Elsinoë spp.

Scientific Name Elsinoë australis Bitancourt & Jenkins [teleom.]
Sphaceloma australis Bitanc. & Jenkins [anam.]
and
Elsinoë fawcettii Bitancourt & Jenkins [teleom.]
Sphaceloma fawcettii Jenkins [anam.]
Synonyms Sphaceloma fawcettii var. viscosa Jenkins (syn. E. australis)
Sphaceloma citri Jenkins
Sphaceloma fawcettii var. fawcettii Jenkins
S. fawcettii var. scabiosa (McAlpine & Tryon)
Ramularia scabiosa
Sporotrichum citri Butler
Common Names English: Citrus scab, common scab of orange, sweet orange scab (E. australis), sour orange scab (E. fawcettii), scab of mango; French: Maladie des verrues des agrumes; German: Zitrusschorf; Spanish: Rona de los citricos / del mango
Description Elsinoë is a genus of the Ascomycota, class Dothideomycetes, order Myriangiales. The teleomorph of both species is only known from Brazil. Ascomata are pulvinate, globose, dark, pseudoparenchymatous, multi-locular, and up to 80 - 120 µm thick. Per locule, up to 20 subglobose or ovoid, bitunicate asci are formed; 12 - 16 µm diameters, with 8 hyaline, ellipsoidal or oblong-ellipsoidal ascospores. The two species are distinguished by the sizes of their ascospores, 5 - 6 x 10 - 12 µm diameters for E. fawcettii, 12 to 20 x 15 - 30 µm for E. australis.

The anamorphs are differentiated by molecular markers, host range, and tissues attacked. Both species form intra-epidermal or sub-epidermal pseudoparenchymatous acervuli from hyaline short-branched mycelium. Hyaline or pale-brown, 2 - 4 septated phialides produce hyaline, one-celled ellipsoid conidia (2 - 4 x 4 - 8 µm).
E. fawcettii produces also spindle-shaped conidia (2 -3 x 10 - 15 µm) on host tissue, E. australis does not. Conidia are capable of reproducing by budding.

In culture, beige to rose or brown colonies grow very slowly; they are well raised above the agar surface and covered by tufts of short, erect hyphae. Four pathotypes of E. fawcettii are differentiated according to the host range: Florida Broad Host Range, Florida Narrow Host Range, Tryon's and Lemon.

Biology

Damage

Initial lesions on young leaves are minute water-soaked spots which develop into light brown or variously bright-colored pustules, a mixture of fungal and host tissue. They grow, coalesce and extend to cover large parts of leaf blades, particularly on the lower surface. The central area of pustules becomes warty, is depressed and turns yellowish brown, grayish and velvety when the fungus sporulates. Old lesions have a rough surface and become cracked and fissured. Symptoms vary with host species and tissue age; scabs are raised on lemons and sour oranges as well as on young tissues, and tend to be flatter on grapefruit and sweet orange and on mature tissues.

Infections cause stunting and malformation of leaves; severe infections may result in defoliation. Warty lesions and corky protuberances are also produced on young twigs, tender shoots and stems. Fruits become infected only in early developmental stages. Raised lesions differing in shape, size and color depending on the species and variety affected are produced on the rind of developed fruits. Scab lesions do not extend into the mesocarp. Severely infected fruits are scarred, distorted and consequently unmarketable.

Scabs due to E. fawcettii are irregular, warty and deeply fissured, E. australis forms larger, smoother, more circular scabs. Scab may be confused with bacterial canker of citrus caused by Xanthomonas campestris pv. citri, melanose due to Diaporthe citri and wind scar.

Lifecycle

Conidia formed abundantly on wet scabs of leaves, twigs or fruits in a nearly saturated atmosphere at 20 - 28 °C, are the inoculum for new infections. Conidia germination and infection requires free water from dew, fog or rainfall; wet periods > 3 h are sufficient for conidial infection. Conidia germinate at temperatures from 13 - 32 °C, infection takes place between 14 - 25 °C. With temperatures of 20 - 21 °C, the incubation period is >5 days. Only young leaves, shoots and fruits are infected.
The fungus survives in scab pustules on fruits remaining on the tree and other plant organs. In resistant varieties, survival on diseased shoots from susceptible rootstocks is possible.

Cause

The pathogen is spread by water droplets (rain or irrigation), insect vectors are of minor importance. The fungus may be spread also by infected nursery stock, ornamental citrus plants, and fruits.

Occurrence

Additional Crop Information

Major host plants of E. fawcettii are sour oranges (Citrus aurantium), grapefruits (C. paradisi), lemons (C. limon), mandarins (C. reticulata), oranges (C. sinensis) and tangelos (C. paradisi x C. reticulata). Other members of Rutaceae include susceptible varieties extending the host range also to ornamental Citrus.

Agricultural Importance

The disease is widespread in wet subtropics and cooler tropics. When young plants of susceptible Citrus species or varieties are grown under favorable environmental conditions - warm and humid weather during fruit set -, the disease may become important also in other regions.Crop losses depend on weather conditions; the disease is not a problem in citrus-growing areas with a dry climate.

E. australis only causes fruit scab, mainly on oranges and mandarins. Lemons, satsumas (C. unshiu), limes (C. aurantifolia), kumquats (Fortunella spp.) and fruits of other Citrus spp. may be also attacked. Some varieties of C. medica, Fortunella spp., C. aurantiifolia and C. sinensis are highly resistant to E. fawcettii. Sour orange is attacked only by the Florida Broad Host Range pathotype also capable of infecting orange fruit. Grapefruit is affected by the Florida Broad and Narrow Host Range pathotypes but not by Tryon's or the Lemon pathotypes. Although E. fawcettii is more widespread, the economic impact of E. australis is larger as it attacks more widely grown citrus species.

Control

Integrated Crop Management

Citrus scab is controlled by using resistant varieties and/or fungicide applications in the nursery and in the orchard. Crop sanitation, proper growth of rootstocks and budwood in nurseries in dry areas or in greenhouses may contribute to produce certified citrus plantings free from disease.

Chemical Control

Protective fungicides (e.g. chlorothalonil, copper, ferbam, thiram) have been the standard for decades to control citrus scab and sweet orange scab.Later, systemic fungicides of the benzimidazole group (benomyl and carbendazim) have been used widely, but resistance development has significantly lowered their reliability.
Meanwhile, QoI fungicides (also known as strobilurins) such as trifloxystrobin have been registered in several countries against citrus scab and are presumably also effective against sweet orange scab.

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