Disease Management in Organic Rabbiteye Blueberries

ABSTRACT

Non-indigenous fruit crops such as peaches, apples, and vinifera grapes have limited potential for organic production in the southeastern United States due to the substantial pest pressures associated with the long growing season and the warm, humid climate. In contrast, rabbiteye blueberries, which comprise a large majority of the blueberry acreage in the Southeast, are native to the region and are well adapted to cope with its challenging climate, poor soils, and extensive pest complexes. Although rabbiteye blueberries are resistant to several important diseases affecting other blueberry species, all rabbiteye cultivars appear susceptible to mummy berry disease and most are affected by one or more foliar diseases such as Septoria leaf spot, Gloeosporium leaf spot, and/or leaf rust. This article briefly reviews old and new approaches for controlling these diseases organically. Mummy berry can be managed by pathogen exclusion, soil cultivation against overwintering primary inoculum, and application of a Bacillus subtilis-based biofungicide against secondary infection of flowers. For foliar diseases, host resistance is the first line of defense, whereas fish oil-based biofungicides have provided promising levels of disease suppression on susceptible cultivars. Knowledge gaps and future research needs are discussed.

KEYWORDS:

• Rabbiteye blueberry
• Vaccinium virgatum
• mummy berry
• Monilinia vaccinii-corymbosi
• Septoria leaf spot
• Septoria albopunctata
• disease management
• organic agriculture

INTRODUCTION

Blueberry production in Georgia has increased exponentially since the mid-1970s to reach a statewide farm gate value of $75 million from more than 4100 hectares in 2006 (Boatright and McKissick, 2007). Thus, blueberry is now the economically most important fruit crop in the Peach State. About 120 hectares of blueberries have been or are currently being converted to organic production, located mostly in economically depressed areas in the southern part of the state. A number of factors indicate considerable near-term potential for expansion of commercial organic blueberry production in Georgia and neighboring states: (1) higher wholesale prices for organic produce in general; (2) the need for alternative sources of income for small and medium-sized farms in a region where traditional cash crops such as tobacco have become unprofitable; (3) increased consumer demand for blueberries fueled by the widely publicized health benefits of blueberry fruit (McCord, 1999); and (4) a serious commitment to organic blueberries by major blueberry marketing organizations in Georgia and Florida. If some important production limitations can be solved, organic production can become a significant part of the blueberry industry in the Southeast.

Rabbiteye blueberries (Vaccinium virgatum Ait. = V. ashei Rehd.), which are grown on more than 90% of the blueberry acreage in Georgia (Scherm and Krewer, 2003), are native to the southeastern United States and well-adapted to the region's warm and humid climate and large complex of potential pest species. The Flatwoods physiographic region of the southeastern Coastal Plains has extensive areas of virgin, sandy soils with a low pH suitable for blueberry production. In addition, rabbiteye blueberries grow well on many low-organic-matter soils throughout the Southeast with the addition of locally available amendments such as pine bark, pine straw, or pine sawdust. This allows for production of the crop on degraded, acidic, sandy clay-loam soils on former cotton farms in the Piedmont, now home to the urban and suburban population centers (Atlanta-Charlotte urban corridor) where the demand for organic produce is greatest.

Rabbiteye blueberries traditionally have been considered disease resistant, especially with regard to root rot, fruit rots, and dieback diseases, which are less prevalent than on other blueberry species (Caruso and Ramsdell, 1995). However, all rabbiteye cultivars appear susceptible to mummy berry disease, caused by the fungus Monilinia vaccinii-corymbosi (J.M. Reade) Honey. The disease is widespread in the Southeast and causes two symptom types: a shoot blight in early spring and fruit mummification in early summer. Even a low incidence of fruit mummies (pseudosclerotia) can lead to the rejection of entire fruit loads in the packinghouse with substantial reductions in economic return to the producer (Scherm and Copes, 1999). In a grower survey in Georgia, mummy berry was ranked the most important blueberry disease, considered a “major” or “moderate” problem by >75% of the participating producers (Scherm et al., 2001). Conventional growers in Georgia control mummy berry successfully with multiple applications of fungicide at the onset of and during bloom, when the infection leading to fruit mummification occurs (Scherm and Stanaland, 2001). In the absence of chemical management options, mummy berry is a major constraint to organic blueberry production.

Besides mummy berry, most rabbiteye blueberry cultivars are also susceptible to one or more foliar diseases caused by pathogenic fungi. Foremost are Septoria leaf spot, caused by Septoria albopunctata Cooke; Gloeosporium leaf spot, caused by Gloeosporium minus Shear; and leaf rust, caused by Thekopsora minima P. & H. Syd. (Scherm et al., 2008a; Fig. 1). Based on the aforementioned survey of production constraints in Georgia (Scherm et al., 2001), foliar diseases are now considered a “major” or “moderate” problem by >50% of blueberry growers. In addition, recent yield loss estimates indicate that foliar diseases are responsible for up to 30% of the total disease-related blueberry losses statewide (Williams-Woodward, 2003). Epidemics reduce photosynthesis of affected plants and lead to premature defoliation in the summer and fall, reduced flower bud production during the fall and winter, and lower return yields in the next year (Ojiambo and Scherm, 2005; Ojiambo et al., 2006, 2007).

FIGURE 1. Prevalence of Septoria leaf spot, Gloeosporium leaf spot, and leaf rust on rabbiteye blueberry cultivars ‘Brightwell’, ‘Climax’, ‘Powderblue’, ‘Premier’, and ‘Tifblue’, based on a field survey in Georgia in summer and fall of 2002 (n = 69) and 2003 (n = 79). Prevalence was calculated based on presence or absence of a particular disease on at least 5% of the leaves in a given sample. Data source: Scherm et al. (2008a).

This article briefly reviews old and new approaches for managing mummy berry and foliar diseases of blueberry organically. It focuses on methods that can be mechanized, allowing their use on a commercial scale.

ORGANIC MANAGEMENT OF MUMMY BERRY

In plantings where mummy berry is not present, pathogen exclusion is the most important management practice. In Georgia, the disease is endemic throughout the commercial blueberry belt in the southeastern part of the state, but there are isolated plantings, especially in south-central Georgia, southwest Georgia, the Piedmont, and the mountain areas, where M. vaccinii-corymbosi has not been established. In such plantings, care must be taken not to introduce the pathogen as pseudosclerotia in nursery plant containers or on field equipment such as mowers or mechanical harvesters. Still, even if these precautions are taken, it is possible that ascospores or conidia of the pathogen are introduced, by wind or by pollinating insects, from wild hosts growing in wooded areas around blueberry plantings. Indeed, of the more than one dozen wild Vaccinium species housed at the University of Georgia herbarium, at least three (in addition to wild rabbiteye blueberry) have been reported as hosts of M. vaccinii-corymbosi, viz. V. elliotti Chapman (mayberry), V. myrsinites Lam. (shiny blueberry), and V. pallidum Ait. (Blueridge blueberry; Batra, 1991). Other common Vaccinium species in Georgia apparently are not attacked by the fungus (e.g., V. arboreum Marsh. = sparkleberry) or are hosts to closely related fungal species that do not infect cultivated blueberries (e.g., V. stamineum L. = deerberry, which is subject to infection by Monilinia polycodii (J.M. Reade) Honey).

In plantings where the disease is endemic, cultural practices have been used with some success against pseudosclerotia on the ground, the only source of primary pathogen inoculum in the spring. Pseudosclerotia are unable to germinate when covered with soil to a depth of at least 2.5 cm (Milholland, 1974); thus, soil cultivation in the aisles between the plant rows in the dormant season reduces disease pressure. Unfortunately, most pseudosclerotia rest near the crowns of the blueberry bushes where cultivation equipment cannot be used without damaging the shallow roots of the plants (Ngugi et al., 2002). Efficiency of mechanical cultivation may be improved either by hand-ranking the pseudosclerotia into the aisle prior to cultivation or by combining shallow-operating implements that have good access to pseudosclerotia under the plant rows (e.g., in-row tillers such as the Weed Badger; Weed Badger Div., Marion, N.D.) with those that result in deep burial in the aisles. Alternatively, regular and aggressive use of a bed-shaper (V-plow) may result in accumulation of a sufficient amount of soil around the crown of the plants to effectively bury the pseudosclerotia (Fig. 2). On sandy soils with a high water table, blueberries are typically grown on raised beds for drainage. These beds erode over time and have to be rebuilt annually or biennially. The soil in the aisle is first loosened with a rotary cultivator and then thrown back on the top and shoulder of the bed using the bed-shaper.

FIGURE 2. Vertical distribution of plastic beads (used as surrogates for pseudosclerotia of Monilinia vaccinii-corymbosi) following soil cultivation methods applied on two commercial blueberry farms. Farm II: bed shaper (V-plow), followed by single passes with rotary cultivator and rotary hoe; Farm III: single pass with rotary cultivator. Values are means and standard errors of four replicate plots with 100 beads per plot. Data source: Ngugi et al. (2002).

Mulches such as pine bark, pine straw, or wheat straw are used commonly in organic blueberry plantings, primarily for weed suppression and moisture retention (Kuepper and Diver, 2004). If applied in a sufficiently thick layer, organic mulches may suppress mummy berry by inhibiting germination of pseudosclerotia on the ground. However, following decomposition or erosion of the mulch, germination of pseudosclerotia may actually be enhanced due to the better water-holding capacity of the mulch compared with bare soil. As such, the efficacy of mulching in suppressing the disease is difficult to predict.

Early-spring soil applications of caustic substances or desiccants such as lime sulfur or soybean oil (Cox and Scherm, 2001) may burn the apothecia of M. vaccinii-corymbosi, the ascospore-producing structures that emerge from pseudosclerotia upon germination. However, this approach is usually not very effective since apothecia develop over an extended period of time, hence requiring frequent applications. In contrast, soil-applied biological control agents could act against pseudosclerotia season-long, not just during the germination period in the spring. Mycoparasites such as Coniothyrium minitans W.A. Campb. have been tested extensively to reduce soil inoculum levels of Sclerotinia sclerotiorum (Lib.) de Bary, a close relative of M. vaccinii-corymbosi (McQuilken et al., 1995), and several formulations of this biocontrol agent are now available for commercial use. Investigating the potential for managing mummy berry with such sclerotial mycoparasites is an important research need for organic blueberry production.

In conventional production, direct application of fungicide to the blueberry bushes is the most widely used and most effective management practice against mummy berry. On rabbiteye blueberries in Georgia, three to five applications may be needed to control both stages of infection. On cultivars on which vegetative bud break occurs before bloom, one or two pre-bloom applications are made to control the shoot blight phase of the disease; this is followed by bloom sprays to control secondary infection of open flowers which leads to fruit mummification. In conditions where bloom occurs before or simultaneously with leaf bud break, only bloom applications are needed (Scherm and Stanaland 2001). Few biofungicides approved for use in organic production have been tested against mummy berry, and even fewer have been evaluated at multiple locations over several years (Table 1). The latter include Citrex (water-soluble organic acids; Citrex Inc., Miami, Fla.), Sulforix (calcium polysulfides; Best Sulfur Products, Fresno, Calif.), and Serenade (QST 713 strain of Bacillus subtilis; AgraQuest, Davis, Calif.). Of the three products, only Serenade has shown fairly consistent control of both shoot blight and fruit mummification in conditions of moderate to high disease pressure (Table 1). Still, the level of disease suppression (around 50% relative to the untreated check in most cases) is lower than that experienced after application of conventional fungicides (usually >80%). Nevertheless, Serenade is currently the most effective biofungicide for mummy berry control and is, therefore, widely used by organic blueberry producers in Georgia.

TABLE 1. Summary of field trials to evaluate the efficacy of the biofungicides Citrex, Sulforix, and Serenade against mummy berry disease of blueberry

Product	State and year	Blueberry cultivar and type ^z	Application timing	Disease pressure ^y		Rel. disease reduction ^x (%)		Reference ^w
				Blight	Mummies	Blight	Mummies
Citrex	GA 2005	Brightwell (R)	4 × in-season	Low	Moderate	NS	NS	61:SMF005
	GA 2005	Tifblue (R)	4 × in-season	Low	Moderate	NS	NS	61:SMF005
	MI 2005	Rubel (H)	4 × in-season	Very low	Very low	NS	NS	61:SMF023
	MI 2004	Jersey (H)	5 × in-season	Very low	Low	57.5	66.1	60:SMF013
Sulforix	MI 2005	Jersey (H)	1 × bud-swell	Low	Low	82.7	NS	61:SMF024
	MI 2004	Jersey (H)	1 × bud-swell	Very low	Low	38.9	NS	60:SMF013
	MI 2004	Jersey (H)	5 × in-season	Very low	Low	68.1	NS	60:SMF013
	NC 2004	Harrison (H)	1 × bud-break	…	High	…	NS	60:SMF045
	NC 2004	Harrison (H)	5 × in-season	…	High	…	NS	60:SMF045
Serenade	MI 2005	Rubel (H)	4 × in-season	Very low	Very low	94.7	NS	61:SMF023
	GA 2004	Brightwell (R)	4 × in-season	Very low	Moderate	NS	58.7	60:SMF031
	MI 2003	Jersey (H)	5 × in-season	Very low	Low	NS	45.2	59:SMF023
	MI 2002	Jersey (H)	5 × in-season	High	Very high	32.6	47.7	58:SMF039
	MI 2001	Jersey (H)	4 × in-season	High	High	NS	NS	57:SMF05
	MI 2000	Rubel (H)	8 × in-season	Moderate	High	56.1	53.4	56:SMF6
^aBlueberry type: H = highbush, R = rabbiteye.
^yDisease pressure based on disease levels observed in the untreated check.
^xRelative disease reduction compared with levels observed in the untreated check.
^wCitations indicate volume and page numbers of the series Fungicide & Nematicide Tests, published by the American Phytopathological Society.
^NSLack of statistically significant difference relative to untreated (α = 0.05).

ORGANIC MANAGEMENT OF FOLIAR DISEASES

Host resistance is the best line of defense against foliar diseases of blueberry. In a 2-year field survey in Georgia (Scherm et al., 2008a), ‘Powderblue’ consistently was least affected by foliar diseases among five commonly planted rabbiteye cultivars (‘Brightwell’, ‘Climax’, ‘Powderblue’, ‘Premier’, and ‘Tifblue’). ‘Climax’ showed the lowest levels of Septoria leaf spot but, based on the survey and subsequent observations, can be affected severely by leaf rust in wet years. ‘Premier’ and ‘Tifblue’ were intermediate in foliar disease levels, whereas ‘Brightwell’ often had high levels of Septoria and/or Gloeosporium leaf spots. On ‘Delite’, a cultivar not formally included in the disease survey, we often observed considerable premature defoliation associated with leaf rust infections; because of its high susceptibility to leaf rust, this cultivar is unsuitable for organic production.

In conventional blueberries in the southeastern United States, synthetic fungicides are applied during summer and fall to control foliar disease epidemics on susceptible cultivars (Cline et al., 2006). These products are, of course, unacceptable in organic production. Since the pathogens causing foliar blueberry diseases can overwinter on current-season shoots (S. albopunctata and G. minus) or on alternate hosts outside blueberry plantings (T. minima), cultural practices such as mulching are not effective disease management strategies. This leaves organic producers with limited options against foliar diseases on susceptible cultivars. Most organic blueberry growers in Georgia utilize the biofungicide Serenade for disease control during the summer. However, a recent field trial (Scherm et al., 2008b) showed that Serenade has limited efficacy against Septoria leaf spot (Fig. 3), the most common component of the foliar disease complex on rabbiteye blueberry. The lack of efficacy of Serenade against Septoria is not surprising when one considers ecological realities: the nutrient-poor leaf surface is not a favorable environment for biocontrol agent buildup and disease control over a 3-month period during harsh weather conditions (summer heat and severe solar radiation), even with repeated applications. In that same trial, however, two fish oil products, Organic Gem (Advanced Marine Technologies, New Bedford, Mass.) and Neptune's Harvest (Neptune's Harvest Fertilizer, Gloucester, Mass.), showed significant efficacy against the disease (Fig. 3). Previous studies on other perennial crops such as grape also reported significant foliar disease suppression following application of fish oils (Cabaleiro et al., 2003; Martin et al., 2005). Since the efficacy of fish oils in a particular crop-disease combination depends critically on the source, type, and part of fish used in the formulation of the fish oil or emulsion, future research on blueberry should test more products and different formulations for their effects on foliar diseases.

FIGURE 3. Effect of four biofungicides, each applied four times during summer and early fall, on severity of Septoria leaf spot of ‘Brightwell’ rabbiteye blueberry in a field trial on a certified organic farm in 2007. Error bars indicate standard errors, and means followed by the same letters are not significantly different according to Fisher's protected LSD test. Data source: Scherm et al. (2008b).

Following further verification of their disease control efficacy, fish oil sprays during the summer may have significant added value by providing readily available macro- and micronutrients to support plant growth and development. For example, Neptune's Harvest and Organic Gem, the two fish oil products used in our aforementioned leaf spot trial, are marketed as organic foliar fertilizers with N-P-K ratios of 2–4–1 and 3–3–0.3, respectively. These potential nutritional benefits need to be documented in future research by measuring foliar nutrient status, plant growth, and flower bud formation in fish oil-treated plots.

CONCLUSIONS

Although organic sales currently account for only 2% of total food sales in the United States, organic agriculture is one of the fastest growing sectors of agriculture nationwide, with growth rates of about 20% per year for the last 15 years (Thilmany, 2006). Growth has been particularly brisk for organic fruits and vegetables (Greene, 2006). In the Southeast, however, organic production has been lagging behind due the substantial pest pressures associated with the long growing season and the warm, humid climate. For this reason, non-indigenous fruit crops such as peaches, apples, and Vitis vinifera grapes have limited potential for commercial organic production in the region. In contrast, rabbiteye blueberries are native to the southeastern United States and are well adapted to the region's challenging climate, poor soils, and extensive pest complexes. Various strategies, ranging from exclusion to cultural practices, host resistance, and application of biofungicides, are available to manage mummy berry and foliar diseases, the key disease problems on rabbiteye cultivars in Georgia and neighboring states. With further improvements in fertility and weed management, organic production can become a significant part of the blueberry industry in the Southeast.

Harald Scherm and Gerard Krewer's research on organic blueberry production is currently supported by the USDA's Integrated Organic Program (award no. 2006-02018).