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Citrus Black Spot in Florida by M Citrus black spot in Florida By M. M. Dewdney, N.A. Peres, M. Ritenour, T. Schubert, R. Atwood, G. England, S. H. Futch, T. Gaver, T. Hurner, C. Oswalt and M. A. Zekri he fungal disease citrus black spot has been found in the TImmokalee area in a small number of groves. The Division of Plant Industry (DPI) and the Citrus Health Response Program (CHRP) are conducting surveys to determine how many additional groves have been affected. A forensic team is attempt- ing to discover how the pathogen may have arrived in Florida, but so far, no real clues have emerged. See www. Figure 1. Mild hard spot symptoms on Valencia fruit in Florida. doacs.state.fl.us/pi/enpp/pathology/ citrus-black-spot.html for maps show- you have suspicious lesions on your ently does not allow citrus to be im- ing the locations of black spot finds fruit, please contact your local CHRP ported from areas known to have this as they are confirmed. If you feel that office to arrange for an inspection and disease, and the movement of fruit into collection of samples. Visit European markets is also restricted. www.doacs.state.fl.us/pi/pec/ Currently, the USDA’s Animal and pec-dist-map.html for CHRP Plant Health Inspection Service and contact information. other federal agencies are developing Black spot is a serious policies for intrastate, interstate and disease that has the potential international trade of Florida citrus to be economically damaging and will communicate these policies to both processing and fresh to growers via Florida Citrus Mutual. fruit. When the disease is not This is a complicated process involv- controlled in the grove, up to ing trade negotiations with many 80 percent of fruit has been countries, and great care is needed to reported to be affected. The make sure that all agreements are fair disease impact comes in the and biologically sound. The disease is form of blemished fruit and currently found in many humid sub- reduced yields from premature tropical countries including Argentina, abscission of heavily infected Australia, Brazil, China, South Africa, fruit. The United States pres- sub-Saharan Africa, Southeast Asia Figure 2 (above). Moderate hard spot symptoms on Valen- cia in Florida. The small red lesions are early virulent spot. Figure 3 (right). Classic hard spot symptoms from Florida. Figure 4 (far right). Cracked spot on Va- lencia from Florida. CITRUS INDUSTRY • July 2010 and southern South America. The limited to three months of the year. cases of black spot resistance to strobi- disease has not been detected within In Brazil, conidia are more impor- lurins in the literature, other countries Mediterranean countries. tant for disease spread as they can have more fungicides with different As with many fungal pathogens, infect young fruit on cultivars that modes of action to use as rotational black spot has a sexual and asexual have multiple or overlapping crops products. We do not want to lose the reproductive phase, and each stage has on the tree, such as Pera or Valencia strobilurin management tool from our a different scientific name: Guignardia oranges. Environmental conditions in disease control arsenal. citricarpa is the sexual stage and Phyl- Florida are most similar to those in losticta citricarpa is the asexual stage. Brazil, which suggests that conidia LEAF LITTER Both names are used in documents are potentially more important in the The major period of leaf drop in referring to black spot. disease cycle. Only through experi- Florida occurs from late January to ence can we be certain, but disease March. Since black spot has a disease DISEASE CYCLE control in Florida will likely be more cycle similar to greasy spot (caused by To aid disease management deci- challenging than it is in South Africa Mycosphaerella citri), it is suspected sions, it is essential to have a basic and Australia. that spore production by Guignardia understanding of the disease cycle. will probably begin in late April and The sexual structures of the fungus, CONTROL extend through mid- to late summer as known as pseudothecia, are formed in In countries with the disease, it does with Mycosphaerella. Inocu- decomposing leaf litter. Approximate- black spot has been controlled most lum can be reduced if the leaf litter ly 50 to 180 days from the time of leaf effectively through the use of fungi- decomposes before spore production drop, the sexual structures mature and cide applications during the summer begins. Application of lime, shred- eject ascospores (sexual spores). Leaf months. Several of the fungicides rec- ding of leaf litter, or frequent light drop can occur at various times dur- ommended for control abroad are not irrigation (approximately five times/ ing the season, but most leaves abscise labeled for use on Florida citrus. This week) will promote litter decomposi- in the spring. Intermittent wetting and leaves two control options: copper and tion. The irrigation should be done drying appears to be essential for the the strobilurin fungicides (Abound®, with microsprinklers as drip emitters maturation of the fungal structures in Gem® and Headline®). An all-copper will not wet the grove floor enough to the fallen leaves, and rain, irrigation program appears to be effective at promote decomposition. Fertilization or even heavy dew can be sources reducing disease incidence and sever- with urea, ammonium sulfate, or other of wetting. ity in Brazil and may be adequate for fertilizers that generate ammonia de- The ascospores are ejected up many processing blocks that are not lays decomposition, but inhibits spore to 0.5 inch (1.2 cm) above the litter concerned with copper phytotoxicity production. In Brazil, mowers are surface, where they are dispersed by to the rind (copper burn). designed to throw weed and ground air to susceptible leaves, fruit and Applications should begin late cover residues from row middles twigs. Optimal conditions for infec- April to early May and continue with under the canopy to cover leaf litter. tion by G. citricarpa are temperatures monthly sprays until late August. If This practice stimulates decomposi- between 70°F and 90°F (21°-32°C) abundant rain is anticipated during tion and provides a physical barrier and leaf wetness for 24 to 48 hours. April, applications should start early to spore dispersal. Fungicide applica- In most regions where black spot is in the month. If a block is already on tions should reduce the leaf and fruit well established, ascospores are the a canker-control program with cop- infection, and in combination with primary inoculum. Conidia (asexual per applications every 21 days, this practices mentioned above for dealing spores), produced in pycnidia (spore- regimen should control black spot. If with litter, could substantially reduce forming structures), are produced on canker sprays are not needed in a fresh inoculum levels. mature fruit, leaf litter and dead twigs. market block or phytotoxicity is a con- Because of the role leaf litter plays The conidia are dispersed over short cern, it would be best to save strobilu- in the disease cycle, minimizing trash distances by splashing water from one rin applications for the hottest months. movement from one location to anoth- fruit to the next or from the leaf litter In these cases, we suggest starting er is essential. The disease is normally to low-hanging fruit and foliage. Infec- with a copper application, followed by asymptomatic on leaves, so it is very tions on fruit from both spore types a strobilurin, then copper, strobilurin easy to inadvertently move inoculum have a 4- to 6-month latent period and a final copper application. from one block or grove to another. before symptoms develop. Fungicide applications for other This is the rationale for the tarping In South Africa and Australia, foliar fungal diseases earlier in the regulations for fruit coming from the where many of the studies on the black spring will be ineffective for black quarantine areas, but other equipment spot disease cycle have been done, spot control because the spores are could also easily move trash from one ascospores from leaf litter are almost unlikely to be ejected until the rains of area to another. exclusively responsible for new infec- late April-early May when leaf decom- Movement of fresh fruit from areas tions; so there is effectively only one position has begun. Remember that a with black spot is restricted to avoid disease cycle per year. Conidia are not total of only four strobilurin applica- introduction of the pathogen to new important in South Africa and Aus- tions are allowed per block per year areas. However, since only asexual tralia because of the characteristics of for all disease problems. We also do spores that are splash-dispersed are these regions: 1) uniform flowering not recommend back-to-back applica- produced on affected fruit, the risk and fruiting because of cool tempera- tions of strobilurins to discourage the of introducing the pathogen into new tures during flower initiation and the development of pathogen resistance. areas with fruit is very low, especially use of drip irrigation; 2) rainfall is Although there have been no reported after fruit have received commercial CITRUS INDUSTRY • July 2010 packinghouse treatments (i.e., wash- ment of black spot symptoms. Appli- shipment of fresh fruit. ing, waxing, fungicide application cation of fungicides shortly before or and grading). after harvest did not reduce subsequent M.M. Dewdney and N. A. Peres are Studies in Argentina showed that disease development. Exposure of fruit assistant professors; M. Ritenour is an an effective black spot control program to high light intensity accelerates the associate professor; R. Atwood, G. Eng- using fungicides during the growing development of symptoms in the field land, S.H.
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