Guignardia citricarpa

Scientific Name Guignardia citricarpa Kiely [teleom.]
Synonyms Phyllosticta citricarpa (McAlpine) Aa [anam.]
Phoma citricarpa McAlpine [anam.]
Phyllostictina citricarpa (McAlpine) Petr. [anam.]
Common Names English: Citrus black spot (CBS), hard spot, shot-hole, freckle spot, speckled blotch of citrus; German: Schwarzfleckenkrankheit; Spanish: Mancha negra de las frutas de cítricos; French: Maladie des taches noires
Description G. citricarpa belongs to the Ascomycota in the class Dothideomycetes.

Biology

Damage

G. citricarpa is a fruit disease; leaf lesions are uncommon on most citrus but may be more frequent on lemons. Fruit symptoms are classified to as hard or shot-hole spot, false melanose, freckle spot and virulent spot. The lesions of hard spot generally occur on mature fruit. They are crater-like with a gray centre, a dark-brown to black ring, and often have a green halo. Pycnidia are often apparent in these lesions. It is a preharvest symptom.

False melanose usually appears on green fruit without pycnidia formation on these lesions. Freckle spots are 1-3 mm in diameter with an orange to red color, and occur late in the season. They are an indicator of a heavy infection spreading irregularly over large areas of the mature fruit. Lesions of virulent spots are small and necrotic with light centre and a dark rim and may have a chlorotic halo.

Lifecycle

Ascospores are carried by wind throughout the canopy and long distances beyond. When ascospores are deposited on fruit or vegetative tissues under moist conditions, they germinate to form an appressorium. The cuticle and epidermis of leaves or fruits is penetrated with an infection peg. Quiescent infections on fruit develop to produce the typical black spot symptom after the fruit attains full size or becomes mature. Fruits are susceptible for at least 4-5 months after petal fall. Pycnidia are found in abundance on dead fallen leaves, and are also produced on fruits and peduncles. They are dark brown to black and 115-190 µm in diameter. Conidia are obovate to elliptical, hyaline, nonseptate, multiguttulate with a colourless appendage and are 5.5-7.0 µm wide by 8.0-10.5 µm long.

The anamorph probably plays only a minor role in the disease cycle. Conidia produced on the leaves and fruit in the canopy are capable of infecting the leaves and fruit. However, conidia produced on dead leaves can only reach susceptible fruit and leaves by splash dispersal into the canopy. Conidia produced on fruit can be washed down through the canopy and infect leaves and younger fruit that are still at the susceptible stage.

Cause

Ascospores from infected, fallen leaves are the major source of inoculum. Pseudothecial development in such leaves occurs from 40 to 180 days after leaf fall, depending on the frequency of wetting and drying. Perithecia are aggregated, globose, non-papillate, and about 100-175 µm in diameter. The ascus is cylindrical, clavate, and each contains eight spores being 4.5 x 6.5 µm wide by 12.5-16 µm long. A colorless appendage occurs at each end. Once ascospores are mature, rainfall or irrigation may trigger their release.

Occurrence

Additional Crop Information

Citrus species are the major host plants of G. citricarpa.

Agricultural Importance

CBS is endemic in South America, Asia, South Africa and Australia but not in North America and in the Mediterranean areas. Accordingly, fruits from affected areas represent a risk for introduction of this pathogen into new regions.
Black spot is one of the most important diseases of Citrus. It occurs in subtropical regions with summer rain fall. Most of the losses have been due to the external blemishes which make fruit unsuitable for the fresh market.

Control

Integrated Crop Management

The removal of off-season fruit may be useful to reduce conidial inoculum. Mechanical removal of leaf litter from the orchard floor reduces disease pressure and facilitates control. Spore trapping and rainfall and dew measurements have been helpful in determining the timing of ascospore release. Sour orange (Citrus aurantium) is one of the few species of citrus that is resistant to black spot. Some attempts have been made to produce tolerant hybrids using sour orange as a source of resistance.

Chemical Control

The most effective means to reduce post-harvest development of symptoms is through preventive application of fungicides during the fruit growing season.
For many years copper and mancozeb have been standard products for the control of citrus black spot and other citrus pathogens.
Benzimidazoles, showing satisfactory effectiveness in the beginning, are meanwhile mostly unreliable due to resistance development.
From the new fungicide groups especially the strobilurins represented for example by trifloxystrobin have gained importance due to their effectiveness and their broad spectrum of activity.

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