• Topics: Plant Pathology | Citrus REC - Lake Alfred | Dewdney, Megan M | Estes, Mark R | Peres, Natalia A | Citrus Black Spot | SP 043 Florida Citrus Pest Management Guide

2012 Florida Citrus Pest Management Guide: Citrus Black Spot¹

Citrus black spot is a disease caused by the fungus Guignardia citricarpa. The disease causes fruit blemishes and significant yield losses, especially on sweet oranges. Black spot can affect all commercial citrus species and cultivars commonly grown in Florida. Lemons are the most susceptible, but sweet oranges—especially mid-late maturing types such as 'Valencia'—are also highly susceptible to this disease. 'Hamlin' sweet oranges and tangerine/mandarin types are moderately susceptible. Grapefruit is thought to be moderately susceptible, but there is little information available. Management is required in groves intended for processing and fresh market fruit in quarantine and surrounding areas and should be considered in all others.

Black spot fruit symptoms are wide ranging and have many different names. Hard spot is the most diagnostic symptom of black spot. The 3–10 mm diameter lesions are depressed and nearly circular, with gray necrotic tissue at the middle that has a brick-red to black margin that can be cracked around the edges. Structures that produce the asexual spores (pycnidia) are often present in the center of lesions and resemble slightly elevated black dots. Hard spot appears as the fruit begins to color before harvest. Lesions first occur on the side of the fruit with the greatest light exposure. False melanose symptoms appear on green fruit early in the season and do not contain pycnidia. The slightly raised lesions are 1–3 mm in diameter and can vary in color from tan to chocolate brown. Under favorable infection conditions, false melanose can resemble the mudcake symptoms of authentic melanose, but are very dark brown rather than rust red. False melanose symptoms can develop into hard spot as the season progresses. Cracked spot is a symptom that has only been observed in the Americas and is reported to be an interaction between rust mites and G. citricarpa. Cracked spots are large, diffuse, smooth lesions that form raised cracks around the center. Hard spots can form in the center of these lesions. The most concerning black spot symptom is virulent spot. Early virulent spot (freckle spot) lesions start as irregularly shaped, sunken lesions with a reddish color. Early virulent spot can either coalesce to cover a large portion of the fruit surface or become hard spot. When spots coalesce, they turn from brown to black, and the older lesion surface becomes leathery. Many pycnidia can be found in early and expanded lesions. Virulent spot occurs on mature, severely infected fruit at the end of the season. Virulent spot symptoms can appear in post-harvest on apparently symptomless fruit, sometimes in transit to markets. Despite the unsightliness of black spot lesions, they rarely cause internal fruit rot so those fruit that have not fallen off the tree are still suitable for processing. Significant fruit drop is a common symptom in heavily infected groves.

Airborne ascospores produced in decomposing leaf litter on the grove floor and blown into the canopy by the wind are the primary inoculum for black spot. These spores germinate and directly infect the leaves and fruit. There is a long latent period for this disease, which means that most symptoms do not appear for several months, usually not until the fruit begins to ripen. The fungus requires a long wetting period of 24–48 hours to infect, and the disease is favored by warm, humid weather such as occurs during the summer months. Major ascospore release usually occurs from April to early September, with favorable infection conditions from May through September. Fruit remains susceptible most of the growing season. It is unknown how long leaves may remain susceptible. The asexual spores (conidia) are formed in fruit lesions, and to a lesser extent in leaf litter and twigs. Conidia spread by rain splash and can infect fruit and leaves.

Monthly fungicide applications of copper and/or strobilurins (Abound, Gem, or Headline) will be needed from early May to mid-September to control black spot. Fungicide applications in April are advised if there is substantial rainfall that month. Our fungicide recommendations are based on efficacy data from trials in other countries with black spot and products registered for use on citrus in Florida. Since only four strobilurin fungicide applications can be used in a season for any purpose, it is recommended to reserve strobilurin fungicides for times when phytotoxicity from copper applications is a concern (temperatures >94°F). This is especially important for fresh fruit. It is recommended that strobilurin fungicides not be applied in two consecutive sprays to manage pathogen resistance. Currently, there are no other rotational fungicides for resistance management.

In addition to chemical control measures, practices to accelerate leaf litter decomposition beneath the trees to reduce the ascospore inoculum may be beneficial. Enhancing leaf litter degradation should commence in mid-March. There are three methods that have reduced the ascospore inoculum of Mycosphaerella citri, the fungus that causes greasy spot. The first is to increase microsprinkler irrigation to at least 5 times a week for approximately a ½ hour per irrigation period for 1.5 months. The leaf litter decomposition will be greater compared to that with the traditional irrigation frequency. One drawback is that leaf litter reduction will be confined to the areas where the microsprinklers reach. A second method is to apply urea (187 lb/treated acre) or ammonium sulfate (561 lb/acre) to the leaf litter. If using ammonium sulfate as a method to control leaf litter inoculum, monitor the soil pH to ensure that it does not become too low. The leaf litter decay will be less than without urea but when tested with M. citri, the number of spore-producing structures was reduced and fewer spores were produced. Nitrate-based fertilizers are ineffective. The final method is to apply dolomitic lime or calcium carbonate (2,226 lb/treated acre) to the leaf litter. The decay rate is greater for litter treated with lime, and inoculum production is reduced. All treatments worked equally well with M. citri, and there is no indication that one method is better than another. Lime or irrigation methods should not be used in conjunction with the high N treatments since they have opposite methods of action.

There are several cultural practices that will aid control and help restrict further spread of black spot. It is essential to minimize plant trash movement among groves and even among blocks within groves. While there are generally few symptoms on leaves, the ascospores, which are the main inoculum, are formed within the leaves. As leaf litter decomposes, the spores form and are forcibly ejected. It is very easy to inadvertently move the fungus from one site to another with symptomless leaves and other trash. This is the basis of the tarping requirement from quarantine areas, but any grove equipment or vehicle can move leaf litter or trash from one location to another.

Declining trees should be removed from a grove regardless of the cause. Trees that are declining will often have off-season bloom as a symptom of stress. Where there is more than one age of fruit present on the tree, the asexual spores on the fruit can be transferred to new fruit, amplifying the disease. This problem is especially troublesome on 'Valencia' when new and old fruit crops overlap. Fruit do not appear to become resistant to infection as they age. In addition, nutritionally stressed trees will often express black spot symptoms first. A good nutritional program (http://edis.ifas.ufl.edu/pdffiles/SS/SS47800.pdf) helps to minimize symptoms and maintain yields.

Where possible, open the tree canopy by skirting to reduce the leaf wetness periods. The fungus requires between 24-48 hours of leaf wetness to infect. It is also important to minimize dead wood in the canopy. Like the melanose pathogen, black spot fungus can colonize and reproduce in dead twigs. Canopies with significant numbers of dead twigs will have more problems with black spot than those without.

Finally, as with all fungal diseases, it is important to use clean nursery stock. Currently, there are no nurseries near known infected groves; however, this may change as disease distribution becomes better understood.

Regulatory Considerations

Care must be exercised in handling and moving citrus fruit, leaves, twigs, and debris from CBS Quarantined Areas since the disease may be easily and unwittingly spread to other citrus trees, nurseries, or groves. The following rules are in addition to stipulations imposed as a result of the statewide citrus canker quarantine:

The U.S. Department of Agriculture Animal and Plant Health Inspection Service (APHIS) issued a Federal Order effective October 14, 2010, to help prevent the spread of the plant pathogen Guignardia citricarpa Kiely, the causal agent of citrus black spot (CBS). The initial CBS Quarantined Areas and Regulated Areas are located in Collier and Hendry Counties and were announced and delineated in the Federal Order (DA-2010-47). An updated Federal Order (DA-2011-29) was released on June 2, 2011 expanding existing quarantines by 6 sections in Collier County and 14 sections in Hendry County. Additional sections have been quarantined since that date, and we anticipate publication of a new Federal Order that will specify these changes. Details of the latest regulations and quarantined areas of record for CBS may be accessed through the state's Citrus Health Response Program (CHRP) website, along with other relevant compliance information, at the following address: http://www.doacs.state.fl.us/pi/enpp/pathology/citrus-black-spot.html.

Growers, Harvesters and Haulers

Citrus growers and fruit handlers must operate under compliance agreements with regulations that serve to protect the citrus industries of Florida, the United States, and international trade partners. When harvesting citrus in the groves, vehicles used to transport fruit from CBS Quarantined Areas must meet the following minimum standards: All conveyances, whether loose-filled with fruit or loaded in pallet boxes or field bins and stacked on trucks or trailers, must be completely covered with no openings greater than 0.5 inches, with the exception of side and rear walls constructed of expanded metal, which must not exceed 0.75 x 1-11/16 inches. Tarpaulins (tarps) used as fruit covers may be of any fabric with a weave of less than 0.5 inches. Details of transport vehicle regulations may be found in the CBS Federal Order DA-2011-29.

Each load of fruit must be identified by issuing a clearly written, serially numbered trip ticket containing the following information: Grove name, Land owner or agent, Lessee, Harvester; License tag number; Number of boxes in load; Harvested grove block, and Grower C/A Number; Destination processing plant or packinghouse; Harvest date. Loads from quarantined areas must have “TARP” and a big letter, “Q,” written near the bottom of the ticket. In addition, the Citrus Fruit Harvesting Permit number must be included on the trip ticket, if issued, indicating that neither CBS nor Citrus Canker (CC) was found in a pre-harvest grove inspection for the specified block.

Prior to departing any citrus grove, all personnel are required to inspect all vehicles and equipment for plant material and debris, as well as clean all vehicles, equipment, picking sacks, and clothing to ensure that they are free of fruit, limbs, leaves, soil, and debris prior to applying microbial decontaminant. All plant material and debris cleaned from said vehicles and equipment are to be left on the grove property, or, if moved, must be transported away from citrus production areas (both nursery and grove) to a location that will not pose a risk to citrus trees or groves, and be treated in accordance with the terms set forth in the Federal Order (DA-2011-29). Grove and harvesting equipement must be decontaminated using one of the following materials:

List A - Equipment Decontaminants

A1. 200 ppm solution of sodium hypochlorite with a pH of 6.0–7.5; or

A2. 0.2% solution of a quaternary ammonium chloride (QAC) cleaner/disinfectant compound; or

A3. Peroxyacetic acid-based sanitizer at labeled rates.

Personnel exposed to citrus fruit, leaves, or plant material in groves must be decontaminated by an appropriate sanitizing method in accordance with Approved Decontamination Products & Methods (see FDACS/ CHRP Grower/ Caretaker Compliance Agreement, "Schedule 11," available on the Division of Plant Industry website at http://www.freshfromflorida.com/pi/chrp).

Processing and Packing Facilities and Haulers

All citrus fruit harvested from a Citrus Black Spot Quarantined Area must move intrastate either directly to a processor operating under a state compliance agreement for processing into a product other than fresh fruit, or to a packinghouse operating under a federal compliance agreement. Distribution of fresh citrus fruit from a CBS Quarantined Area directly to markets within Florida is prohibited.

Packers and processors receiving whole fruit from CBS Quarantine Areas for packing or processing, respectively, must operate under a state or federal compliance agreement. Each load of fruit harvested from a quarantined area is required to be covered by a tarp in accordance with federal regulations to preclude the loss of leaves, fruit, and debris in transit to a packing or processing facility. The load must arrive tarped at the receiving facility, and all quarantined fruit, leaves, and debris in the truck or trailer must be unloaded and handled in compliance with Federal Order DA-2010-47. The vehicle must be thoroughly cleaned out and decontaminated prior to departing the receiving facility. If any citrus leaves or other citrus waste material is to be moved from a receiving facility (or from a grove), it must be placed in bags or be covered in transit in order to prevent the loss of leaves, fruit, or debris. Once emptied and cleaned of all leaves and plant debris, all trailers, truck beds, field boxes or bins must be disinfected by using one of the decontaminant materials in List A (above).

Disposal of Citrus Debris

All leaves, culled or eliminated fruit, and other plant debris originating from a CBS Quarantined Area, cleaned from trailers, tarpaulins, field boxes or field bins at a receiving facility, or hauled from a CBS Quarantined Area, must be moved in an enclosed or covered conveyance as stipulated in the Federal Order. When citrus plant material comes in contact with a vehicle, the vehicle must be decontaminated following movement with a sanitizer in List A (above), in accordance with the Federal Order.

Citrus leaves, fruit, and plant debris must be handled or treated by one of the following methods subject to monitoring by an inspector:

a. Heat treated to a minimum of 180°F for at least one hour;

b. Incinerated;

c. Buried at a landfill or other APHIS or State approved disposal site, and covered with dirt at the end of every day that dumping occurs;

d. Fed to livestock as feed, only by Special Permit (DACS-08126) issued by a Division of Plant Industry official, in a fenced pasture located a minimum of 1,900 feet from the nearest citrus tree or grove. Each disposal site must be inspected and approved prior to use, and re-inspected each year.

Additional Rules for Fresh Citrus Packers and Shippers

Since a Harvesting Permit is required for shipments of quarantine fruit to the European Union (EU) with respect to citrus canker, the grove may be inspected at the same time for symptoms of CBS, and the Harvesting Permit may serve as a single instrument in declaring the grove as visually free of both canker and CBS. In addition, fruit from said grove must be inspected and found free of CBS (and canker) in the packinghouse. As an option, the Federal Order for CBS allows a statement that the source grove has been “subjected to appropriate treatments against CBS,” and the harvested fruit must also be inspected and found free of CBS in the packinghouse.

Fresh fruit from groves within a CBS Quarantined Area is eligible for interstate movement under federal certificate to all states under the following conditions:

The fruit must be washed and brushed, then treated with a disinfectant from List B (below), then treated at label rates of imazalil and/or thiabendazole at time of packing. The fruit must be waxed in a packinghouse operating under an APHIS-approved packinghouse procedure prior to shipment. The fruit must be free of leaves and other plant material, and attached stems must be less than 1 inch in length. The fruit must be disinfested and packed in a packinghouse, having a signed APHIS compliance agreement. The fruit must be accompanied by a federal certificate issued by a person or inspector operating under compliance with APHIS. And the certificate must be present on the packed cartons or containers of fruit and the accompanying paperwork.

Fresh fruit from groves within a CBS Quarantined Area is eligible for movement interstate under limited permit to noncommercial citrus-producing states under the following conditions:

The fruit must be washed, brushed, and treated with a disinfectant from List B (below) in a packinghouse operating under an APHIS-approved packinghouse procedure prior to shipment. The fruit must be free of leaves and other plant material, and attached stems must be less than 1 inch in length. The fruit must be packed in a packinghouse with a signed APHIS compliance agreement. The fruit must be accompanied by a limited permit issued by a person or inspector operating under compliance with APHIS. And the limited permit must be present on the packed cartons or containers of fruit and the accompanying paperwork.

List B – Chemical Treatments – Must be applied in accordance with APHIS-approved Packinghouse procedures for Citrus Black Spot (http://www.aphis.usda.gov/plant_health/plant_pest_info/citrus/black_spot.shtml):

B1. Sodium hypochlorite solution at 200 PPM for at least 2 minutes; or

B2. Sodium o-phenyl phenate (SOPP) solution at 1.86 to 2.0 % total solution; or

B3. Peroxyacetic acid (PAA) solution at 85 PPM for at least 1 minute.

Exports

Regulated fruit from groves in a CBS Quarantined Area that is not eligible for interstate movement under the conditions stated for consumption in the US may be moved through Florida or interstate only for immediate export. Regulated fruit for export must be accompanied by a “Limited Permit for Export Only” issued by an inspector or a trained individual operating under a compliance agreement and must be moved in a container under APHIS seal directly to the port of export. No trans-loading will be allowed at ports located in citrus-producing states.

Regulatory Remarks

Although there is currently no requirement for covering loads of fruit prior to movement or in transit to receiving facilities from perimeter areas around CBS Quarantines, covering this fruit is highly recommended as a precautionary measure for the protection of citrus groves in non-quarantine areas of our state.

Please check for updates and information on federal quarantines, regulations, and the interstate movement of citrus at the APHIS Citrus Health Response Program website: http://www.aphis.usda.gov/plant_health/plant_pest_info/citrus/index.shtml.

Recommended Chemical Controls

READ THE LABEL.

See Table 1.

Rates for pesticides are given as the maximum amount required to treat mature citrus trees unless otherwise noted. To treat smaller trees with commercial application equipment, including handguns, mix the per acre rate for mature trees in 250 gallons of water. Calibrate and arrange nozzles to deliver thorough distribution and treat as many acres as this volume of spray allows.

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, Dean.

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