Bill Cline, North Carolina State University, and Jonathan Oliver, University of Georgia
Organic blueberry production is practiced worldwide with varying degrees of success. In the southeastern U.S. our warm, humid climate is one of the most challenging due to increased pest pressure when compared to drier climates in California and elsewhere. The studies cited here were conducted in North Carolina (2022) and Georgia (2020, 2021, 2022) to determine the efficacy of organic fungicides for controlling blueberry diseases. These diseases (Figs. 1-4) included anthracnose ripe rot (Colletotrichum sp.), mummy berry (Monilinia vaccinii-corymbosi), Septoria leaf spot (Septoria albopunctata), and blueberry leaf rust (Thekopsora minima).
What are Organic Fungicides?
Organic fungicides are non-conventional products that are listed by the Organic Materials Review Institute (OMRI) and are generally accepted by organic reviewers as qualifying for use in organic production systems. The products evaluated in these trials (Table 1) include oils, plant or bacterial extracts, or biological control agents (microorganisms) as their active ingredient. Not all products were evaluated in all trials, and not all products were tested against all pathogens. Table 2 gives a summary of the number of times each product was evaluated in the various trials, and against which pathogens.
Table 1. Organic fungicides evaluated on blueberry in Georgia and North Carolina, 2020-2022
|Trade Name||Active Ingredient|
|Actinovate AG||Streptomyces lydicus WYEC 108|
|Double Nickel LC||Bacillus amyloliquefacens strain D747|
|Ecoswing||Extract of Swinglea glutinosa|
|Howler||Pseudomonas chlororaphis strain AFS009|
|LifeGard WG||Bacillus mycoides isolate J|
|OSO 5% SC||Polyoxin D zinc salt|
|Oxidate 5.0||Hydrogen peroxide 27%, peroxyacetic acid 5%|
|PureSpray Green||Mineral oil|
|Serenade Opti||QST 713 strain of Bacillus subtilis|
|Theia||Bacillus subtilis strain AFS032321|
|Thyme Guard||Thyme oil 23%|
|Timorex Act||Tea tree oil|
Fungicides were evaluated under field conditions in randomized, replicated trials using CO2 backpack sprayers applying the equivalent of 50, 75 or 100 gallons of spray solution per acre. Products were applied repeatedly every 7-14 days from bud break through late bloom, except for the NC leafspot trials, where products were applied at two-week intervals five times after harvest. Mummy berry primary (shoot strike) stage and secondary (fruit infection) stage were evaluated in the field based on visible symptoms. Fruit rot diseases were evaluated by hand harvest of ripe fruit every seven days for two pickings. Harvested berries were held at room temperature for 48-72 hours, then sorted and evaluated based on the presence of characteristic symptoms (spores, mold, soft or leaky fruit). Leaf spot and leaf rust data consisted of incidence ratings (number of leaves infected), severity (number of spots per leaf), defoliation, and in the case of leaf rust in NC on the cultivar Vernon, a visual estimation of bud set as an indicator of early defoliation, since bushes that defoliate prematurely produce fewer flower buds for the next year’s crop. All data were subjected to statistical analysis. All trials included an untreated control. Exact dates, locations, and results for each trial are available in the references cited at the end of this article.
Table 2 shows a summary of results from the seven organic fungicide trials compiled in this review article.
Table 2. Summary of seven organic fungicide trials in Georgia and North Carolina, 2020-2022.“o” = not tested, “-“ = not effective, “+” = significant disease control (as compared to the untreated check). Multiple symbols in a cell indicate the number of times a product was evaluated against that pathogen over the three years.
|Product and rate/acre||Number of trials||Mummy Berry||Anthracnose fruit rot||All fruit rots combined||Septoria leaf spot||Blueberry rust|
|Actinovate AG 12 oz||1||–||o||o||o||–|
|Double Nickel LC 4.5 pt||6||–||– –||– +||– +||– – – –|
|Ecoswing 1.5 pt||1||+||o||o||o||o|
|Ecoswing 2 pt||2||+||o||o||o||–|
|Howler 3 lb||1||–||o||o||o||–|
|Howler 5 lb||6||– +||– –||+ +||– +||– + –|
|LifeGard WG 2.5 oz||2||–||o||o||o||– –|
|OSO 5% SC 6.5 fl oz||5||–||+||+||– +||– – – –|
|Oxidate 5.0 26 fl oz||1||–||o||o||o||–|
|PureSpray Green 1.5 gal*||5||o||+ +*||– –||– +||– + +|
|Serenade Opti 20 oz||5||– +||–||–||–||– – –|
|Theia 3 lb||3||+||–||–||+||+|
|Thyme Guard 1.5 qt||3||o||–||+||– –||– – –|
|Timorex ACT 17 fl oz||3||o||–||+||–||– –|
|*PureSpray Green was effective against anthracnose fruit rot but also caused unacceptable darkening at the calyx end of ‘Farthing’ berries in the NC trial, and for that reason is not recommended for pre-harvest use.
**Various products with known efficacy against specific diseases were used in some trials for comparison as a “conventional control”, including Cevya, Indar 2F, Orbit 3.6E, Miravis Prime, and pyraziflumid.
Organic fungicides are not as consistently effective as conventional fungicides, but in many cases the organic products performed significantly better than the untreated control, as shown by the multiple “+” symbols in Table 2. In some instances, both the “+” and the “-“ symbol appear in the same cell, indicating that the product worked in some trials but not in others. Often the failure of control occurred under a high disease pressure, whereas control was achieved when disease pressure was light. Products that have one or more “+” symbols in the table are those that organic growers might consider for use against that pathogen. Many cells in the above table contain only the “o” symbol. These are experiments that need to be conducted in the future (or results sourced from prior work) in order to fill out the blanks and provide organic growers with the information needed to make good disease control decisions. This review article is limited in scope to the seven trials conducted in a three-year period in Georgia and North Carolina, however these experiments provide real-world results in a very challenging disease control environment and should be useful as guidance to growers in the southeastern US and elsewhere.
Oliver, J.E., Cline, W.O. and Curry, S. 2022. Evaluating organic fungicides for control of blueberry diseases in the southeastern U.S. (pdf) Research report to the Southern Region Small Fruit Consortium.
Oliver, J.E., 2022. Evaluation of fungicides for organic disease control in 2020 & 2021. Dixie Blueberry News, 11 March 2022.