Citrus disease Huang Jiang Hua 31 March, 2008

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.

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.

 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( 无花果 ), mangos, olive, avocado, and many other plants.

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).

 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.

 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( 花萼 ) (button) surrounded by distorted leaves.

 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( 苯菌灵 ) or captafol help reduce fruit drop.

 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 mm diameter that become sunken, wrinkled and dark, with concentric rings of fungal fruiting bodies.

 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.

 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( 叶柄 ), 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.

 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.

 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( 开花 ) 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.

 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.

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

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

 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