Fungal Diseases of Citrus Fruit and Foliage Megan Dewdney PLP 5115c Foliar Fungal Diseases to be Covered oGreasy spot/Rind blotch oMelanose oCitrus Black Spot oPostbloom Fruit Drop oAlternaria Brown Spot (and leaf spot of rough lemon) oScab Diseases oPseudocercospera Fruit and Leaf Spot Greasy Spot oCausal agent: Zasmidium citri-griseum Synonyms: Mycosphaerella citri; Stenella citri-grisea; Cercospora citri-grisea oOther similar diseases described around world but caused by other Mycosphaerellaceae Amycosphaerella africana in Africa and Europe Mycosphaerella horii in Japan M. lageniformis in California oImportant disease on most types of citrus o1915 First described in Florida and Cuba Greasy Spot cont. oAlso occurs in Texas, the Caribbean, Central and South America, and parts of Asia oPrimary effect is to cause defoliation which can lead to decreases of yield and fruit size Up to 25% on sweet orange in Florida Up to 45% on grapefruit Zasmidium citri-griseum – sexual stage oLoculoascomycete Pseudothecia up to 90 µm Found in leaf litter Ascospores fusiform and hyaline with one septum (2-3 x 6-12 µm) Zasmidium citri-griseum – asexual stage oConidia are pale olive brown, cylindrical with indistinct septae that can be in chains oTwo types of conidiophores Most common simple, smooth, dark and erect Rare, in clusters (fasciculate) found in necrotic areas on leaves Mycelium oEpiphytic hyphae Highly branched Rough walls Olive brown color when young but darken with age and the walls become smooth oApressoria formed in stomatal chambers oMycelia within leaf grow intercellularly and are not very branched Tissue Susceptibility oHighly susceptible cultivars Grapefruit, Pineapple, Hamlin, and Tangelos oLess susceptible cultivars Valencia, Temple, Murcott, and most tangerines oYoung and mature leaves susceptible to infection oImmature fruit susceptible Symptoms Greasy Spot Disease Cycle Caused by Zasmidium citri-griseum Pseudothecia Maturation - Moisture Mondal and Timmer, 2002 Pseudothecia Maturation - Temperature Mondal and Timmer, 2002 Optimal Temperatures for Ascospore Production o20 °C = 68.0 °F o24 °C = 75.2 °F o28 °C = 82.4 °F o32 °C = 89.6 °F Mondal and Timmer, 2002 Peak Ascospore Ejection The peak ascospore ejection 35 o 2000-01 30 2001-02 period has shifted to earlier in 1969 season 25 20 Why? 15 10 Is this beneficial? Percent ascospores of total 5 0 A M J J A S O N D J F M Months Epiphytic Growth oOccurs during the wet summer months oAscospore dose does not determine level of epiphytic growth oSimilar patterns on fruit and leaves Mondal and Timmer, 2005 Epiphytic Growth and Infection oTissue penetrated only through stomata oHigh density of penetration required for symptoms oRequires high humidity oSymptoms caused by swelling stimulated by hyphae Cultural Controls oReduce leaf litter in winter and early spring Disking Frequent irrigation to promote decomposition Mulch leaf litter Put urea or lime on the leaf litter oProblem with this approach Not enough of the leaf litter is decomposed Spray Timing oLess susceptible cultivars One spray between May and June often sufficient especially in Northern production regions oIn South Florida, more susceptible cultivars and in groves with severe defoliation Two sprays; one mid-May – June, the second once flush has expanded A third and final spray may be needed for fresh grapefruit in a grove that was heavily infested the previous year Spray Timing Effects 5 Unsprayed control May spray 4 July spray August spray 3 May+June spray 2 1 Epiphytic mycelial growth growth mycelial Epiphytic 0 May Jun Jul Aug Sep Oct Nov Dec Jan Melanose oCausal agent: Diaporthe citri Synonym: Phomopsis citri oDisease is present in most citrus producing countries oImportant only where fresh fruit is produced in humid areas oCauses lesions on fruit and leaves oAll citrus susceptible but grapefruit and lemons are the most susceptible Diaporthe citri – sexual stage oAscospores formed in perithecia Spherical with flattened base (125-160 µm) Long tapered beaks (200-800 µm) oAscospores are hyaline 2 cells each with 2 oil droplets (guttulae) 3.2-4.5 x 11.5-14.2 µm Diaporthe citri – asexual stage oPycnidia are dark, ovoid and erumpent with thick walls Found scattered on dead twigs 200-450 µm Spores are extruded in a tendril (cirrhus) Diaporthe citri – asexual stage cont. oTwo forms of hyaline conidia o α-conidia are unicellular 2 oil droplets (biguttulate) 2.5 -4 x 5-9 µm o β -conidia Filiform and hooked Don’t germinate and are predominant form in older pycnidia Likely a spermatial spore for mating 0.7-1.5 x 20-30 µm Tissue Susceptibility oSpring flush usually not severely infected oOn summer flush infection can lead to defoliation especially after dieback oLeaves become resistant once fully expanded oFruit resistant 12 weeks after petal fall and when infection occurs later during the 12 weeks, lesions are smaller Grapefruit are susceptible until 7-10 cm in diameter Symptoms Melanose Disease Cycle Caused by Diaporthe citri Pycnidia Production Mondal et al., 2004 Pycnidia Production oWetting period, twig diameter, temperature and disease severity on the twig all had significant effects on pycnidia formation oFormation takes between 3-5 months in field and can occur on dead twigs Mondal et al., 2004 Conidia Production oMost of the inoculum is produced on twigs that die between January and April oConidia produced at low %RH are viable for several weeks to months Mondal et al., 2007 Infection oConidia germinate 6 hrs at 16°C 4 hrs 20 to 28°C oLiterature has varying times and temperatures needed for infection oOptimum temp determined to be 24-28°C Agostini et al., 2003 Cultural Controls oSelect younger groves for fresh fruit Less dead wood for inoculum production oRemove dead wood from canopy oClear out brush piles Spray Timing oOranges and Tangerines First spray mid to late April One to two applications sufficient oGrapefruit (fresh market) First application when fruit ¼ to ½ inch Copper to be applied every 3 weeks until fruit resistant in late June to early July There is a model to determine whether copper residues are sufficient to control disease based on weathering of copper and the growth rate of fruit Black Spot oCausal agent: Phyllosticta citricarpa Synonyms: Guignardia citricarpa; Phoma citricarpa oHosts : Citrus species and hybrids oSweet oranges, mandarins and tangerines, lemons, grapefruit o‘Tahiti’ lime and some sour orange cultivars have non- symptomatic infection Other Phyllosticta spp. and citrus oPathogenic species P. citriasiana – Described in 2009 from pomelo (tan spot) P. citrichinaensis – Described in 2011 from pomelo, lemon, mandarin, and sweet orange with minor symptoms P. paracitricarpa – Described in 2017 in Europe only on lemon leaf litter oEndophytic species P. capitalensis - Ubiquitous P. citribraziliensis - Described in 2011 from citrus leaves in Brazil P. paracapitalensis – Described in 2017 in Europe on Citrus spp. Glienke et al., 2011; Guarnaccia et al., 2017; Wang et al., 2011; Wulandari e al., 2009 Black Spot cont. oRind spots cause the most economic damage Internal quality unaffected oReduces fruit value for the fresh market oRestricts export of fresh fruits Mostly to European countries and formerly the U.S oCauses premature fruit drop reducing yield Particularly with late harvest cultivars World Distribution o Occurs mostly in summer rainfall areas or areas with prolonged dew or fog in warm weather Current Quarantine Areas oThree Counties with Disease Collier Hendry Polk Lee Charlotte Phyllosticta citricarpa – sexual stage oNever found in fruit – in leaf litter oForm aggregated ascomata - peritheciod pseudothecium 100-175 µm diameter oAscopores are aseptate, hyaline, multiguttulate and cylindrical with swollen middles 4.5 -6.5 x 12.5-16 µm Tran et al., 2017 Baayen et al. 2002 Phyllosticta citricarpa – asexual stage oForms pycnidia Dark brown or black Form on fruit, leaves, twigs and fruit pedicles 115-190 µm oConidia are obovate, hyaline, aseptate and multiguttalate 5.5-7 x 8-10.5 µm oSpermatia are dumbbell shaped 51 -8 x 0.5- Glienke et al. 2011 Tissue Susceptibility oHosts include Citrus species and hybrids oSymptomatic hosts: Sweet oranges, mandarins and tangerines, lemons, grapefruit oNon-symptomatic host: ‘Tahiti’ lime Produces ascospores from leaves oFruit are susceptible for 5-6 months post-petal fall Leaf susceptibility period still uncertain Symptoms Black Spot Disease Cycle Caused by Phyllosticta citricarpa Inoculum Basics oMajor source of inoculum: decomposing infected leaves on orchard floor (ascospores) oAdditional source of inoculum: lesions on infected fruits, leaves and branches (conidia) oMeans of spread: Wind (ascospores); Water splash (ascospores and conidia) Infection Conditions oOptimal conditions for infection: Temp 21 – 32 °C Wetting period 24 - 48h oSymptom expression: 1 – 12 months Very long latent period oSurvival of the fungus: leaves, twigs, fruits, peduncles, and leaf litter What is Needed for Ascospores oRequirement for two opposite mating types to be present to get ascospores oOnly one mating type found in Florida MAT1-2 Wang et al. 2016 The Lonely Peninsula oWanted to know the proportion of mating types in Florida compared to Queensland, Au Number of isolates State, Total Location Hetero Country number Alpha HMG -karyon Collier 50 0 50 0 Florida, Hendry 62 0 62 0 US Polk 1 0 1 0 Total 113 0 113 0 Beerwah 2 0 2 0 Queens Emerald 1 1 0 0 -land Gayndah 4 1 3 0 Australia Mundubbera 16 8 5 3 Tiaro 3 2 1 0 Total 26 12 11 3 Wang et al. 2016 Still Only One Mating Type oGlobal population study found 2 mating types in all locations except Florida oGreatest genetic diversity from Australia and China oSouth African and Brazil had low genetic diversity Likely founder effect o Florida population was clonal oCarstens et al. 2017 Role of P. citricarpa Spermatia oSpermatia are commonly found in culture and leaf litter samples Role in life cycle hypothesized to be male gametes oFirst ascospores produced in culture Demonstrated meiosis via multilocus genotyping of ascospores Needed direct contact of isolates carrying opposite idomorphs or a spermatia suspension oTran et al.