineffective for controlling flyspeck, bitter rot, black rot and white rot on apples during July and August, but sulfur may provide adequate suppression of these diseases in more northern areas.
Liquid lime sulfur is generally more effective than elemental sulfur, but is a caustic material that must be used with caution. Whereas wettable sulfur has no post-infection activity, liquid lime sulfur provides 60–70 hours of post-infection activity against apple scab (counting from the beginning of an infection period). Liquid lime sulfur is also useful to “burn out” scab infections when they appear on leaves, but it has no activity against scab during the incubation period between 70 hours post-infection and appearance of symptoms. Liquid lime sulfur applied at 2 qt/100 gal on a 21-day interval or at 1 qt/100 gal on a 10-day interval provided good control of flyspeck in a 2006 trial in New York’s Hudson Valley. However, the liquid-lime sulfur sprays did not control summer fruit decays, and may have increased summer fruit rots in Vermont because it damages fruit lenticels. Caution should be used when applying liquid lime sulfur, because it is extremely caustic and an eye and respiratory irritant. Care should also be taken to thoroughly clean spray equipment after use because the material is corrosive to steel and other sprayer materials. Unfortunately, research has shown that both sulfur and lime sulfur can suppress photosynthesis which can reduce yield (Burell, 1945; Palmiter and Smock, 1954). Therefore, the number of sprays should be kept to a minimum.
Note: both sulfur and lime sulfur are phytotoxic if applied within 7-10 days of horticultural oil.
Copper fungicides (e.g., Champ, Nu Cop) also control many tree fruit diseases, but copper causes phytotoxicity under certain conditions. When applied in a full-orchard spray between silver tip and ¼” green tip, copper is effective in reducing overwintering fire blight inoculum and provides control of apple scab for about seven days following treatment. Copper is extremely phytotoxic to foliage on sweet cherries. On apples, copper applied between half-inch green and bloom usually causes fruit russeting. Copper applied between bloom and roughly July 4 may cause blackening of the lenticels. Copper applied later in July will provide excellent control of sooty blotch and flyspeck on red apple cultivars, but July applications may still cause severe fruit discoloration of yellow cultivars.
Summer applications of copper fungicides have been used effectively to control bacterial leaf spot on peaches, but care is required to avoid a build-up of copper residues that can result in severe leaf injury on peaches. Repeated summer applications of copper on peaches should be avoided unless rainfall has removed the residue from the previous application. Copper has also been used to control cherry leaf spot on tart cherry.
Some newer formulations of copper fungicides (e.g., Cueva, Badge X2) are available and labeled for use during the summer against multiple diseases including Brooks Spot, fruit rots, and Sooty Blotch. These materials contain different formulations of copper than many older materials, and the amount of available copper ions in applied rates may be substantially less than those materials. Research in Vermont has shown good control from using Cueva in summer against rust, rots, Brooks spot, and sooty blotch on fruit, but fruit russet was increased by the treatment. However, less severity of russet was observed than is typical of that resulting from older, higher-rate materials, and the use of lower-rate copper materials in summer is worthy of further on-farm evaluation.
Bacillus subtilis (Serenade) is a biofungicide labeled for control of fire blight, apple scab and powdery mildew. Serenade is a wettable powder formulation of the bacterium Bacillus subtilis, a common soil resident. The bacterium acts by releasing cell contents during growth in order to eliminate or reduce competitors in its immediate environment. Serenade is ineffective for controlling fungal diseases under the climatic conditions that exist in the Northeast. When used alone, Serenade provides partial control of fire blight when applied to blossoms prior to or immediately after (i.e. within 24 hours) an infection period. Serenade should be applied as a preventive and can be applied up to and including the day of harvest.
Extract of Reynoutria sachalinensis (Regalia) is a plant defense promoter with translaminar but not systemic activity, and therefore must be applied preventatively on a 10-14 day schedule. It is not effective against apple scab, but is labeled for fire blight suppression and management of sooty blotch, fly speck, and powdery mildew. Research on its effectiveness against these diseases is limited in New England, so its use is not recommended beyond trial applications at this time.
Bacillus amyloliquefaciens strain D747 (Double Nickel 55, Double Nickel LC) is a biofungicide labeled for fire blight, sooty blotch, flyspeck, and summer rots and suppression of apple scab on pome fruit. Preliminary research has shown good efficacy against the shoot stage of fire blight when combined with low-rate copper products such as Cueva or Badge X2 in summer sprays. Caution is advised, however, regarding fruit finish problems that may result from this treatment. Field data on efficacy against other diseases is limited, so trial use only is recommended at this time.
Potassium bicarbonate (e.g., Kaligreen, MilStop, Armicarb) has variable activity as a fungicide. This material does not have post-infection activity and therefore needs to be applied prior to infection. In addition, it has a short residual period and repeated applications are necessary. Bicarbonate products may provide some control of diseases, but have been insufficient in trials when used alone. When combined or used in rotation with wettable sulfur, potassium bicarbonate products have been effective in managing apple scab in several European studies.
Hydrogen dioxide (Hydrogen peroxide) (OxiDate) kills fungi and bacteria via surface contact with the organism. Hydrogen peroxide does not have residual activity, nor will it control fungi or bacteria that have already penetrated host tissue. Thus, it must be applied after pathogens have been deposited on plant surfaces but before they can initiate infections. Field applications to apples are not recommended because OxiDate can cause severe fruit russetting under certain conditions. Controlled inoculation trials indicate no significant effect of OxiDate on fire blight infection of apple.
Streptomycin (Agri-mycin) is a bactericide formerly used for control of fire blight of apples and pears, but its use in certified-organic production systems has been disallowed since October 2014. Older reccomendations for the use of streptomycin to manage fire blight should be ignored.
Fire blight management in organic orchards may be challenging in orchards with susceptible cultivars and a history of the disease given the removal of streptomycin from the approved materials list. Growers are encouraged to adopt an integrated fire blight management program in organic orchards, including: 1) use of cultivars with reduced susceptibility to the disease; 2) conservative application of nitrogen fertilizers to reduce excessive, succulent growth that is more susceptible to infection; 3) aggressive removal of fire blight cankers in winter; 4) application of fixed copper between silver tip and ¼” green tip to all orchard blocks and varieties; 5) use of disease modeling software (i.e. Maryblyt™ or Cougarblight, also available through NEWA (newa.cornell.edu)) to predict infections; 6) application of lime sulfur or biocontrol materials (e.g., Serenade, Bloomtime Biological) during bloom when infections are predicted; and 7) frequent removal of diseased tissue during dry weather in summer. More information on non-antibiotic management of fire blight may be found at: https://www.youtube.com/watch?v=NuKxKCWKl28 (link is external).