Diaporthe citri

Scientific Name Diaporthe citri F. A. Wolf [teleomorph], Phomopsis citri H. Fawcett non (Sacc.) Traverso & Spessa [anamorph]
Synonyms Diaporthe medusaea Nitschke [syn.]
Phomopsis caribaea W. T. Horne [obs.]
Phomopsis californica H. S. Fawcett [obs.]
Phomopsis cytosporella Penz. & Sacc. [obs.]
Common Names English: Melanose of Citrus, stem-end rot, Phomopsis stem-end rot, gummosis, Mango dieback; French: Pourriture phomopsienne des agrumes; German: Zitrus-Gummose und -Fruchtfäule; Spanish: Melanosis de los citricos
Description The ascomycete Diaporthe citri belongs to the class Sordariomycetes, order Diaporthales. Ascomata are ostiolate perithecia, embedded singly or in small groups in a stroma composed of host (bark and partly wood of dead branches) and fungal tissues. They are globose (300 - 800 µm diameter) with ostiolar necks erumpent through outer host tissues. Unitunicate asci produce hyaline, 2-celled ellipsoid to fusoid ascospores (11 - 16 x 3 - 5 µm).

The Phomopsis anamorph (form order Sphaeropsidales) produces 1-celled hyaline conidia in dark ostiolate, nearly globose pycnidia (up to 600 µm diameter), which are scattered or clustered, immersed, later erumpent. Simple phialids produce two types of conidia, fusiform to ellipsoidal (6 - 10 x 2 - 3 µm, alpha), and filiform, curved or bent (20 - 30 x 0.5 - 1 µm) beta conidia.

Biology

Damage

Melanose occurs on nearly mature fruits and consists of small reddish-brown to dark-brown specks (0.2 - l.5 mm) around the oil glands. These specks are hardly visible, but in aggregation appear as unsightly blemishes. Infections induce the formation of wound scab tissue and cracking; fruits are stunted and fall prematurely. With 85 - 90 % relative humidity, the spots are smooth, slightly sunken and typically dark-brown.
On grapefruit pustules are larger and more raised than on oranges and tangerines. The minute spots on fruits can be distinguished from mite damage as they feel like sandpaper when touched. On mature fruits D. citri causes fruit rot which typically develops from the stem-end or button. The rind of the point of peduncle insertion becomes dark brown and tougher without losing its flexibility. Peduncle rotting may cause fruit fall.

Stem-end rot is usually seen in storage. Growth is slower than that of Botryosphaeria rhodina, the other cause of stem-end rot. The fungus also affects immature leaves, young branches, stalks and stems. Under conditions of high moisture, on both leaf surfaces small dark and sunken spots with chlorotic haloes are produced, which develop into raised, corky, superficial, necrotic areas, > 1 mm diameter. Leaves lose the green color, become distorted and fall prematurely.

Lifecycle

The role of D. citri ascospores in epidemiology is uncertain. The pathogen survives as mycelium and pycnidia on the bark of weakened or dead branches and stems. With rain and temperatures of 20 - 24 °C, pycnidia exude alpha- and beta-conidia in white to yellowish cirrhi. Mature conidia in these tendrils can endure desiccation and are splash-dispersed to leaves, branchlets and fruits. After 10 - 12 h under wet conditions, the alpha-conidia produce germ tubes which rapidly infect the fruit. Leaves may be infected within 2 - 3 weeks after emergence, fruits from petal-fall until they reach a diameter of 2 - 3 cm.

Cause

In seasons with rainy picking periods D. citri causes fruit rot. Stem-end rot extensively develops in areas where the rainy season concurs with picking. Moisture favors fungal infections through the abscission zone of the button and D. citri rapidly colonizes the button tissues. Stem-end rot occurs on the tree when fruits are overripe or weakened. In commercial fruit the disease generally develops after harvest. The fruit rind is colonized by the fungus growing from contaminated buttons. Fruits rot 7 - 10 days after harvest and may cause extensive spread in oranges.

Occurrence

Additional Crop Information

Species of Citrus aurantiifolia (lime), C. aurantium (sour orange), C. limon (lemon), C. maxima (pummelo), C. medica (citron), C. reticulata (mandarin), C. sinensis (orange), and Citrus x paradisi (grapefruit) are commonly attacked.

Agricultural Importance

Melanose, caused by Diaporthe citri, is especially a thread in those years or regions where long wetting of young fruit coincidences with relatively high temperatures. It is important where inoculum is abundant and rainfall occurs during early fruit development. This disease causes severe damage to leaves, twigs and young fruits of citrus under conditions of prolonged high moisture. Under Mediterranean conditions, the dry climate prevailing during the fruit growth and maturation seasons makes infection by this fungus rare and D. citri is a weak pathogen.

Control

Integrated Crop Management

Methods include careful pruning and sanitation of young trees that have suffered from freeze injury. Centres of infection are removed, dried branches cut out and dead wood and rotten fruits disposed of.

Chemical Control

Fungicide sprays, especially with copper compounds have been commonly used in citrus production for many decades. Timing and number of fungicide applications depend on the local climate.
Pre-harvest sprays have partially proven also effective against storage disease and are reported to be more effective than post-harvest dipping in controlling storage rots caused by D. citri and other pathogens.
Beside copper fungicides which continue to be a standard tool in melanose control, more recently the strobilurin fungicide trifloxystrobin has become available in some countries for improved melanose control.

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