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Fungal Diseases of Citrus Fruit and Foliage

Fungal Diseases of Citrus Fruit and Foliage. Megan Dewdney PLP 5115c. Foliar Fungal Diseases to be Covered. Alternaria Brown Spot (and leaf spot of rough lemon) Greasy spot Melanose Scab Diseases Postbloom Fruit Drop Black Spot Pseudocercospera Fruit and Leaf Spot. Note on Terminology.

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Fungal Diseases of Citrus Fruit and Foliage

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  1. Fungal Diseases of Citrus Fruit and Foliage Megan Dewdney PLP 5115c

  2. Foliar Fungal Diseases to be Covered • Alternaria Brown Spot (and leaf spot of rough lemon) • Greasy spot • Melanose • Scab Diseases • Postbloom Fruit Drop • Black Spot • PseudocercosperaFruit and Leaf Spot

  3. Note on Terminology • Many Ascomycete fungi have what is know as an anamorph and teleomorph • Anamorph: asexual or imperfect stage • Teleomorph: sexual or perfect stage • Have separate names • Fungi where no sexual stage has been identified are grouped into the ‘Fungi Imperfecti’ • Synonym:Deuteromycte • Some Basidiomycetes have also lost sexual stage • Ex. Rhizoctonia spp.

  4. Alternaria Brown Spot • Causal agent: Alternariaalternata • Synonyms Alternariacitri and A. alternatapv. citri • No known teleomorph • Important disease on tangerines and tangelos • 1903 First described in Australia on Emperor mandarin

  5. Alternaria Brown Spot cont. • 1974 First identified in Florida • Also found in South Africa, Turkey, Israel, Spain, Colombia and other countries • Host specific toxin • Isolates from tangerines and tangelos do not infect rough lemon except in rare circumstances • Disease on rough lemon same organism with different toxin • Considered separate pathotypes of A. alternata

  6. Alternariaalternata • No sexual stage known • Necrotrophic • Conidia are small, thick walled, pigmented and multicellular • The conidiophores are determinate and pigmented • Conidia are borne in chains • Hyphae penetrate host tissue directly; No appresorium

  7. Tissue Susceptibility • Highly susceptible cultivars • Dancy, Minneola, Orlando, Sunburst, Murcott, Nova and Lee • Leaves susceptible from formation to when fully expanded and hardened • Fruit are susceptible from petal fall to 5 cm (2 inch) in diameter

  8. Symptoms

  9. Alternaria Brown Spot Disease Cycle Caused by Alternariaalternata

  10. When are the Conidia Released? • Conidia released by rain events or sudden changes in relative humidity • In field trapping number of conidia in the air related to leaf wetness duration • Number of airborne conidia not related to infection severity Timmer et al., 1988

  11. Infection Conditions • Optimum temperatures 23-27°C • Can get infection between 17-32°C • Infection can occur with as little as 4-6 hours of leaf wetness but disease severity increases with leaf wetness Canihos et al., 1999

  12. Alternaria Brown Spot Disease Cycle Caused by Alternariaalternata

  13. Cultural Controls • Disease-free nursery trees • Careful choice of planting site • Air drainage important • Wider spacing • No vigorous rootstocks • No over-fertilization or over-watering • Hedge in late March • No overhead irrigation

  14. Fungicides • Copper –Works well for fruit but not leaves • Ferbam – Only moderately effective • Strobulurins – Most effective but specific MOA • Azoxystrobin • Trifloxystrobin • Pyraclostrobin • First spray when spring flush ¼-1/2 full expansion; high inoculum another before full expansion or at petal fall • Rest of the year maintain protective coating

  15. ALTER-RATER: A Forecasting System • Weather-based point system to better time fungicide applications • Points assigned based on: • Rainfall and leaf wetness • Average daily temperature • Thresholds vary by cultivar susceptibility • More information found at: http://edis.ifas.ufl.edu/ch183

  16. The ALTER-RATERSuggested Threshold Scores

  17. ALTER- RATER Daily Points

  18. Greasy Spot • Causal agent: Mycosphaerellacitri • Anamorph: Stenellacitri-grisea • Anamorph synonym Cercosporacitri-grisea • Other similar diseases described around world but caused by other Mycosphaerella spp. • Important disease on most types of citrus • 1915 First described in Florida and Cuba

  19. Greasy Spot cont. • Also occurs in Texas, the Caribbean, Central and South America, and parts of Asia • Primary 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

  20. Mycosphaerellacitri • Loculoascomycete • Pseudothecia up to 90 µm • Found in leaf litter • Ascospores fusiform and hyaline with one septum (2-3 x 6-12 µm)

  21. Stenellacitri-grisea • Conidia are pale olive brown, cylindrical with indistinct septae that can be in chains • Two types of conidiophores • Most common simple, smooth, dark and erect • Rare, in clusters (fasciculate) found in necrotic areas on leaves

  22. Mycelium • Epiphytic hyphae • Highly branched • Rough walls • Olive brown color when young but darken with age and the walls become smooth • Appressoria formed in stomatal chambers • Mycelia within leaf grow intercellularly and are not very branched

  23. Tissue Susceptibility • Highly susceptible cultivars • Grapefruit, Pineapple, Hamlin, and Tangelos • Less susceptible cultivars • Valencia, Temple, Murcott and most tangerines • Young and mature leaves susceptible to infection • Immature fruit susceptible

  24. Symptoms

  25. Greasy Spot Disease Cycle Caused by Mycosphaerellacitri

  26. Pseudothecia MaturationMoisture Mondal and Timmer, 2002

  27. Pseudothecia MaturationTemperature

  28. Optimal Temperatures for Ascospore Production • 28 °C = 82.4 °F Mondal and Timmer, 2002

  29. Peak Ascospore Ejection • The peak ascospore ejection period has shifted to earlier in season • Why? • Is this beneficial?

  30. Epiphytic Growth • Occurs during the wet summer months • Ascospore dose does not determine level of epiphytic growth • Similar patterns on fruit and leaves Mondal and Timmer, 2005

  31. Epiphytic Growth and Infection • Tissue penetrated only through stomata • High density of penetration required for symptoms • Requires high humidity • Symptoms caused by swelling stimulated by hyphae

  32. Greasy Spot Disease Cycle Caused by Mycosphaerellacitri

  33. Cultural Controls • Reduce leaf litter in winter and early spring • Disking • Frequent irrigation to promote decomposition • Mulch leaf litter • Put urea or lime on the leaf litter • Problem with this approach • Not enough of the leaf litter is decomposed

  34. Fungicides • Petroleum oil – gives adequate control on less susceptible cultivars • Copper – more consistent control than oil • Strobilurins – same concerns about MOA • Azoxystrobin • Trifloxystrobin • Pyraclostrobin • Fenbuconazole – moderate risk for resistance • DMI fungicide or sterol biosynthesis inhibitors

  35. Spray Timing • Less susceptible cultivars • One spray between May and June often sufficient especially in Northern production regions • In 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

  36. Spray Timing Effects

  37. Melanose • Causal agent: Diaporthecitri • Anamorph:Phomopsis citri • Disease is present in most citrus producing countries • Important only where fresh fruit is produced in humid areas • Causes lesions on fruit and leaves • All citrus susceptible but grapefruit and lemons are the most susceptible

  38. Diaporthecitri • Ascospores formed in perithecia • Spherical with flattened base (125-160 µm) • Long tapered beaks (200-800 µm) • Ascospores are hyaline • 2 cells each with 2 oil droplets (guttulae) • 3.2-4.5 x 11.5-14.2 µm

  39. Phomopsiscitri • Pycnidia are dark, ovoid and erumpent with thick walls • Found scattered on dead twigs • 200-450 µm • Spores are extruded in a tendril (cirrhus)

  40. Phomopsiscitri cont. • Two forms of hyaline conidia • α-conidia are unicellular • 2 oil droplets (biguttulate) • 2.5-4 x 5-9 µm • β-conidia • Filiform and hooked • Don’t germinate and are predominant form in older pycnidia • 0.7-1.5 x 20-30 µm

  41. Tissue Susceptibility • Spring flush usually not severely infected • On summer flush infection can lead to defoliation especially after dieback • Leaves become resistant once fully expanded • Fruit 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

  42. Symptoms

  43. Melanose Disease Cycle Caused by Diaporthecitri

  44. Pycnidia Production Mondal et al., 2004

  45. Pycnidia Production • Wetting period, twig diameter, temperature and disease severity on the twig all had significant effects on pycnidia formation • Formation takes between 3-5 months in field and can occur on dead twigs Mondal et al., 2004

  46. Most of the inoculum is produced on twigs that die between January and April • Conidia produced at low %RH are viable for several weeks to months Mondal et al., 2007

  47. Infection • Conidia germinate • 6 hrs at 16 °C • 4 hrs 20 to 28 °C • Literature has varying times and temperatures needed for infection • Optimum temp determined to be 24-28 °C Agostiniet al., 2003

  48. Melanose Disease Cycle Caused by Diaporthecitri

  49. Cultural Controls • Select younger groves for fresh fruit • Less dead wood for inoculum production • Remove dead wood from canopy • Clear out brush piles

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