Fusicladium effusum
(pecan scab)

Pictures

Picture	Title	Caption	Copyright
Title Pecan scab Caption Pecan scab on pecan nuts of cv. Wichita.|Pecan scab on 'Wichita' cultivar pecan nuts. Copyright Charles J. Graham	Pecan scab	Pecan scab on pecan nuts of cv. Wichita.|Pecan scab on 'Wichita' cultivar pecan nuts.	Charles J. Graham

Identity

Preferred Scientific Name

• Fusicladium effusum G. Winter

Preferred Common Name

• pecan scab

Other Scientific Names

• Cladosporium caryigenum (Ellis & Langl.) Gottwald
• Cladosporium caryigenum var. caryigenum
• Cladosporium effusum (G. Winter) Demaree
• Fusicladium caryigenum Ellis & Langl.
• Fusicladosporium effusum (G. Winter) Partridge & Morgan-Jones

International Common Names

• Spanish: rona de la pacana
• French: tavelure du pacanier
• Portuguese: sarna da nogueira pecã

Local Common Names

• Germany: Schorf: Pecannuss

EPPO code

• CERCEF (Cercospora effusa)
• CLADCA (Fusicladium effusum)

Summary of Invasiveness

F. effusum is a fungal pathogen that causes pecan scab, which can result in severe economic losses on susceptible cultivars and resulting harm to the pecan industry in areas with high rainfall where pecan is grown. The disease develops on leaves, fruits and shoots and results in loss of photosynthetic area and reduced fruit size and quality. Pecan scab can also lead to reduced fruit set in the following year due to plant stress. Fungicides used to control pecan scab are costly. It is introduced to new areas through movement of infected host material. Despite quarantine restrictions, it is likely that human-mediated transfer has occurred between the native habitats in south-eastern USA and Mexico, and locations where pecan is grown as an exotic in South America, South Africa and New Zealand. F. effusum overwinters as stroma and conidia in lesions on shoots and fruit shucks, and the conidia are dispersed in wind and rain splash. The pathogen is a threat to all pecan-growing regions with a humid, wet environment.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Fungi
•         Phylum: Ascomycota
•             Subphylum: Pezizomycotina
•                 Class: Dothideomycetes
•                     Subclass: Pleosporomycetidae
•                         Order: Pleosporales
•                             Family: Venturiaceae
•                                 Genus: Fusicladium
•                                     Species: Fusicladium effusum

Notes on Taxonomy and Nomenclature

Pecan scab is the name for the disease associated with Fusicladium effusum. It was first described from Carya tomentosa (=C. alba) by Winter (1885) as F. effusum Wint. Subsequently, Ellis and Everhart (1888) described what they believed to be a different fungus on Carya illinoiensis, which they named Fusicladium caryigenum Ellis and Lang. In 1928 Demaree reassigned the fungus as Cladosporium effusum (G. Winter) Demaree. Sixty-two years later, using developments in the taxonomy of the Hyphomycetes, Gottwald (1982) re-evaluated the fungus based on conidial morphologies, development and epidemiological characteristics and concluded it should be assigned Cladosporium caryigenum (Ellis and Lang.) Gottwald. No teleomorph has been identified for F. effusum, but recently Schnabel et al. (1999), Schubert et al. (2003) and Beck et al. (2005) used molecular (rDNA ITS sequences) and morphological criteria to demonstrate that Venturia species and their anamorphs (including Fusicladium species) formed a monophyletic clade (including F. effusum). The anamorphs were assigned to the single genus Fusicladium nom. cons. prop. (Braun et al., 2002). Venturia anamorphs with catenate conidia, previously referred to as Cladosporium (which included the anamorph on Carya spp.) were assigned to Fusicladium based on the process of conidiogenesis and the structure of the conidiogenous loci (scars) and conidial hila, which agreed with those of other Fusicladium species (Beck et al. 2005). Recent molecular characterization based on the cytochrome-b gene has confirmed that F. effusum is a close relative of Venturia inaequalis (Seyran et al., 2010a).  

A similar fungus (Fusicladium effusum var. carpineum Ellis and Everh.) was observed on Carpinus spp. in North America. But differences in symptoms, condiophore and conidia size resulted in this fungus eventually being considered a separate species (Fusicladium carpineum (Ellis and Everh.) U. Braun and K. Schub.) (Schubert et al., 2003).

Description

Mycelium:  The mycelium is composed of branched, septate, olivaceous brown to brown hyphae (1-3 µm wide). On the host, the fungus colony is hypophyllous and maculicolous. The stromata may be poorly developed on leaves, but well developed on fruit shucks (husks) and twigs (Gottwald, 1982; Partridge and Morgan-Jones, 2003; Schubert et al., 2003).

Conidiophores: The conidiophores are solitary, semi-macronemateous, dark brown and ascending, mostly erect and occasionally branched. They are smooth, cylindrical, septate with thick walls (22-130 µm long and 4-6 µm wide). The conidiogenous cells (10-40 µm long) are polyblastic, integrated and indeterminate with a single or several denticle-like conidiogenous loci (1.5-3 µm wide). They are reported to detach and act as conidia (ramaconidia) (Gottwald, 1982; Partridge and Morgan-Jones, 2003; Schubert et al., 2003).   

Conidia: The conidia are borne in catenulate, frequently branched chains, with 100 or more conidia being produced in acropetal succession. Colour is pale olive-brown to brown. Conidia are pyriform, subcylindrical, ellipsoid to fusiform in shape, with a smooth surface but often bear one or more protuberant scars. They are 10-24 × 5-10 µm in size and are borne on both surfaces of the leaf, on fruit surfaces and on shoots of the host (Gottwald, 1982; Partridge and Morgan-Jones, 2003; Schubert et al., 2003).

Distribution

Pecan scab is now widely distributed (Schubert and Braun, 2002; CABI/EPPO, 2012). The first report was from the USA (Winter, 1885), where it is believed to have originated, but the pathogen has been effectively disseminated along with its host to all areas where pecan is commercially produced and where conditions are humid and wet during the growing season. Apart from the USA, it has been reported in Mexico (Garza-Lopez et al., 1996), South America (Mendes et al., 1998; Kobyashi, 1984; Mantz et al., 2009) and South Africa (Doidge et al., 1953; Crous et al., 2000). There is a report from Canada, but this is has not been confirmed (CABI/EPPO, 2012). In the USA it occurs predominantly in the south-eastern states where weather conditions are conducive to the survival and spread of the pathogen, but Cole (1953) claimed that it has been reported in all states where pecan is grown, including the dry south-western states. Pecan scab has not been reported in Australia (Anon., 1984).

A record of F. effusum in New Zealand (Pennycook, 1989; CABI/EPPO, 2012) published in previous versions of the Compendium is invalid. The source of the record (Pennycook, 1989) is based on earlier interception or quarantine records where the imported material was destroyed and the fungus eradicated (NZFUNGI2, 2019). There are no further detections from New Zealand and no evidence that F. effusum is established (Ministry for Primary Industries, New Zealand, communication to CABI, 2019).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

F. effusum was first reported on Carya infecting Carya tomentosa in North America in 1885 (Winter, 1885). Subsequently pecan (Carya illinoiensis) has become a widely cultivated crop throughout the southern states in the USA, and pecan scab is widely distributed through the south-eastern USA and in Mexico where pecan is grown commercially. Pecan has been introduced as an exotic crop to other countries, and significant industries exist in South America, South Africa and Australia. There are confirmed reports of scab on pecan from South Africa and several countries in South America, but no report of F. effusum in Australia (Anon., 1984). Pecan is cultivated as a minor crop in several other countries (e.g. Israel and Spain) but the presence of scab in these countries is unknown.

Risk of Introduction

F. effusum can be spread on living, infected host material as lesions on the leaves or stems. Infected nursery material might be used for planting in new locations, or diseased scion wood might be grafted onto rootstocks. It is not known to be seedborne. Australia does not have pecan scab, and has strict phytosanitary regulations in place that limits introductions of potentially diseased material.

Habitat

The pathogen needs substantial moisture to complete its life-cycle (rain and leaf wetness), hence its significance as a problem in wet, humid, pecan-growing regions (Sparks et al., 2009). Rain is important for dispersal and production of conidia, and surface wetness lasting several hours is crucial for the infection process (Gottwald and Bertrand, 1982; Gottwald, 1985).

Habitat List

Hosts/Species Affected

From an economic viewpoint pecan (Carya illinoinensis) is the most important host. The other susceptible species are wild hosts in the same genus as pecan (Carya) that occupy similar habitats in the USA (Schubert et al., 2003; http://nt.ars-grin.gov/fungaldatabases). There are reports of its occurrence on Carpinus spp., but the causal agent in now considered a separate species (Schubert et al., 2003). There is an unconfirmed report of F. effusum causing disease on Juglans regia from Brazil (Mendez et al., 1998). No other species are reported to be hosts, or susceptible to infection by F. effusum.

Symptoms

F. effusum affects the fruit, stems, leaves, dormant buds and catkins (Nolen, 1926; Demaree, 1924, 1928; Littrell, 1980). Defoliation and nut drop can occur if infection is severe. The symptoms are similar on all infected plant parts.

On the leaves, dark brown to black spots of scab can be observed on both the abaxial and adaxial surface of the lamina shortly after bud break, and are often associated with the veins or midrib. Spots vary in size from <1 to 7 mm in diameter, which can coalesce into larger spots. When young, the spots have a velvety appearance. As the infection ages, it turns hard and forms a dark grey or silvery to brown spot that extends through the leaf. These senescent spots can crack and drop out of the leaf, resulting in a shot-holed appearance on older leaves. The leaf lesions are a source of inoculum for the young fruit (Nolen, 1926; Demaree, 1928; Schubert et al., 2003).

Small, olive-green to black spots can develop on the young fruit early in the season. If conditions are favourable, the lesions can increase in size, coalesce and cause large areas of disease, often with a velvety appearance while the lesions are young. If the lesions are particularly large the surface can become brown and cracked. If infection is very severe and the pathogen penetrates deeper it can cause the shuck (husk) to cling to the shell of the nut. The spots may be slightly raised. On fruit, black fungal stromata may form on the spot, which can produce a dark, velvety growth of conidiophores the following spring (Nolen, 1926; Demaree, 1928; Schubert et al., 2003). 

On the shoots or twigs the symptoms are similar to those on the leaves or fruit. The edges of the lesions may be slightly raised, with dark fungal growth in the centre. Twig lesions may also form stroma and overwinter, producing conidia the following spring, similar to those zone the fruit shuck (husk) (Nolen, 1926; Demaree, 1928). 

Symptoms on the pedicels and bracts of the catkins and on dormant buds are reported to be slight and typified as small, black spots (Demaree, 1924).

List of Symptoms/Signs

Biology and Ecology

Genetics and Reproductive Biology

The pathogen is known only by an asexual phase, producing conidia which infect the host and cause disease. F. effusum exhibits pathogenic diversity on pecan (Demaree and Cole, 1929; Converse, 1960; Graves, 1975; Conner and Stevenson, 2004). Strains can be pathogenic on one cultivar, or perhaps a few but not on others (Conner and Stevenson, 2004) and there is a history of cultivars previously resistant to pecan scab developing the disease as the pathogen adapts to host resistance genes (Goff et al., 1996).

Epidemiology

Conidia are produced early in the spring on stromata in the previous season’s lesions on twigs and shucks (Demaree, 1924). The conidia are dispersed by wind and rain (Gottwald, 1982; Gottwald and Bertrand, 1982; Latham, 1982) and they are spread to the young leaves after bud break, and subsequently from lesions that develop on the leaves and fruit (Gottwald and Bertrand, 1982). Although wet locations or seasons are particularly prone to severe scab on susceptible cultivars, pecan grown at higher altitude has less scab in the USA (Sparks et al., 2009). The disease is polycyclic with a latent period of 7-18 days, so multiple generations can occur within a season if conditions are suitable (Gottwald and Bertrand, 1982; Turechek and Stevenson, 1998).  Optimum temperature for infection is 15-25°C, with a period of leaf wetness of <10-48 h. Foliage is most susceptible 7-21 days after emergence, becoming relatively resistant to infection when fully expanded (Gottwald and Bertrand, 1982; Turechek and Stevenson, 1998). Resistance in older leaves might be due to changes in phenolics and trichome structure on the leaf surface (Wetzstein and Sparks, 1983). Nuts are susceptible to infection throughout the season (Gottwald and Bertrand, 1983). The disease can spread throughout the canopy of the tree (Bock et al., 2013), although it is often most severe in the lower canopy. In dry areas where scab is grown under irrigation (e.g. the south-western USA) pecan scab is not an issue, even on susceptible cultivars.

Process of Infection and Host Colonization

Under ideal infection conditions, conidia germinate within 3 h of inoculation (Latham and Rushing, 1988; Rushing and Latham, 1991) and after 12 h the hyphae that originate from the aspersoria penetrate the host cuticle. The hyphae grow subcuticulary along the anticlinal walls of the epidermal cells as well as ramifying into the leaf interior through the middle lamella of adjacent epidermal cells. Hyphal growth is intercellular, and melanised, bulbous cells develop on the branches of the subcuticular hyphae by 144 h. The melanised, bulbous cells push through the cuticle and support the conidiophore of the fungus. Conidia develop by 168 h after germination. Various phylloplane chemicals can influence germination of conidia (Wood et al., 1988). But Yates et al. (1996) found little difference in germination of conidia of F. effusum on the leaves of resistant, susceptible or non-host leaves, but on resistant cultivars they observed no subcuticular development of hyphae, while on susceptible subcuticular colonization of the host occurred. Fatty acids in the fruit affected sporulation of F. effusum (Gottwald and Wood, 1984).

F. effusum will infect pecan from the seedling stage throughout the life of the tree, which can exceed 100 years in commercial orchards. The disease has been reported to infect shoots, leaves, fruit and catkins (Nolen, 1926; Demaree and Cole, 1929).

Climate

Latitude/Altitude Ranges

Air Temperature

Notes on Natural Enemies

No natural enemies of F. effusum have been described.

Means of Movement and Dispersal

Natural dispersal: The conidia of F. effusum are dispered by wind and rain (Gottwald and Bertrand, 1982; Latham; 1982).

Accidental introduction: The disease has been unintentionally introduced to locations other than its native range in North America. The pathway for the introductions is not known, but is probably associated with the transport of diseased plants and scion wood. Pecan cultivars are often disseminated as graftwood.

Seedborne Aspects

There is no evidence that F. effusum is seedborne.

Plant Trade

Impact Summary

Impact

Pecan scab causes yield and nut quality losses on susceptible cultivars (Gottwald and Bertrand, 1988; Stevenson and Bertrand, 2001). Nut drop can occur and total yield loss is possible if the disease is severe (Hunter, 1983). The effect of infection is to reduce net photosynthetic and dark respiration rates of both fruit and foliage (Gottwald and Wood, 1985) reduce nut number, nut weight, percent oil, moisture and protein content (Gottwald and Bertrand, 1983). Apart from a direct impact on yield and quality, the disease affects foliage and can result in loss of photosynthetic area and leaf drop. Severe scab can induce alternate bearing, the single most important problem facing the pecan industry in North America (Smith and Weckler, 2011). Reports of the impact of scab on pecan are most numerous in North America, but reports of its occurrence in South America and South Africa suggest that it has the potential to cause epidemics in these geographic areas. Fungicides are used to reduce the impact (Gottwald and Bertrand, 1988) but these are costly.

Economic Impact

Economic losses occur as a result of reduced yield and quality of susceptible cultivars. The cost of fungicides that must be applied several times a season to control the disease on these cultivars also adds to the burden imposed by pecan scab. Pecan growers typically apply chemical fungicides 7 to 10 times per season on susceptible cultivars (Brenneman et al., 1998; Seyran et al., 2010b).

Social Impact

Increased use of pesticides and loss of income might threaten sustainability of pecan production.

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Reproduces asexually

• Negatively impacts agriculture
• Negatively impacts cultural/traditional practices
• Negatively impacts livelihoods

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant
• Difficult/costly to control

Diagnosis

F. effusum can be detected and identified by taking a sample of spores from the symptoms and observing them under a microscope to confirm the identity of the pathogen (Schubert et al., 2003). It can also be isolated from diseased plant material and grown on a wide range of media (Barnes, 1964), but potato dextrose agar or oatmeal agar are very commonly used. The fungus is slow-growing and takes approximately 3 weeks to reach a diameter of 2.5 cm, even on preferred media. Once mature, the colony can be inspected and samples of conidia observed under a microscope to confirm the identity.

Detection and Inspection

Pecan scab is relatively easily detected based on visual inspection for characteristic symptoms of the disease on the foliage, fruit and shoots of its host (Nolen, 1926; Demaree, 1924, 1928; Littrell, 1980; Goff et al., 1996). Characteristic symptoms include dark black spots of scab with a velvety appearance on the surface of the affected organ.  

Similarities to Other Species/Conditions

This is the only species of Fusicladium known to cause disease on pecan and other species in the genus Carya. Morphologically, F. effusum bears some similarity to several other Fusicladium spp. (Schubert and Braun, 2002; Partridge and Morgan-Jones, 2003; Schubert et al., 2003) but differences in spore size, morphology, and colony growth habit can generally be used to differentiate F. effusum in culture. There are no other pathogens that infect pecan and cause the same (or very similar) symptoms.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Regulatory Control

There are no specific regulatory control measures for F. effusum in the USA beyond the standard regulations pertaining to the import and export of plant material. Australia does not have the disease but has strict phytosanitary regulation on the import of material from other countries (Anon., 1984).

Cultural Control

No cultural control measures are particularly effective for managing pecan scab. However, ensuring that the trees in an orchard are adequately spaced to ensure good sunlight penetration and air-flow will help reduce the severity on susceptible cultivars (Cooper and Johnson, 1986). Planting susceptible cultivars in locations that are low-lying and wet should be avoided. Ensuring old shucks (husks) are removed from trees after harvest can also reduce sources of primary inoculum, although being a polycyclic disease this is probably of very limited value.

Biological Control

Although not strictly biological control, Bacillus mycoides is a microbial agent that is thought to induce a systemically acquired resistance response and significantly reduces the severity of pecan scab (Brenneman, 2009).

Chemical Control

Fungicides are the primary method to manage pecan scab on susceptible cultivars. The earliest fungicide used to control scab was Bordeaux mixture (Demaree, 1925; Large, 1965), which can still be used to control scab for organic production of pecans in the USA. Since the 1960s, modern fungicides have been successfully used to reduce the impact of scab (Large, 1965). In  the USA there are now several classes of fungicide that can be used including dithiocarbamates, benzimidazoles, triazoles, strobilurins, guanadines, dialkyldithiocarbamates and organometals (Hudson et al., 2012) and most recently phosphites (Bock et al., 2013), which are thought to elicit a systemically acquired resistance response. However, fungicide resistance is an issue with some of these fungicides (Litrell, 1976; Littrell and Bertrand, 1981; Reynolds et al., 1997; Stevenson, 1999; Stevenson et al., 2004; Seyran et al., 2010b). To ensure prolonged efficacy against pecan scab, either alternate sprays or mixes of fungicides with different activity are advised (Isakeit, 2010). One issue with fungicides is obtaining sufficient spray coverage in mature pecan trees, which can grow to a height of >30 m. Powerful air-blast sprayers provide disease control to a height of 10 m, but above that control of scab is diminished (Bock et al., 2013). In areas where scab is a problem aerial application of fungicide might reduce disease (Bertrand and Brenneman, 2001; Sumner, 2004; Reilly et al., 2007) and reduce the risk of fungicide resistance developing where it might otherwise be exposed to a gradient in fungicide.

Forecasting

There are two forecasting system for pecan scab. The first, AU-PECAN is based on recorded rain events and the 5-day average forecast of more rain (Brenneman et al., 1999; Bertrand and Brenneman, 2001). The second, the Pecan Scab Risk Assessment Tool is available on Pecan ipmPIPE (http://pecan.ipmpipe.org/map/scab/index.cfm) and uses weather data to estimate the accumulation of hours of relative humidity of 90% or more at an air temperature of 70 degrees Fahrenheit or above to produce an assessment of the risk of pecan scab (Broembsen et al., 1998; Payne and Smith, 2012).

Host Resistance and Tree Health

Several cultivars are resistant to scab. There is a history of the pathogen adapting to resistant cultivars, resulting in a loss of resistance (Goff et al., 1996). Current scab cultivars with excellent scab resistance in the southeastern USA include Elliott, Kanza, Curtis, Gloria Grand, Barton and Excel. Cultivars with good resistance to scab include Sumner. Cultivars that are particularly susceptible to scab and which should be avoided in scab-prone areas include Wichita, Desirable and Schley (http://www.caes.uga.edu/commodities/fruits/pecanbreeding/cultivars/scab_resistant.html; http://aggie-horticulture.tamu.edu/carya/index.htm). The genetics of resistance to scab in pecan suggests some additive gene action (Thompson and Grauke, 1994) and recent work has identified various markers (RAPDs, AFLPs and SSRs) that might be useful in helping breed for resistance using marker-assisted selection (Grauke et al., 2003; Beednagari et al., 2005). 

Tree health might influence scab susceptibility. Recent work has suggested that Ni nutrition might play a role in reducing the severity of scab on susceptible pecan (Wood et al., 2012).

Gaps in Knowledge/Research Needs

Epidemiology

Although some good information exits on fundamental aspects of spore production and the environmental factors influencing it, there is little information on the spread of the pathogen and disease within or among trees. How important is inter-tree spread in epidemics? Rainfall  that results in free moisture on susceptible tissue can result in an infection cycle. With multiple wet periods numerous cycles can occur but additional information is needed on the exact parameters of rain events, temperature, humidity, and tissue growth stage influence on epidemics.How do rain events and the temperature and humidity associated with them influence epidemic development? Can further detailed information on these aspects be used to improve the existing forecasting methods?

Genetic diversity and population biology

F. effusum has been characterized as pathogenically diverse, but there is no information available on the molecular genetics or population diversity, but tools (RFLPs, SSRs etc) exist and these could be applied to provide much useful information on these aspects of the pathogen. Armed with better knowledge of the pathogen populations, resistance breeding can be improved.

Fungicide resistance

Fungicide resistance is a real management problem in some areas, and more research is needed to understand the adapting populations of F. effusum. New tools are needed to identify and monitor the fungicide resistant strains to allow appropriate management decisions.

Reproduction

No teleomorph (sexual stage) has been identified, if it exists. Confirming the existence of a teleomorph (or not) should be a priority.

Resistance breeding

Some molecular markers have been developed in pecan. However, more are needed to identify the parts of the genome and the alleles responsible for resistance (eventually for marker assisted selection, which can help speed up the process of breeding with this long-lived and slow-growing crop). Improved, reliable and uniform screening methods also need to be developed, including scab severity assessment methods.

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Links to Websites

Organizations

USA: Pecan Breeding Laboratory (ARS-USDA PBL-CGRU), USDA-ARS Pecan Breeding, 10200 FM 50, Somerville, TX 77879, http://aggie-horticulture.tamu.edu/carya/index.htm

USA: South-Eastern Fruit and Tree Nut Research Laboratory (USDA-ARS-SEFTNRL), 21 Dunbar Rd., Byron, GA 31008, http://www.ars.usda.gov/Main/docs.htm?docid=5039

USA: University of Georgia College of Agricultural and Environmental Sciences, 2360 Rainwater Road, Tifton, GA 31793-5737, http://www.caes.uga.edu/commodities/fruits/pecan/

Contributors

04/06/13 Original text by:

Clive H. Bock,  USDA-ARS-SEFTNRL, 21 Dunbar Rd., Byron, GA 31008, USA

Distribution Maps