Citrus disease

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Citrus disease

• Huang Jiang Hua
• 31 March, 2008

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Anthracnose diseases

• Anthracnoses are diseases of the foliage, stems,
  or fruits that typically appear as dark-colored
  spots or sunken lesions with a slightly raised
  rim.
• Some cause twig or branch dieback.
• In fruit infections, anthracnoses often have a
  prolonged latent stage.
• In some fruit crops, the spots are raised and
  have corky surfaces.
• Anthracnose diseases of fruit often result in
  fruit drop and fruit rot.

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Anthracnose diseases

• Anthracnose are caused by fungi that produce
  conidia within black acervuli. Four ascomycetous
  fungi, Diplocarpon, Elsinoe, Glomerella, and
  Gnominia, are responsible for most anthracnose
  diseases.
• They are found in nature mostly in their conidial
  stage and can overwinter as mycelium or conidia.

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• Colletotrichum (Gloeosporium), causing
  anthracnose of cereals and grasses (C.
  lagenarium), anthracnose or fruit rot of eggplant
  and of tomato (C. phomoides), anthracnose of
  strawberry (C. acutatum), red rot of sugarcane
  (C. falcatum), onion smudge (C. circinans), and
  anthracnose of citrus, fig(???), mangos, olive,
  avocado, and many other plants.

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Colletotrichum (Gloeosporium) diseases

• Colletotrichum diseases are the most common
  anthracnoses and are very similar, if not
  identical, to the diseases caused by Glomerella.
• The latter is probably the sexual stage of most
  or all species of Colletotrichum (Gloeosporium).

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• Anthracnose diseases, particularly those caused
  by Colletotrichum (Gloeosporium) or Glomerella
  fungi, are very common and destructive on
  numerous crop and ornamental plants.
• Although severe everywhere, anthracnose diseases
  cause their most significant losses in the
  tropics and subtropics.

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• In citrus, Colletotrichum infections cause the
  Key lime anthracnose and postharvest anthracnose
  of some susceptible tangerine cultivars.
• A more serious disease, however, called citrus
  postbloom fruit drop, is caused by a slow-growing
  strain of Colletotrichum gloeosporioides.
• This fungus infects citrus flowers.
• It produces orange to peach-colored spots on the
  petals or affects entire flower clusters.
• Such infections induce newly formed fruitlets to
  drop, leaving behind a persistent calyx(??)
  (button) surrounded by distorted leaves.

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• Postbloom fruit drop affects most citrus species
  in Florida, the Caribbean, and Central America.
• In moist weather, abundant conidia are produced
  in acervuli on diseased petals, and these are
  splashed to healthy flowers by rain.
• In prolonged damp or rainy weather, over 90
  percent of the blossoms may be destroyed by
  Colletotrichum within a few days.
• Control has been difficult in wet weather.
• Sprays with benomyl(???) or captafol help reduce
  fruit drop.

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• On cucurbits, leaf spots are often large, about
  10 mm in size and pale-brown to gray in color,
  with distinct margins.
• The lesions on fruit appear as brownish
  discolorations, often 20-30 mm diameter that
  become sunken, wrinkled and dark, with concentric
  rings of fungal fruiting bodies.

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• Infected ripe tomato fruit has small,
  watersoaked, sunken, circular spots that may
  increase in size up to 1.2 cm in diameter.
• As it ages, the center of an older spot becomes
  blackish and emits gelatinous pink spore masses.
  The anthracnose lesions easily attract other
  rotting organisms to completely rot the infected
  fruit.
• Infection may also occur on unripe fruits, stems,
  leaves, and roots.
• Infected unripe fruits do not show symptoms until
  ripening.
• Infected roots, called dot root rot, only become
  evident when the fruits begin to ripen.
• The root lesions become brown and are dotted with
  fruiting bodies.
• The outer layer of the infected roots is
  completely rotten.

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• Anthracnose on beans appears on leaves at all the
  growth stages of a plant but often appears in the
  early reproductive stages on stems, petioles(??),
  and pods.
• It generally appears first as small and irregular
  yellow, brown, dark-brown, or black spots.
• The spots can expand and merge to cover the whole
  affected area.
• The color of the infected part darkens as it
  ages.
• The diseases can also produce cankers on petioles
  and on stems, causing severe defoliation.

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• On mango green fruit, tiny brown spots develop
  that will only enlarge after harvest.
• The spots enlarge on a ripening fruit and found
  anywhere on the peel in tear-shaped patterns.
• Eventually, the whole fruit rots and fungal
  fruiting bodies are formed on the rotten surfaces.

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• The infected leaves and fruits of pepper have
  small or large lesions, or purplish or brown
  patches without the formation of definite
  lesions.
• The stems and petioles have girdles and the
  inflorescences(??) turn yellow, causing dieback
  and shrivelling.
• The fruit usually develops lesions during the
  ripening process, but lesions may develop on
  fruit of any size and possibly on the foliage and
  stems at later stages of infection.

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• As the fruit ripens, its susceptibility to
  infection increases. The lesions on the fruit are
  circular and may reach 3 cm in diameter or bigger
  on the larger fruit. The concentric rings at the
  center of the lesion may be tan or orange to
  black. Initial infections are undefined tanned
  colored lesions that may appear in a matter of a
  few days after infection.

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Conditions that favor development

• Infected seeds and infected plant debris left in
  the field after harvest
• Moist and warm weather during the reproductive
  stages. Plants are most susceptible during the
  flowering stage from bloom to post harvest
• Badly drained soil
• Wet periods of about 12 hours or more favors the
  occurrence of infection
• Nutritionally stressed or unhealthy crops

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Prevention and control

• Proper seed and planting materials selection. Sow
  only disease-free seeds
• Proper field sanitation
• Seed treatment
• Transplant only healthy seedlings
• Remove and destroy infected parts but avoid
  touching other plant parts, especially when these
  are wet
• Harvest unripe but mature fruits
• Plow under all the plant debris after harvest

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• Practice crop rotation. Take notes of plants that
  are susceptible to anthracnose disease and rotate
  these with those that are resistant
• Keep area free of weeds
• Baking soda spray
• Bordeaux mix
• Sulfur spray