Elsinoe fawcettii Bitanc. & Jenkins 1936

On most host species and cultivars, Elsinoё fawcettii appears as warty pustules that are raised on citrus leaves and fruit. The lesions typically form along the main veins and change appearance as they age. Leaf lesions are initially semi-translucent dots that become papillate (small nipple-like projections) with a cream to a pale yellow color. Overtime, they can increase up to 3 mm in diameter and develop a scab-like appearance. Older scab lesions are typically cinnamon to honey in color, warty, deeply cracked and will split as they age. Fruit and young shoots can develop similar symptoms, but symptoms on young shoots are not observed as often (3).The stromatic portion of the pustules are the site of the fungal spore production (5,7).

E. fawcettii is slow-growing on PDA (potato dextrose agar), but it forms characteristic, raised colonies from single spores or small mycelia fragments after about 4-6 days (8). Unfortunately, they can be easily overgrown by faster-growing organisms before a pure culture can be obtained. This problem can typically be avoided by culturing the organism on a selective media that consists of PDA, dodine, streptomycin sulfate, and tetracycline hydrochloride (7). On PDA, the colors of the colonies vary among isolates ranging from pale ochraceous (ochre colored) to dark vinaceous (red colored) in the center (8) (Image 1).

E. fawcettii’s ascospores (5–6 mm X 10–12 mm) are produced directly on the wet scab pustules that occur on the leaves and fruit. It only takes one to two hours of a high moisture environment to induce spore production. The spores are dispersed via water splash to healthy tissue. Only three to four additional hours of high moisture conditions are needed to cause an infection in healthy tissue (5). The ascocarps (fruiting bodies) are scattered, cushion-shaped (pulvinate), circular to elliptical, dark brown and up to 120 μm wide. They are composed of pseudoparenchymatous tissue containing several locules with asci (microscopic sacs that contain the sexual spores). The asci are subglobose (not quite round) to elliptical and thick-walled at the top. They contain 8 spores and are about 12–16 μm in size (3).

E. fawcettii produces single-celled, hyaline, elliptical conidia (asexual spores) that are about 3–4 mm X 4–8 mm in size and they are typically propagated through budding (4). Conidia are formed directly from the upper cells of the pseudoparenchyma or from 0-2 septet conidiophores (structures that bear the conidia). The conidiophores are monophialidic to polyphialdic, terminal, integrated, hyaline to pale brown, and are typically 12–22 × 3–4 μm (3). It has also been known to form intrahyphal hyphae, also known as endohyphae, which is phenomenon where hyphae form within hyphae. Endohyphae serve as a protective, physical barrier to harsh environments, parasitic attacks, and antimicrobial compounds (13).

E. fawcettii and E. australis cause similar scab diseases on citrus, but they can be distinguished from one another in a number of ways. E. fawcettii produces smaller ascospores compared with those that are produced by E. australis (5–6 mm X 10–12 mm vs. 12–20 mm X 15–30 mm respectively). Only E. fawcettii produces dark pigmented and spindle-shaped conidia on scab lesions (4) (image 2). Although it has been documented that they can be distinguished by their spore shape and size, Timmer suggests that they can be more reliably be differentiated by pathogenicity (8). The host range and symptoms are considered as the best distinguishing factors between E. fawcettii and E. australis. Only E. fawcettii can cause raised scab lesions on leaves which appear as the papillate pustules described earlier. Additionally, E. fawcettii is not typically found on sweet oranges (Citrus sinesis) (6). They can also be differentiated from one another by restriction analysis of amplified ITS DNA with HaeIII, CfoI, and TaqI. The results of the sequence analysis determined that the ITS region of E. australis is 28 bp shorter than the ITS region of E. fawcettii (534 bp) (11).

Elsinoё fawcettii is a pathogen of citrus that causes scab-like symptoms on fruit and leaves. It obtains its energy from the host plant by growing inter- and intracellularly within citrus host cells. Once inside the host cells, the fungus reproduces rapidly and it can initiate new infections if the environmental conditions are favorable (4). As mentioned in the morphology section, the ascospores are produced directly on wet scab pustules and it only takes a few hours of a moist environment to induce spore production. Therefore, water is an essential component of E. fawcettii’s life cycle in terms of spore production and spore dispersal (5). Timmer states that “the optimum temperatures for spore formation, germination, and infection are 75-82°F (24-28°C) (17).” Although, it has been documented that infection can still occur at temperatures below 75°F if the wet or high moisture periods are long (17). Common scab overwinters on the tree canopy in the form of conidia and its full life cycle can also be seen in image 2 (12). It is known to infect various citrus species which are either classified as a major or minor host. The major hosts include: Citrus aurantium (sour orange), Citrus jambhiri (rough lemon), Citrus limon (lemon), Citrus sinesis (navel orange), and Citrus x paradisi (grapefruit). The minor host include: Citrus hystrix (Kaffir lime), Citrus limonia (mandarin lime), Citrus madurensis (calamondin), Citrus noblis (tangor), Citrus unshiu(Satsuma), and Poncirus trifoliata (Trifoliate orange) (10).

E. fawcettii is most problematic in wet subtropical and cooler tropical regions. It does not appear to thrive in areas with a limited annual rainfall of less than 1300mm (51.1811 inches), long-lasting hot seasons, or dry summers (10). It is widespread in many citrus growing areas where suitable conditions occur (3). It has been confirmed as being present in Russia, the Canary Islands, various countries in Asia and India, Indonesia, Japan, North and South Korea, Africa, Central America and the Caribbean, North America, South America, and Oceania region (10).

First discovered in Brazil in 1935, Elsinoё fawcettii is a pathogen that causes scab lesions on citrus fruit, leaves, and twigs. Published in 1936, Bitancourt and Jenkins described it as being the perfect stage of its anamorph Sphaceloma fawcettii var. fawcettii (1). It is also known by the following common names: Common Citrus Scab, Sour Orange Scab, Citrus Scab, and many others in various languages. E. fawcettii affects many different varieties of citrus (2). It has a distinctive morphology from other species in its genus and it is economically significant in any wet, tropical or subtropical climates where citrus is grown (4).

As described earlier, fawcettii causes scab lesions on citrus fruit. Common citrus scab rarely decreases the crop yield when plants are treated with commercial fungicides. Copper fungicides, difenoconazole, and dithianon are three fungicides that have been tested for control of common citrus scab. Although copper fungicides were more commonly used at the time of the study, difenoconazole controlled scab more effectively than the other two tested fungicides (18). If E. fawcettii is left untreated, then it can reduce the value of the fresh market fruit by as much as 50% (4). Infected fruit will not be sold for the fresh market because consumers will not want to purchase unsightly, diseased fruit. Since the pathogen does not cause disease in humans, the infected fruit are typically sold for juice production. If the fruit is sold for juicing, then the growers are typically paid 50% less than what they would receive if the fruit were sold whole. Therefore, E. fawcettii is economically relevant for citrus production in areas that are known to have conditions that are conducive for disease spread and that have susceptible cultivars (9). However, E. fawcettii does not appear to be culturally relevant to human history. So far it has not been used by humans in terms of antimicrobials or as a food source.

E. fawcettii does produce a specialized phytotoxin that is critical to its pathogenicity and is a characteristic that is shared by other Elsinoё sp. As described previously, E. fawcettii typically produces red or brown pigments in culture. These pigments have been identified as a peryleneequinone-containing phytotoxin called elsinochrome. Elsinochrome is nonhost-selective, but its production is required for E. fawcettii to have full fungal virulence and to initiate lesion formation. The elsinochrome toxin operates by reacting with oxygen after absorbing light energy and then generating ROS (a toxic reactive oxygen species) to kill host cells. It may also provide E. fawcettii access to nutrients and allow the fungus to live within host tissues (4, 16).

Elsinoё fawcettii was first discovered in 1935 on the ripe rind of a Satsuma orange, Citrus nobilis unshiu Sw., in M’Boy, São Paulo, Brazil by A.O. Martins. The specimen was sent to Bitancourt and Jenkins who were the first to describe the disease and published their findings in 1936 (1). It is associated with its anamorph (asexual stage) Sphaceloma fawcettii var. fawcettii. It is also known by the following common names in various languages: Citrus scab, common citrus scab, sour orange scab, Gale commune des agrumes, Zitrusschorf, Verrugose dor citros, and Costra o roña de los ágrios. This fungus belongs to the phylum Ascomycota, the Class Dothideomycetes, the order Myriangiales, the Family Elsinoacea, and the genus Elsinoё(2). E. fawcettii has not yet been included in a phylogenetic study on a genus or species level. However, the class Dothideomycetes and the order Myriangiales have been included in a few phylogenetic studies (14, 15).

E. fawcettii is in the class Dothideomycetes. The Dothideomycetes are considered the most phylogenetically diverse class within the Ascomycetes. This class contains a heterogeneous (diverse) group of fungi that can exist in almost every fungal niche around the world (15). They are most typically found as pathogens, endophytes, or epiphytes of living plants. Additionally, they have also been found as saprobes in dead or partially degraded plant matter in leaf litter or in dung degrading cellulose and other complex carbohydrates. But their nutritional modes are not just restricted to associations with plants. Several species are described as being lichenized, but others in the class occur as parasites on fungi or on animals (14).

The order Myriangiales has also been included in these studies; it is reported to be most closely related to the order Dothideales. In these studies, the node that supports the Dothideales, Capnodiales, Myriangiales, and Mycosphaerellaceae is heavily supported. The order Myriangiales includes organisms that are saprobes, epiphytes (including some rock inhabiting fungi), and biotrophs (14, 15). The anamorphs in this order, like the anamorph of Elsinoё sp., are typically acervular coelomycetes which are asexual fungi that form conidia in pycnidia (cavity) or a mat-like cushion of hypae (acervuli) (12, 14).

Due to the lack of information regarding the phyologenetics of this Elsinoё species, it is unclear which species E. fawcettii is most closely related to. Regardless, it is important to note when diagnosing citrus fruit with scab lesions that Elsinoё australis also causes a scab disease of citrus. The two are continually compared to one another in most of the references listed. Like E. fawcettii, E. australis causes a scab disease of citrus known as Sweet Orange Scab. They do vary in pathogenicity, morphology, and host range. The morphology section contains the characteristics that can be used to distinguish E. fawcettii from E. australis and other species.

Elsinoë fawcettii is a species of fungus in the Elsinoaceae family. It is a plant pathogen that causes citrus scab.