February is the time to startthinking about fungal diseasecontrol for the upcoming sea-

son. Protecting the spring flush to sus-tain high yields is important. Fungalfoliar diseases are most important forthose producing fresh market citrusbecause of fruit blemishes, but somediseases like greasy spot are a concernfor all growers. These days whenHuanglongbing (HLB) is everyone’smajor concern, fungal foliar diseasescan seem like a minor problem. But itmust be remembered that fungal foliardiseases can weaken a tree through de-foliation and cause fruit drop, furtherlowering yields. Defoliation can leadto an increase in the number of flushesover the summer when it has becomeimportant to manage the trees for a re-duced number of flushes to make themless attractive to psyllids.

GREASY SPOT

Greasy spot, caused byMyco-sphaerella citri, is a problem on allcitrus cultivars for both processing andfresh market fruit. Ascospores (sexualstage spores) are produced in the leaflitter during the winter months. Changein irrigation practices to microsprinklershas caused the ascospore release periodto take place earlier in the spring thanhistorically — a phenomenon previ-ously documented by Pete Timmer, re-tired University of Florida plantpathologist. The majority of ascosporesis now ejected between April and June.

Despite the earlier ejection pe-riod, the epiphytic growth (onthe surface of the plant) and in-fection conditions are still opti-mal between June and Septem-ber with warm and humid sum-mer nights. Just before this per-iod is when the fungus is mostvulnerable, which suggests thebest spray timing. Once the fun-gus has become well establishedwithin the leaf tissue, controlbecomes difficult.In these times when many

growers also need to control cit-rus canker, the use of copper forgreasy spot is being promoted infavor of other products in an attempt tominimize production costs. Copper alsohas a greater residual effect on fruitthan most other products. Because theM. citri is present on the underside ofthe leaf, slower tractor speeds (2.5mph) and higher spray volumes (250

gallons/acre) are recom-mended when spraying forgreasy spot than for otherdiseases or pests to ensuregood coverage. On process-ing Valencias, a single ap-plication of copper orcopper and oil when appliedin mid-May to June willgive sufficient control pro-viding the disease was notsevere the year before. Onmore susceptible cultivarssuch as grapefruit or earlyseason oranges, at least twocopper applications will benecessary at mid-May toJune and after the summer

flush is expanded. For fresh fruitwhere rind blotch has been problem-atic, a third application may be neces-sary in August. If canker is not aproblem or insufficient greasy spotcontrol is achieved with copper, thehorticultural oils, strobilurin fungi-cides (Abound, Gem or Headline) orfenbuconazole (Enable) are still verygood options for one application, espe-cially for the May-June timing.MELANOSE

Melanose can be problematic on allcitrus cultivars, but is especially severeon fresh grapefruit. Since the inoculum

from Diaporthe citri (asexual stagePhomopsis citri) is found in dead twigs,groves that are less than 10 years oldgenerally do not have serious problemswith melanose. The fungus is able toproduce asexual spores (known as coni-dia) in flask-shaped pycnidia embeddedin the wood. Spores are produced on in-fected twigs from the previous seasonor twigs that were healthy when theydied and subsequently colonized. Be-cause conidia are produced in deadwood, the disease can be especially badin years where a freeze has killed manysmall branches. If HLB-infected treesare not removed and left to decline,they will also become sources of sporesin a grove. Conidia ooze from the open-ing of the pycnidia in a tendril duringperiods of high humidity or rain and aresplashed onto susceptible young leavesand fruit.Ten to 12 hours of leaf wetness,

which is not uncommon in Florida es-pecially in the center of a canopy, arerequired for an infection to take placeif temperatures are between 70-80°F(21-27°C). Longer leaf wetness peri-ods of up to 24 hours are needed iftemperatures are cooler. No spores areproduced from infections that occur onthe leaves and fruit. Copper is the mosteconomical option for melanose con-trol because of the long residual activ-ity, but residues decline with fruitexpansion and rainfall. Residue levelscan be estimated using the coppermodel available for download athttp://www.crec.ifas.ufl.edu/crec_web-sites/ fungal/dmodel.htm.April is usually a dry month and

CITRUS INDUSTRY • February 2009

By Megan Dewdney

Greasy spot symptoms on grapefruit leaves

Melanose on grapefruit

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CITRUS INDUSTRY • February 2009

fruit is expanding rapidly, so mostmelanose sprays are not initiated untilmid-late April. For less susceptiblecultivars such as oranges and tanger-ines, one or two applications are usu-ally sufficient for control unless thereis substantial dead wood in a grove.For more susceptible fresh marketgrapefruit, melanose sprays should beinitiated when the fruit are one-quar-ter to one-half inch. On average, cop-per applications are continued everythree weeks until the fruit become re-sistant in early July. If copper is ap-plied in early June, it can also serveas the first greasy spot application. Ifthere is concern about fruit burn withcopper in hot weather, strobilurinfungicides give good control, butshould never be used more than twicein a row or in a season for melanosebecause of possible development ofresistance to the fungicide.

ALTERNARIA BROWN SPOT

Alternaria alternata is most prob-lematic on fresh market tangerines andtangelos where it can cause blemishesin addition to considerable fruit andleaf drop in severe situations. Occa-sionally alternaria is a problem forgrapefruit as well. Low regions ingroves generally have more problemswith Alternaria brown spot (ABS) be-cause of longer leaf wetness periods.Conidia are produced on infectedleaves in the canopy, twigs, recentlyfallen leaves and last season’s fruitthat are still on the tree. Spores be-come airborne when there is a suddenchange in the humidity or a rain event

greater than 0.1 inch(2.5mm). Leaf infection canoccur in as little as four tosix hours, but disease sever-ity increases with longerwetting periods. Optimumtemperatures for infectionare 73-80°F (23-27°C), butinfection can occur at anytemperature between 63-90°F (17-32°C).Copper and strobilurin

fungicides are available forABS control. A first appli-cation should be made whenthe spring flush is one-quar-ter to one-half expanded. Ina particularly bad year, an-other spray at full expansionmay be necessary. A petalfall spray will be needed and a stro-bilurin is recommended at this point,especially if there are also problems

with scab. If rains are fre-quent in April and May,sprays may be needed every10 days, but as few as once amonth is possible in a dryyear and on more tolerantcultivars. Two spray applica-tions are typically required inJune because of frequent rainand by mid-July, the fruit be-come resistant. In most cases,an attempt to reduce the num-ber of applications based ongrove disease history andweather forecasts is desired.The Alter-Rater, a diseaseforecasting model, has beendeveloped for this purposeand is available on the same

Web site as the copper model.It is important to mention that a

case of A. alternata resistance tostrobilurins has recently been docu-mented on citrus in Florida. Re-stricted use of strobilurin to the limitindicated on the label is importantand never use strobilurins more thanthree times per year or apply strobil-urins more than twice in a row. Onceresistance has developed, the onlyremaining control options for ABSare copper and ferbam.CITRUS SCAB

Citrus scab is a concern for freshmarket citrus only because it is un-

sightly. The scab affects Temples,Murcotts, tangelos, some tangerinehybrids and grapefruit. Conidia areproduced in one to two hours on thescab pustules that occur on leavesand fruit. Infection can occur in an-other three to four hours. Spores aresplashed relatively short distances byrain or overhead irrigation, usuallywithin a tree. Depending on the dis-ease severity of the previous season,two or three sprays may be neededfor scab. For adequate scab control inan affected block, a spray at petal falland another approximately threeweeks later are needed. With highdisease severity, an additional sprayat one-quarter spring flush expansionwill improve control. Copper is onlymoderately effective for citrus scaband is best used for the last spray.Strobilurins, ferbam or fenbucona-

zole are recommended for the springflush or the petal fall sprays, but not forboth because of resistance concerns.Further information on the control

and biology of all of the fungal foliardiseases is available on the CRECWebsite as well as in the 2009 Florida Cit-rus Pest Management Guide and EDIS(http://edis.ifas.ufl.edu/). The FloridaCitrus Pest Management Guide can bepurchased from your local county Ex-tension agent or is online for no chargeat http://www.crec.ifas.ufl.edu/extension/pest/index.htm.

Megan Dewdney (mmdewdney@ufl.edu)is assistant professor, University of Florida,Citrus Research and Education Center inLake Alfred.

Alternaria brown spot on Minneola

Citrus scab on Temples

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