Preferred scientific name
Septoria citri Pass. 1877
Preferred common name
Septoria spot of citrus
Alternative common name
Common host plants
The citrus species lemon (Citrus limon) and grapefruit (C. paradisi) are the most frequently damaged crops worldwide, although all citrus cultivars are susceptible (Menge 2000).
In Australia, in addition to these crops, S. citri has been also reported in the eastern states on orange (C. sinensis) cultivars, sour orange (C. auriantium), and native Rutaceae species of the genera Coleonema and Correa (Plant Health Australia 2001).
Coleonema has species members that are naturalised in eastern Australia, Western Australia (WA) or both; and Correa has endemic species in both the eastern states and WA (Australian Plant Census 2012).
Plant part affected
- New South Wales (Plant Health Australia 2001)
- South Australia (Plant Health Australia 2001, D Cartwright 2004, pers. comm. 22 March 2014)
- Tasmania (Plant Health Australia 2001)
- Victoria (Washington and Nancarrow 1983, Plant Health Australia 2001)
Status in Western Australia
Septoria citri is considered to be absent from Western Australia and is a quarantine pest. It is a prohibited organism under section 12 of the Biosecurity and Agriculture Management Act 2007.
To confirm the current status check the Western Australian Organism List. For more information on prohibited organisms please see frequently asked questions about the BAM Act and WAOL.
Biology and ecology
S. citri can remain in infected orchards as a saprobe, with pycnidia often forming on dead twigs and leaves (Menge 2000). Conidia (spores) are easily dispersed from pycnidia by water splash, onto healthy leaves and fruits. There they can germinate and penetrate tissues but persist only as latent infections when disease conditions are unfavourable.
Latent infections usually occur during cool, damp weather in late summer or autumn and when the fruits are still green (Donovan 2007; Cooke et al. 2009). These infections may remain latent for up to six months with disease progression often occurring after cool, frosty (Menge 2000, Cooke et al. 2009) or cold windy weather (Donovan 2007).
Fruit symptoms generally appear as the fruits start to colour during late winter and early spring (Cooke et al. 2009). Lesions begin as small, round, light tan-coloured lesions, 1-2mm in diameter, with a narrow green margin. These depressions, or pits, are usually confined to the flavedo (outer region of the rind).
As the fruit matures, these pits become red-brown to pale-brown and small black spots that are the fruiting bodies of the fungus (pycnidia), barely visible to the naked eye, may form in them. When frost occurs, or during storage, fruit spots may enlarge and merge to form brown-to-black sunken blotches. These may be several centimetres in diameter and extend to the albedo (inner rind) and occasionally into the fruit segments. In severe infections, fruits develop an off-flavour and drop prematurely. Symptoms may not appear until fruit is in storage.
Leaf symptoms are confined to the lower surface of leaves and consist of small, blister-like brown-to-black spots, 1-4mm in diameter, that are surrounded with yellow halos. Leaf spots eventually become necrotic and pale brown. After leaf fall the spots turn brown with a dark margin and the fruiting bodies of the fungus (pycnidia) form in them.
Where under-canopy irrigation is used, leaves with symptoms are often observed closer to the irrigation system (Menge 2000). Infection may result in severe leaf drop in the lower parts of trees.
Menge (2000) notes that disease is usually more severe in years when rainfall levels are higher than normal, and also indicates that low or rapidly fluctuating temperatures may affect susceptibility of citrus tissues to S. citri and also encourage development of disease symptoms.
Septoria spot is generally considered a disease of minor significance, except for where fruit are grown for the fresh market (Menge 2000). Aesthetically, fruit lesions reduce fruit quality, grade and marketability, and are of concern to growers aiming for high value fresh markets.
Septoria spot is an economically important disease in citrus production zones in Victoria, South Australia and southern New South Wales (Cooke et al. 2009). Grapefruit (C. paradisi), lemon (C. limon) and navel orange (C. sinensis 'Washington') are the most susceptible hosts in these Australian growing zones.
Juicing orange (C. sinensis 'Valencia') can be affected, although this cultivar is usually less susceptible to infection (Cooke et al. 2009). S. citri has been also reported on sour orange (C. auriantium) in the eastern states (Plant Health Australia 2001), although the extent of crop losses was not reported.
The susceptibility of fruits is related to the maturity of the rind at the time of infection. Good disease control practices are essential for export markets (Cooke et al. 2009), as further symptoms may develop during transit.
What do I do if I find it?
Septoria citri is a prohibited organism for Western Australia. It is important that suspected disease occurrence is reported. Please contact the Pest and Disease Information Service (PaDIS) to report this pest.
Australian Plant Census 2012, IBIS database, Centre for Australian National Biodiversity Research, Council of Heads of Australasian Herbaria, viewed 8 April 2014, www.chah.gov.au/apc/index.html.
Cooke T, Persley D, House S 2009, Diseases of Fruit Crops in Australia, CSIRO Publishing, Collingwood.
Donovan N 2007, 'Managing septoria spot in citrus', Primefact 753, New South Wales Department of Primary Industries.
Menge JA 2000, 'Septoria Spot' in LW Timmer, SM Garnsey, JH Graham (eds), Compendium of Citrus Diseases, 2nd edn, The American Phytopathological Society, Minnesota, pp. 32-33.
Plant Health Australia 2001, Australian Plant Pest Database, online database, accessed 8 April 2014.
Washington WS, Nancarrow RJ 1983, List of diseases recorded on fruit and vegetable crops in Victoria before June 30, 1980, Department of Agriculture, Victoria.